java - launch a Java application
To launch a class file:
java [options] mainclass
[args …]
To launch the main class in a JAR file:
java [options] -jar
jarfile [args …]
To launch the main class in a module:
java [options] -m
module[/mainclass] [args …]
or
java [options] --module
module[/mainclass] [args …]
To launch a source-file program:
java [options] source-file
[args …]
classname are the arguments for the main method.
-jar jarfileMain-Class:classname that defines the
class with the public static void main(String[] args)
method that serves as your application’s starting point. When you use
-jar, the specified JAR file is the source of all user
classes, and other class path settings are ignored. If you’re using JAR
files, then see jar.
-m or --module
module[/mainclass]Executes the main class in a module specified by mainclass if it is given, or, if it is not given, the value in the module. In other words, mainclass can be used when it is not specified by the module, or to override the value when it is specified.
-jar jarfile, and -m or
--module module/mainclass
are passed as arguments to the main class.
The java command starts a Java application. It does this
by starting the Java Virtual Machine (JVM), loading the specified class,
and calling that class’s main() method. The method must be
declared public and static, it must not return
any value, and it must accept a String array as a
parameter. The method declaration has the following form:
public static void main(String[] args)
In source-file mode, the java command can launch a class
declared in a source file. See Using
Source-File Mode to Launch Source-Code Programs for a description of
using the source-file mode.
Note: You can use the
JDK_JAVA_OPTIONSlauncher environment variable to prepend its content to the actual command line of thejavalauncher. See Using the JDK_JAVA_OPTIONS Launcher Environment Variable.
By default, the first argument that isn’t an option of the
java command is the fully qualified name of the class to be
called. If -jar is specified, then its argument is the name
of the JAR file containing class and resource files for the application.
The startup class must be indicated by the Main-Class
manifest header in its manifest file.
Arguments after the class file name or the JAR file name are passed
to the main() method.
javawWindows: The javaw command is identical
to java, except that with javaw there’s no
associated console window. Use javaw when you don’t want a
command prompt window to appear. The javaw launcher will,
however, display a dialog box with error information if a launch
fails.
To launch a class declared in a source file, run the
java launcher in source-file mode. Entering source-file
mode is determined by two items on the java command
line:
The first item on the command line that is not an option or part of an option. In other words, the item in the command line that would otherwise be the main class name.
The --source version option, if
present.
If the class identifies an existing file that has a
.java extension, or if the --source option is
specified, then source-file mode is selected. The source file is then
compiled and run. The --source option can be used to
specify the source version or N of the source code.
This determines the API that can be used. When you set
--source N, you can only use the public API that
was defined in JDK N.
Note: The valid values of N change for each release, with new values added and old values removed. You’ll get an error message if you use a value of N that is no longer supported. The supported values of N are the current Java SE release (
@@VERSION_SPECIFICATION@@) and a limited number of previous releases, detailed in the command-line help forjavac, under the--sourceand--releaseoptions.
If the file does not have the .java extension, the
--source option must be used to tell the java
command to use the source-file mode. The --source option is
used for cases when the source file is a “script” to be executed and the
name of the source file does not follow the normal naming conventions
for Java source files.
In source-file mode, the effect is as though the source file is compiled into memory, and the first class found in the source file is executed. Any arguments placed after the name of the source file in the original command line are passed to the compiled class when it is executed.
For example, if a file were named HelloWorld.java and
contained a class named HelloWorld, then the source-file
mode command to launch the class would be:
java HelloWorld.java
This use of source-file mode is informally equivalent to using the following two commands:
javac -d <memory> --source-path <source-root> HelloWorld.java
java --class-path <memory> HelloWorldwhere <source-root> is computed
In source-file mode, any additional command-line options are processed as follows:
The launcher scans the options specified before the source file for any that are relevant in order to compile the source file.
This includes: --class-path, --module-path,
--add-exports, --add-modules,
--limit-modules, --patch-module,
--upgrade-module-path, and any variant forms of those
options. It also includes the new --enable-preview option,
described in JEP 12.
No provision is made to pass any additional options to the
compiler, such as -processor or
-Werror.
Command-line argument files (@-files) may be used in
the standard way. Long lists of arguments for either the VM or the
program being invoked may be placed in files specified on the
command-line by prefixing the filename with an @
character.
In source-file mode, compilation proceeds as follows:
Any command-line options that are relevant to the compilation
environment are taken into account. These include:
--class-path/-classpath/-cp,
--module-path/-p, --add-exports,
--add-modules, --limit-modules,
--patch-module, --upgrade-module-path,
--enable-preview.
The root of the source tree, <source-root> is
computed from the package of the launched class. For example, if
HelloWorld.java declared its classes to be in the
hello package, then the file HelloWorld.java
is expected to reside in the directory somedir/hello/. In
this case, somedir is computed to be the root of the source
tree.
The root of the source tree serves as the source-path for
compilation, so that other source files found in that tree and are
needed by HelloWorld could be compiled.
Annotation processing is disabled, as if -proc:none
is in effect.
If a version is specified, via the --source option,
the value is used as the argument for an implicit --release
option for the compilation. This sets both the source version accepted
by compiler and the system API that may be used by the code in the
source file.
If a module-info.java file exists in the
<source-root> directory, its module declaration is
used to define a named module that will contain all the classes compiled
from .java files in the source tree. If
module-info.java does not exist, all the classes compiled
from source files will be compiled in the context of the unnamed
module.
The source file that is launched should contain one or more
top-level classes. If the first top-level class in the file has a
main entry point, that will be the class to be executed;
otherwise, the class to be executed is the top-level class that has the
same name as the base name of the source file.
For source file that is launched source file, the compiler does
not enforce the optional restriction defined at the end of JLS 7.6, that
a type in a named package should exist in a file whose name is composed
from the type name followed by the .java
extension.
If a source file contains errors, appropriate error messages are written to the standard error stream, and the launcher exits with a non-zero exit code.
In source-file mode, execution proceeds as follows:
If the first top-level class found in the source file contains an
entry main method, then that is the class to be executed;
otherwise, the class to be executed is the top-level class that has the
same name as the base name of the source file. The file to be executed
must have an entry main method.
The compiled classes are loaded by a custom class loader, that delegates to the application class loader. This implies that classes appearing on the application class path cannot refer to any classes declared in source files.
If a module-info.java file exists in the
<source-root> directory, then all the classes
compiled from .java files in the source tree will be in
that module, which will serve as the root module for the execution of
the program. If module-info.java does not exist, the
compiled classes are executed in the context of an unnamed module, as
though --add-modules=ALL-DEFAULT is in effect. This is in
addition to any other --add-module options that may be have
been specified on the command line.
Any arguments appearing after the name of the file on the command line are passed to the main method in the obvious way.
It is an error if there is a class on the application class path whose name is the same as that of the class to be executed.
See JEP 458: Launch Multi-File Source-Code Programs for complete details.
JDK_JAVA_OPTIONS prepends its content to the options
parsed from the command line. The content of the
JDK_JAVA_OPTIONS environment variable is a list of
arguments separated by white-space characters (as determined by
isspace()). These are prepended to the command line
arguments passed to java launcher. The encoding requirement
for the environment variable is the same as the java
command line on the system. JDK_JAVA_OPTIONS environment
variable content is treated in the same manner as that specified in the
command line.
Single (') or double (") quotes can be used
to enclose arguments that contain whitespace characters. All content
between the open quote and the first matching close quote are preserved
by simply removing the pair of quotes. In case a matching quote is not
found, the launcher will abort with an error message.
@-files are supported as they are specified in the command
line. However, as in @-files, use of a wildcard is not
supported. In order to mitigate potential misuse of
JDK_JAVA_OPTIONS behavior, options that specify the main
class (such as -jar) or cause the java
launcher to exit without executing the main class (such as
-h) are disallowed in the environment variable. If any of
these options appear in the environment variable, the launcher will
abort with an error message. When JDK_JAVA_OPTIONS is set,
the launcher prints a message to stderr as a reminder.
Example:
$ export JDK_JAVA_OPTIONS='-g @file1 -Dprop=value @file2 -Dws.prop="white spaces"'
$ java -Xint @file3is equivalent to the command line:
java -g @file1 -Dprop=value @file2 -Dws.prop="white spaces" -Xint @file3The java command supports a wide range of options in the
following categories:
Standard Options for Java: Options guaranteed to be supported by all implementations of the Java Virtual Machine (JVM). They’re used for common actions, such as checking the version of the JRE, setting the class path, enabling verbose output, and so on.
Extra Options for Java:
General purpose options that are specific to the Java HotSpot Virtual
Machine. They aren’t guaranteed to be supported by all JVM
implementations, and are subject to change. These options start with
-X.
The advanced options aren’t recommended for casual use. These are
developer options used for tuning specific areas of the Java HotSpot
Virtual Machine operation that often have specific system requirements
and may require privileged access to system configuration parameters.
Several examples of performance tuning are provided in Performance Tuning Examples.
These options aren’t guaranteed to be supported by all JVM
implementations and are subject to change. Advanced options start with
-XX.
Advanced Runtime Options for Java: Control the runtime behavior of the Java HotSpot VM.
Advanced JIT Compiler Options for java: Control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.
Advanced Serviceability Options for Java: Enable gathering system information and performing extensive debugging.
Advanced Garbage Collection Options for Java: Control how garbage collection (GC) is performed by the Java HotSpot
Boolean options are used to either enable a feature that’s disabled
by default or disable a feature that’s enabled by default. Such options
don’t require a parameter. Boolean -XX options are enabled
using the plus sign (-XX:+OptionName) and disabled
using the minus sign (-XX:-OptionName).
For options that require an argument, the argument may be separated
from the option name by a space, a colon (:), or an equal sign (=), or
the argument may directly follow the option (the exact syntax differs
for each option). If you’re expected to specify the size in bytes, then
you can use no suffix, or use the suffix k or
K for kilobytes (KB), m or M for
megabytes (MB), or g or G for gigabytes (GB).
For example, to set the size to 8 GB, you can specify either
8g, 8192m, 8388608k, or
8589934592 as the argument. If you are expected to specify
the percentage, then use a number from 0 to 1. For example, specify
0.25 for 25%.
The following sections describe the options that are deprecated, obsolete, and removed:
Deprecated Java Options: Accepted and acted upon — a warning is issued when they’re used.
Obsolete Java Options: Accepted but ignored — a warning is issued when they’re used.
Removed Java Options: Removed — using them results in an error.
These are the most commonly used options supported by all implementations of the JVM.
Note: To specify an argument for a long option, you can use either
--name=value or--name value.
-agentlib:libname[=options]Loads the specified native agent library. After the library name, a
comma-separated list of options specific to the library can be used. If
the option -agentlib:foo is specified, then the JVM
attempts to load the library named foo using the platform
specific naming conventions and locations:
Linux and other POSIX-like platforms: The JVM
attempts to load the library named libfoo.so in the
location specified by the LD_LIBRARY_PATH system
variable.
macOS: The JVM attempts to load the library
named libfoo.dylib in the location specified by the
DYLD_LIBRARY_PATH system variable.
Windows: The JVM attempts to load the library
named foo.dll in the location specified by the
PATH system variable.
The following example shows how to load the Java Debug Wire Protocol (JDWP) library and listen for the socket connection on port 8000, suspending the JVM before the main class loads:
-agentlib:jdwp=transport=dt_socket,server=y,address=8000
-agentpath:pathname[=options]-agentlib but uses the full path
and file name of the library.
--class-path classpath,
-classpath classpath, or -cp
classpathSpecifies a list of directories, JAR files, and ZIP archives to search for class files.
On Windows, semicolons (;) separate entities in this
list; on other platforms it is a colon (:).
Specifying classpath overrides any setting of the
CLASSPATH environment variable. If the class path option
isn’t used and classpath isn’t set, then the user class path
consists of the current directory (.).
As a special convenience, a class path element that contains a base
name of an asterisk (*) is considered equivalent to specifying a list of
all the files in the directory with the extension .jar or
.JAR . A Java program can’t tell the difference between the
two invocations. For example, if the directory mydir contains
a.jar and b.JAR, then the class path element
mydir/* is expanded to A.jar:b.JAR, except that the order
of JAR files is unspecified. All .jar files in the
specified directory, even hidden ones, are included in the list. A class
path entry consisting of an asterisk (*) expands to a list of all the
jar files in the current directory. The CLASSPATH
environment variable, where defined, is similarly expanded. Any class
path wildcard expansion that occurs before the Java VM is started. Java
programs never see wildcards that aren’t expanded except by querying the
environment, such as by calling
System.getenv("CLASSPATH").
--disable-@files@filename expansion. This option stops
expanding @-argfiles after the option.
--enable-preview--enable-native-access
module[,module…]ALL-UNNAMED to indicate
code on the class path. When this option is present, any use of
restricted methods by code outside the specified modules causes an
IllegalCallerException.
--finalization=value--module-path modulepath… or -p
modulepathSpecifies where to find application modules with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.
On Windows, semicolons (;) separate path elements in
this list; on other platforms it is a colon (:).
--upgrade-module-path modulepath…Specifies where to find module replacements of upgradeable modules in the runtime image with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.
On Windows, semicolons (;) separate path elements in
this list; on other platforms it is a colon (:).
--add-modules
module[,module…]ALL-DEFAULT,
ALL-SYSTEM, and ALL-MODULE-PATH.
--list-modules-d module_name or
--describe-module module_name--dry-run--dry-run option might be useful for validating the
command-line options such as the module system configuration.
--validate-modules-Dproperty=value-Dfoo="foo bar").
-disableassertions[:[packagename]…|:classname]
or
-da[:[packagename]…|:classname]Disables assertions. By default, assertions are disabled in all
packages and classes. With no arguments, -disableassertions
(-da) disables assertions in all packages and classes. With
the packagename argument ending in ..., the switch
disables assertions in the specified package and any subpackages. If the
argument is simply ..., then the switch disables assertions
in the unnamed package in the current working directory. With the
classname argument, the switch disables assertions in the
specified class.
The -disableassertions (-da) option applies
to all class loaders and to system classes (which don’t have a class
loader). There’s one exception to this rule: If the option is provided
with no arguments, then it doesn’t apply to system classes. This makes
it easy to disable assertions in all classes except for system classes.
The -disablesystemassertions option enables you to disable
assertions in all system classes. To explicitly enable assertions in
specific packages or classes, use the -enableassertions
(-ea) option. Both options can be used at the same time.
For example, to run the MyClass application with assertions
enabled in the package com.wombat.fruitbat (and any
subpackages) but disabled in the class
com.wombat.fruitbat.Brickbat, use the following
command:
java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
-disablesystemassertions or -dsa-enableassertions[:[packagename]…|:classname]
or
-ea[:[packagename]…|:classname]Enables assertions. By default, assertions are disabled in all
packages and classes. With no arguments, -enableassertions
(-ea) enables assertions in all packages and classes. With
the packagename argument ending in ..., the switch
enables assertions in the specified package and any subpackages. If the
argument is simply ..., then the switch enables assertions
in the unnamed package in the current working directory. With the
classname argument, the switch enables assertions in the
specified class.
The -enableassertions (-ea) option applies
to all class loaders and to system classes (which don’t have a class
loader). There’s one exception to this rule: If the option is provided
with no arguments, then it doesn’t apply to system classes. This makes
it easy to enable assertions in all classes except for system classes.
The -enablesystemassertions option provides a separate
switch to enable assertions in all system classes. To explicitly disable
assertions in specific packages or classes, use the
-disableassertions (-da) option. If a single
command contains multiple instances of these switches, then they’re
processed in order, before loading any classes. For example, to run the
MyClass application with assertions enabled only in the
package com.wombat.fruitbat (and any subpackages) but
disabled in the class com.wombat.fruitbat.Brickbat, use the
following command:
java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
-enablesystemassertions or -esa-help, -h, or -?--help-javaagent:jarpath[=options]java.lang.instrument.
--show-version-showversion--show-module-resolution-splash:imagepathShows the splash screen with the image specified by
imagepath. HiDPI scaled images are automatically supported and
used if available. The unscaled image file name, such as
image.ext, should always be passed as the argument to the
-splash option. The most appropriate scaled image provided
is picked up automatically.
For example, to show the splash.gif file from the
images directory when starting your application, use the
following option:
-splash:images/splash.gif
See the SplashScreen API documentation for more information.
-verbose:class-verbose:gc-verbose:jni-verbose:module--version-version-X--help-extra@argfileSpecifies one or more argument files prefixed by @ used
by the java command. It isn’t uncommon for the
java command line to be very long because of the
.jar files needed in the classpath. The
@argfile option overcomes command-line length
limitations by enabling the launcher to expand the contents of argument
files after shell expansion, but before argument processing. Contents in
the argument files are expanded because otherwise, they would be
specified on the command line until the --disable-@files
option was encountered.
The argument files can also contain the main class name and all
options. If an argument file contains all of the options required by the
java command, then the command line could simply be:
java @argfile
See java Command-Line
Argument Files for a description and examples of using
@-argfiles.
The following java options are general purpose options
that are specific to the Java HotSpot Virtual Machine.
-Xbatch-Xbatch flag
disables background compilation so that compilation of all methods
proceeds as a foreground task until completed. This option is equivalent
to -XX:-BackgroundCompilation.
-Xbootclasspath/a:directories|zip|JAR-filesSpecifies a list of directories, JAR files, and ZIP archives to append to the end of the default bootstrap class path.
On Windows, semicolons (;) separate entities in this
list; on other platforms it is a colon (:).
-Xcheck:jniPerforms additional checks for Java Native Interface (JNI) functions.
The following checks are considered indicative of significant problems with the native code, and the JVM terminates with an irrecoverable error in such cases:
JNIEnv belonging
to another thread.jfieldID, or jmethodID, is detected as
being invalid. For example:
ReleaseXXX function on a parameter
not produced by the corresponding GetXXX functionThe following checks only result in warnings being printed:
Lname;) when
it should not be.NULL parameter is allowed, but its use is
questionable.Get/ReleasePrimitiveArrayCritical or
Get/ReleaseStringCriticalExpect a performance degradation when this option is used.
-Xcomp-Xdebug-Xdiag-Xint-Xinternalversion-version option, and then exits.
-Xlog:option-Xmixed-Xint to
switch off.
-Xmn sizeSets the initial and maximum size (in bytes) of the heap for the
young generation (nursery) in the generational collectors. Append the
letter k or K to indicate kilobytes,
m or M to indicate megabytes, or
g or G to indicate gigabytes. The young
generation region of the heap is used for new objects. GC is performed
in this region more often than in other regions. If the size for the
young generation is too small, then a lot of minor garbage collections
are performed. If the size is too large, then only full garbage
collections are performed, which can take a long time to complete. It is
recommended that you do not set the size for the young generation for
the G1 collector, and keep the size for the young generation greater
than 25% and less than 50% of the overall heap size for other
collectors. The following examples show how to set the initial and
maximum size of young generation to 256 MB using various units:
-Xmn256m
-Xmn262144k
-Xmn268435456Instead of the -Xmn option to set both the initial and
maximum size of the heap for the young generation, you can use
-XX:NewSize to set the initial size and
-XX:MaxNewSize to set the maximum size.
-Xms sizeSets the minimum and the initial size (in bytes) of the heap. This
value must be a multiple of 1024 and greater than 1 MB. Append the
letter k or K to indicate kilobytes,
m or M to indicate megabytes, or
g or G to indicate gigabytes. The following
examples show how to set the size of allocated memory to 6 MB using
various units:
-Xms6291456
-Xms6144k
-Xms6mIf you do not set this option, then the initial size will be set as
the sum of the sizes allocated for the old generation and the young
generation. The initial size of the heap for the young generation can be
set using the -Xmn option or the -XX:NewSize
option.
Note that the -XX:InitialHeapSize option can also be
used to set the initial heap size. If it appears after -Xms
on the command line, then the initial heap size gets set to the value
specified with -XX:InitialHeapSize.
-Xmx sizeSpecifies the maximum size (in bytes) of the heap. This value must be
a multiple of 1024 and greater than 2 MB. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. The default value is chosen at
runtime based on system configuration. For server deployments,
-Xms and -Xmx are often set to the same value.
The following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
-Xmx83886080
-Xmx81920k
-Xmx80mThe -Xmx option is equivalent to
-XX:MaxHeapSize.
-Xnoclassgc-Xnoclassgc at startup, the class objects in the
application are left untouched during GC and are always be considered
live. This can result in more memory being permanently occupied which,
if not used carefully, throws an out-of-memory exception.
-XrsReduces the use of operating system signals by the JVM. Shutdown hooks enable the orderly shutdown of a Java application by running user cleanup code (such as closing database connections) at shutdown, even if the JVM terminates abruptly.
Non-Windows:
The JVM catches signals to implement shutdown hooks for
unexpected termination. The JVM uses SIGHUP,
SIGINT, and SIGTERM to initiate the running of
shutdown hooks.
Applications embedding the JVM frequently need to trap signals
such as SIGINT or SIGTERM, which can lead to
interference with the JVM signal handlers. The -Xrs option
is available to address this issue. When -Xrs is used, the
signal masks for SIGINT, SIGTERM,
SIGHUP, and SIGQUIT aren’t changed by the JVM,
and signal handlers for these signals aren’t installed.
Windows:
The JVM watches for console control events to implement shutdown
hooks for unexpected termination. Specifically, the JVM registers a
console control handler that begins shutdown-hook processing and returns
TRUE for CTRL_C_EVENT,
CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, and
CTRL_SHUTDOWN_EVENT.
The JVM uses a similar mechanism to implement the feature of
dumping thread stacks for debugging purposes. The JVM uses
CTRL_BREAK_EVENT to perform thread dumps.
If the JVM is run as a service (for example, as a servlet engine
for a web server), then it can receive CTRL_LOGOFF_EVENT
but shouldn’t initiate shutdown because the operating system doesn’t
actually terminate the process. To avoid possible interference such as
this, the -Xrs option can be used. When the
-Xrs option is used, the JVM doesn’t install a console
control handler, implying that it doesn’t watch for or process
CTRL_C_EVENT, CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT, or
CTRL_SHUTDOWN_EVENT.
There are two consequences of specifying -Xrs:
Non-Windows: SIGQUIT thread dumps
aren’t available.
Windows: Ctrl + Break thread dumps aren’t available.
User code is responsible for causing shutdown hooks to run, for
example, by calling System.exit() when the JVM is to be
terminated.
-Xshare:modeSets the class data sharing (CDS) mode.
Possible mode arguments for this option include the following:
autoonNote: The
-Xshare:onoption is used for testing purposes only. It may cause the VM to unexpectedly exit during start-up when the CDS archive cannot be used (for example, when certain VM parameters are changed, or when a different JDK is used). This option should not be used in production environments.
off-XshowSettings-XshowSettings:categoryShows settings and continues. Possible category arguments for this option include the following:
alllocalepropertiesvmsystem-Xss sizeSets the thread stack size (in bytes). Append the letter
k or K to indicate KB, m or
M to indicate MB, or g or G to
indicate GB. The actual size may be rounded up to a multiple of the
system page size as required by the operating system. The default value
depends on the platform. For example:
Linux/x64: 1024 KB
Linux/Aarch64: 2048 KB
macOS/x64: 1024 KB
macOS/Aarch64: 2048 KB
Windows: The default value depends on virtual memory
The following examples set the thread stack size to 1024 KB in different units:
-Xss1m
-Xss1024k
-Xss1048576This option is similar to -XX:ThreadStackSize.
--add-reads
module=target-module(,target-module)*ALL-UNNAMED to read all unnamed modules.
--add-exports
module/package=target-module(,target-module)*ALL-UNNAMED to export to all
unnamed modules.
--add-opens
module/package=target-module(,target-module)*--limit-modules
module[,module…]--patch-module
module=file(;file)*--source versionThe following extra options are macOS specific.
-XstartOnFirstThreadmain() method on the first (AppKit) thread.
-Xdock:name=application_name-Xdock:icon=path_to_icon_fileThese java options can be used to enable other advanced
options.
-XX:+UnlockDiagnosticVMOptionsUnlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options aren’t available.
Command line options that are enabled with the use of this option are not supported. If you encounter issues while using any of these options, it is very likely that you will be required to reproduce the problem without using any of these unsupported options before Oracle Support can assist with an investigation. It is also possible that any of these options may be removed or their behavior changed without any warning.
-XX:+UnlockExperimentalVMOptionsThese java options control the runtime behavior of the
Java HotSpot VM.
-XX:ActiveProcessorCount=xOverrides the number of CPUs that the VM will use to calculate the size of thread pools it will use for various operations such as Garbage Collection and ForkJoinPool.
The VM normally determines the number of available processors from
the operating system. This flag can be useful for partitioning CPU
resources when running multiple Java processes in docker containers.
This flag is honored even if UseContainerSupport is not
enabled. See -XX:-UseContainerSupport for a description of
enabling and disabling container support.
-XX:AllocateHeapAt=pathTakes a path to the file system and uses memory mapping to allocate the object heap on the memory device. Using this option enables the HotSpot VM to allocate the Java object heap on an alternative memory device, such as an NV-DIMM, specified by the user.
Alternative memory devices that have the same semantics as DRAM, including the semantics of atomic operations, can be used instead of DRAM for the object heap without changing the existing application code. All other memory structures (such as the code heap, metaspace, and thread stacks) continue to reside in DRAM.
Some operating systems expose non-DRAM memory through the file
system. Memory-mapped files in these file systems bypass the page cache
and provide a direct mapping of virtual memory to the physical memory on
the device. The existing heap related flags (such as -Xmx
and -Xms) and garbage-collection related flags continue to
work as before.
-XX:-CompactStringsDisables the Compact Strings feature. By default, this option is enabled. When this option is enabled, Java Strings containing only single-byte characters are internally represented and stored as single-byte-per-character Strings using ISO-8859-1 / Latin-1 encoding. This reduces, by 50%, the amount of space required for Strings containing only single-byte characters. For Java Strings containing at least one multibyte character: these are represented and stored as 2 bytes per character using UTF-16 encoding. Disabling the Compact Strings feature forces the use of UTF-16 encoding as the internal representation for all Java Strings.
Cases where it may be beneficial to disable Compact Strings include the following:
When it’s known that an application overwhelmingly will be allocating multibyte character Strings
In the unexpected event where a performance regression is observed in migrating from Java SE 8 to Java SE 9 and an analysis shows that Compact Strings introduces the regression
In both of these scenarios, disabling Compact Strings makes sense.
-XX:ErrorFile=filenameSpecifies the path and file name to which error data is written when
an irrecoverable error occurs. By default, this file is created in the
current working directory and named
hs_err_pidpid.log where pid
is the identifier of the process that encountered the error.
The following example shows how to set the default log file (note
that the identifier of the process is specified as %p):
-XX:ErrorFile=./hs_err_pid%p.log
Non-Windows: The following example shows how to
set the error log to /var/log/java/java_error.log:
-XX:ErrorFile=/var/log/java/java_error.log
Windows: The following example shows how to set
the error log file to C:/log/java/java_error.log:
-XX:ErrorFile=C:/log/java/java_error.log
If the file exists, and is writeable, then it will be overwritten. Otherwise, if the file can’t be created in the specified directory (due to insufficient space, permission problem, or another issue), then the file is created in the temporary directory for the operating system:
Non-Windows: The temporary directory is
/tmp.
Windows: The temporary directory is specified by
the value of the TMP environment variable; if that
environment variable isn’t defined, then the value of the
TEMP environment variable is used.
-XX:+ExtensiveErrorReportsErrorFile. This option can be turned on in environments
where maximal information is desired - even if the resulting logs may be
quite large and/or contain information that might be considered
sensitive. The information can vary from release to release, and across
different platforms. By default this option is disabled.
-XX:FlightRecorderOptions=parameter=value
(or)
-XX:FlightRecorderOptions:parameter=valueSets the parameters that control the behavior of JFR. Multiple parameters can be specified by separating them with a comma.
The following list contains the available JFR
parameter=value entries:
globalbuffersize=sizememorysize. Change the memorysize parameter to
alter the size of global buffers.
maxchunksize=sizem or M to specify the size in megabytes
(MB), or g or G to specify the size in
gigabytes (GB). By default, the maximum size of data chunks is set to 12
MB. The minimum allowed is 1 MB.
memorysize=sizeglobalbuffersize and numglobalbuffers
parameters based on the size specified. Append m or
M to specify the size in megabytes (MB), or g
or G to specify the size in gigabytes (GB). By default, the
memory size is set to 10 MB.
numglobalbuffersmemorysize
parameter to alter the number of global buffers.
old-object-queue-size=number-of-objectspreserve-repository={true|false}repository=pathretransform={true|false}stackdepth=depththreadbuffersize=size-XX:LargePageSizeInBytes=sizeSets the maximum large page size (in bytes) used by the JVM. The
size argument must be a valid page size supported by the
environment to have any effect. Append the letter k or
K to indicate kilobytes, m or M
to indicate megabytes, or g or G to indicate
gigabytes. By default, the size is set to 0, meaning that the JVM will
use the default large page size for the environment as the maximum size
for large pages. See Large Pages.
The following example describes how to set the large page size to 1 gigabyte (GB):
-XX:LargePageSizeInBytes=1g
-XX:MaxDirectMemorySize=sizeSets the maximum total size (in bytes) of the java.nio
package, direct-buffer allocations. Append the letter k or
K to indicate kilobytes, m or M
to indicate megabytes, or g or G to indicate
gigabytes. If not set, the flag is ignored and the JVM chooses the size
for NIO direct-buffer allocations automatically.
The following examples illustrate how to set the NIO size to 1024 KB in different units:
-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576-XX:-MaxFDLimit-XX:NativeMemoryTracking=modeSpecifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:
off-XX:NativeMemoryTracking
option.
summarydetailCallSite, individual virtual memory
region and its committed regions.
-XX:+NeverActAsServerClassMachineEnable the “Client VM emulation” mode which only uses the C1 JIT
compiler, a 32Mb CodeCache and the Serial GC. The maximum amount of
memory that the JVM may use (controlled by the -XX:MaxRAM=n
flag) is set to 1GB by default. The string “emulated-client” is added to
the JVM version string.
By default the flag is set to true only on Windows in
32-bit mode and false in all other cases.
The “Client VM emulation” mode will not be enabled if any of the following flags are used on the command line:
-XX:{+|-}TieredCompilation
-XX:CompilationMode=mode
-XX:TieredStopAtLevel=n
-XX:{+|-}EnableJVMCI
-XX:{+|-}UseJVMCICompiler-XX:ObjectAlignmentInBytes=alignmentSets the memory alignment of Java objects (in bytes). By default, the value is set to 8 bytes. The specified value should be a power of 2, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.
The heap size limit in bytes is calculated as:
4GB * ObjectAlignmentInBytes
Note: As the alignment value increases, the unused space between objects also increases. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes.
-XX:OnError=stringSets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.
Non-Windows: The following example shows how the
-XX:OnError option can be used to run the
gcore command to create a core image, and start the
gdb debugger to attach to the process in case of an
irrecoverable error (the %p designates the current process
identifier):
-XX:OnError="gcore %p;gdb -p %p"
Windows: The following example shows how the
-XX:OnError option can be used to run the
userdump.exe utility to obtain a crash dump in case of an
irrecoverable error (the %p designates the current process
identifier). This example assumes that the path to the
userdump.exe utility is specified in the PATH
environment variable:
-XX:OnError="userdump.exe %p"
-XX:OnOutOfMemoryError=stringOutOfMemoryError exception is first thrown. If the
string contains spaces, then it must be enclosed in quotation marks. For
an example of a command string, see the description of the
-XX:OnError option.
-XX:+PrintCommandLineFlags-XX:+PreserveFramePointer-XX:-PreserveFramePointer) and using the RBP register to
hold the frame pointer of the currently executing method
(-XX:+PreserveFramePointer . If the frame pointer is
available, then external profiling tools (for example, Linux perf) can
construct more accurate stack traces.
-XX:+PrintNMTStatistics-XX:NativeMemoryTracking). By default, this option is
disabled and native memory tracking data isn’t printed.
-XX:SharedArchiveFile=pathSpecifies the path and name of the class data sharing (CDS) archive file
-XX:SharedArchiveConfigFile=shared_config_file-XX:SharedClassListFile=file_nameSpecifies the text file that contains the names of the classes to
store in the class data sharing (CDS) archive. This file contains the
full name of one class per line, except slashes (/) replace
dots (.). For example, to specify the classes
java.lang.Object and hello.Main, create a text
file that contains the following two lines:
java/lang/Object
hello/MainThe classes that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths.
-XX:+ShowCodeDetailsInExceptionMessagesNullPointerException messages.
When an application throws a NullPointerException, the
option enables the JVM to analyze the program’s bytecode instructions to
determine precisely which reference is null, and describes
the source with a null-detail message. The null-detail message is
calculated and returned by
NullPointerException.getMessage(), and will be printed as
the exception message along with the method, filename, and line number.
By default, this option is enabled.
-XX:+ShowMessageBoxOnError-XX:StartFlightRecording=parameter=valueStarts a JFR recording for the Java application. This option is
equivalent to the JFR.start diagnostic command that starts
a recording during runtime. You can set the following
parameter=value entries when starting a
JFR recording:
delay=times to specify the time in
seconds, m for minutes, h for hours, or
d for days (for example, specifying 10m means
10 minutes). By default, there’s no delay, and this parameter is set to
0.
disk={true|false}dumponexit={true|false}filename is not entered, the recording is
written to a file in the directory where the process was started. The
file name is a system-generated name that contains the process ID,
recording ID, and current timestamp, similar to
hotspot-pid-47496-id-1-2018_01_25_19_10_41.jfr. By default,
this parameter is disabled.
duration=times to
specify the time in seconds, m for minutes, h
for hours, or d for days (for example, specifying
5h means 5 hours). By default, the duration isn’t limited,
and this parameter is set to 0.
filename=pathSpecifies the path and name of the file to which the recording is written when the recording is stopped, for example:
recording.jfr/home/user/recordings/recording.jfrc:\recordings\recording.jfrIf %p and/or %t is specified in the filename, it expands to the JVM’s PID and the current timestamp, respectively.
name=identifiermaxage=timedisk parameter is set to
true. Append s to specify the time in seconds,
m for minutes, h for hours, or d
for days (for example, specifying 30s means 30 seconds). By
default, the maximum age isn’t limited, and this parameter is set to
0s.
maxsize=sizedisk
parameter is set to true. The value must not be less than
the value for the maxchunksize parameter set with
-XX:FlightRecorderOptions. Append m or
M to specify the size in megabytes, or g or
G to specify the size in gigabytes. By default, the maximum
size of disk data isn’t limited, and this parameter is set to
0.
path-to-gc-roots={true|false}Specifies whether to collect the path to garbage collection (GC) roots at the end of a recording. By default, this parameter is disabled.
The path to GC roots is useful for finding memory leaks, but
collecting it is time-consuming. Enable this option only when you start
a recording for an application that you suspect has a memory leak. If
the settings parameter is set to profile, the
stack trace from where the potential leaking object was allocated is
included in the information collected.
settings=pathSpecifies the path and name of the event settings file (of type JFC).
By default, the default.jfc file is used, which is located
in JAVA_HOME/lib/jfr. This default settings file collects a
predefined set of information with low overhead, so it has minimal
impact on performance and can be used with recordings that run
continuously.
A second settings file is also provided, profile.jfc, which provides more data than the default configuration, but can have more overhead and impact performance. Use this configuration for short periods of time when more information is needed.
You can specify values for multiple parameters by separating them with a comma. Event settings and .jfc options can be specified using the following syntax:
option=valueJAVA_HOME/bin/jfr tool.
event-setting=value<event-name>#<setting-name>=<value>. To
add a new event setting, prefix the event name with ‘+’.
You can specify values for multiple event settings and .jfc options by separating them with a comma. In case of a conflict between a parameter and a .jfc option, the parameter will take precedence. The whitespace delimiter can be omitted for timespan values, i.e. 20ms. For more information about the settings syntax, see Javadoc of the jdk.jfr package.
-XX:ThreadStackSize=sizeSets the Java thread stack size (in kilobytes). Use of a scaling
suffix, such as k, results in the scaling of the kilobytes
value so that -XX:ThreadStackSize=1k sets the Java thread
stack size to 1024*1024 bytes or 1 megabyte. The default value depends
on the platform. For example:
Linux/x64: 1024 KB
Linux/Aarch64: 2048 KB
macOS/x64: 1024 KB
macOS/Aarch64: 2048 KB
Windows: The default value depends on virtual memory
The following examples show how to set the thread stack size to 1 megabyte in different units:
-XX:ThreadStackSize=1k
-XX:ThreadStackSize=1024This option is similar to -Xss.
-XX:-UseCompressedOopsDisables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used. This will automatically limit the maximum ergonomically determined Java heap size to the maximum amount of memory that can be covered by compressed pointers. By default this range is 32 GB.
With compressed oops enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes smaller than the compressed oops pointer range. This option works only for 64-bit JVMs.
It’s possible to use compressed pointers with Java heap sizes greater
than 32 GB. See the -XX:ObjectAlignmentInBytes option.
-XX:-UseContainerSupportLinux only: The VM now provides automatic container
detection support, which allows the VM to determine the amount of memory
and number of processors that are available to a Java process running in
docker containers. It uses this information to allocate system
resources. The default for this flag is true, and container
support is enabled by default. It can be disabled with
-XX:-UseContainerSupport.
Unified Logging is available to help to diagnose issues related to this support.
Use -Xlog:os+container=trace for maximum logging of
container information. See Enable
Logging with the JVM Unified Logging Framework for a description of
using Unified Logging.
-XX:+UseLargePagesEnables the use of large page memory. By default, this option is disabled and large page memory isn’t used.
See Large Pages.
-XX:+UseTransparentHugePages-XX:+AllowUserSignalHandlers-XX:VMOptionsFile=filenamejava -XX:VMOptionsFile=/var/my_vm_options HelloWorld.
-XX:UseBranchProtection=modeLinux AArch64 only: Specifies the branch protection
mode. All options other than none require the VM to have
been built with branch protection enabled. In addition, for full
protection, any native libraries provided by applications should be
compiled with the same level of protection.
Possible mode arguments for this option include the following:
nonestandardpac-retThese java options control the dynamic just-in-time
(JIT) compilation performed by the Java HotSpot VM.
-XX:AllocateInstancePrefetchLines=linesSets the number of lines to prefetch ahead of the instance allocation pointer. By default, the number of lines to prefetch is set to 1:
-XX:AllocateInstancePrefetchLines=1
-XX:AllocatePrefetchDistance=sizeSets the size (in bytes) of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point.
Negative values denote that prefetch distance is chosen based on the
platform. Positive values are bytes to prefetch. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. The default value is set to
-1.
The following example shows how to set the prefetch distance to 1024 bytes:
-XX:AllocatePrefetchDistance=1024
-XX:AllocatePrefetchInstr=instructionSets the prefetch instruction to prefetch ahead of the allocation pointer. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0:
-XX:AllocatePrefetchInstr=0
-XX:AllocatePrefetchLines=linesSets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code. The default value is 1 if the last allocated object was an instance, and 3 if it was an array.
The following example shows how to set the number of loaded cache lines to 5:
-XX:AllocatePrefetchLines=5
-XX:AllocatePrefetchStepSize=sizeSets the step size (in bytes) for sequential prefetch instructions.
Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes,
g or G to indicate gigabytes. By default, the
step size is set to 16 bytes:
-XX:AllocatePrefetchStepSize=16
-XX:AllocatePrefetchStyle=styleSets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3:
0123-XX:+BackgroundCompilation-XX:-BackgroundCompilation (this is equivalent to
specifying -Xbatch).
-XX:CICompilerCount=threadsSets the number of compiler threads to use for compilation. By default, the number of compiler threads is selected automatically depending on the number of CPUs and memory available for compiled code. The following example shows how to set the number of threads to 2:
-XX:CICompilerCount=2
-XX:+UseDynamicNumberOfCompilerThreads-XX:CICompilerCount. This option is enabled by default.
-XX:CompileCommand=command,method[,option]Specifies a command to perform on a method. For
example, to exclude the indexOf() method of the
String class from being compiled, use the following:
-XX:CompileCommand=exclude,java/lang/String.indexOf
Note that the full class name is specified, including all packages
and subpackages separated by a slash (/). For easier
cut-and-paste operations, it’s also possible to use the method name
format produced by the -XX:+PrintCompilation and
-XX:+LogCompilation options:
-XX:CompileCommand=exclude,java.lang.String::indexOf
If the method is specified without the signature, then the command is
applied to all methods with the specified name. However, you can also
specify the signature of the method in the class file format. In this
case, you should enclose the arguments in quotation marks, because
otherwise the shell treats the semicolon as a command end. For example,
if you want to exclude only the indexOf(String) method of
the String class from being compiled, use the
following:
-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I"
You can also use the asterisk (*) as a wildcard for class and method
names. For example, to exclude all indexOf() methods in all
classes from being compiled, use the following:
-XX:CompileCommand=exclude,*.indexOf
The commas and periods are aliases for spaces, making it easier to
pass compiler commands through a shell. You can pass arguments to
-XX:CompileCommand using spaces as separators by enclosing
the argument in quotation marks:
-XX:CompileCommand="exclude java/lang/String indexOf"
Note that after parsing the commands passed on the command line using
the -XX:CompileCommand options, the JIT compiler then reads
commands from the .hotspot_compiler file. You can add
commands to this file or specify a different file using the
-XX:CompileCommandFile option.
To add several commands, either specify the
-XX:CompileCommand option multiple times, or separate each
argument with the new line separator (\n). The following
commands are available:
breakcompileonly-XX:CompileOnly option,
which lets you specify several methods.
dontinlineexcludehelp-XX:CompileCommand option.
inlinelog-XX:+LogCompilation
option) for all methods except for the specified method. By default,
logging is performed for all compiled methods.
optionPasses a JIT compilation option to the specified method in place of
the last argument (option). The compilation option is set
at the end, after the method name. For example, to enable the
BlockLayoutByFrequency option for the append()
method of the StringBuffer class, use the following:
-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency
You can specify multiple compilation options, separated by commas or spaces.
printquietInstructs not to print the compile commands. By default, the commands
that you specify with the -XX:CompileCommand option are
printed; for example, if you exclude from compilation the
indexOf() method of the String class, then the
following is printed to standard output:
CompilerOracle: exclude java/lang/String.indexOf
You can suppress this by specifying the
-XX:CompileCommand=quiet option before other
-XX:CompileCommand options.
-XX:CompileCommandFile=filenameSets the file from which JIT compiler commands are read. By default,
the .hotspot_compiler file is used to store commands
performed by the JIT compiler.
Each line in the command file represents a command, a class name, and
a method name for which the command is used. For example, this line
prints assembly code for the toString() method of the
String class:
print java/lang/String toString
If you’re using commands for the JIT compiler to perform on methods,
then see the -XX:CompileCommand option.
-XX:CompilerDirectivesFile=fileAdds directives from a file to the directives stack when a program starts. See Compiler Control.
The -XX:CompilerDirectivesFile option has to be used
together with the -XX:UnlockDiagnosticVMOptions option that
unlocks diagnostic JVM options.
-XX:+CompilerDirectivesPrintPrints the directives stack when the program starts or when a new directive is added.
The -XX:+CompilerDirectivesPrint option has to be used
together with the -XX:UnlockDiagnosticVMOptions option that
unlocks diagnostic JVM options.
-XX:CompileOnly=methodsSets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods are compiled.
-XX:CompileOnly=method1,method2,...,methodN is an alias
for:
-XX:CompileCommand=compileonly,method1
-XX:CompileCommand=compileonly,method2
...
-XX:CompileCommand=compileonly,methodN-XX:CompileThresholdScaling=scaleCompileThresholdScaling option has a
floating point value between 0 and +Inf and scales the thresholds
corresponding to the current mode of operation (both tiered and
nontiered). Setting CompileThresholdScaling to a value less
than 1.0 results in earlier compilation while values greater than 1.0
delay compilation. Setting CompileThresholdScaling to 0 is
equivalent to disabling compilation.
-XX:+DoEscapeAnalysis-XX:-DoEscapeAnalysis.
-XX:InitialCodeCacheSize=sizeSets the initial code cache size (in bytes). Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. The default value depends on the
platform. The initial code cache size shouldn’t be less than the
system’s minimal memory page size. The following example shows how to
set the initial code cache size to 32 KB:
-XX:InitialCodeCacheSize=32k
-XX:+Inline-XX:-Inline.
-XX:InlineSmallCode=sizeSets the maximum code size (in bytes) for already compiled methods
that may be inlined. This flag only applies to the C2 compiler. Append
the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or
g or G to indicate gigabytes. The default
value depends on the platform and on whether tiered compilation is
enabled. In the following example it is set to 1000 bytes:
-XX:InlineSmallCode=1000
-XX:+LogCompilationEnables logging of compilation activity to a file named
hotspot.log in the current working directory. You can
specify a different log file path and name using the
-XX:LogFile option.
By default, this option is disabled and compilation activity isn’t
logged. The -XX:+LogCompilation option has to be used
together with the -XX:UnlockDiagnosticVMOptions option that
unlocks diagnostic JVM options.
You can enable verbose diagnostic output with a message printed to
the console every time a method is compiled by using the
-XX:+PrintCompilation option.
-XX:FreqInlineSize=sizeSets the maximum bytecode size (in bytes) of a hot method to be
inlined. This flag only applies to the C2 compiler. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. The default value depends on the
platform. In the following example it is set to 325 bytes:
-XX:FreqInlineSize=325
-XX:MaxInlineSize=sizeSets the maximum bytecode size (in bytes) of a cold method to be
inlined. This flag only applies to the C2 compiler. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. By default, the maximum bytecode
size is set to 35 bytes:
-XX:MaxInlineSize=35
-XX:C1MaxInlineSize=sizeSets the maximum bytecode size (in bytes) of a cold method to be
inlined. This flag only applies to the C1 compiler. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. By default, the maximum bytecode
size is set to 35 bytes:
-XX:MaxInlineSize=35
-XX:MaxTrivialSize=sizeSets the maximum bytecode size (in bytes) of a trivial method to be
inlined. This flag only applies to the C2 compiler. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. By default, the maximum bytecode
size of a trivial method is set to 6 bytes:
-XX:MaxTrivialSize=6
-XX:C1MaxTrivialSize=sizeSets the maximum bytecode size (in bytes) of a trivial method to be
inlined. This flag only applies to the C1 compiler. Append the letter
k or K to indicate kilobytes, m
or M to indicate megabytes, or g or
G to indicate gigabytes. By default, the maximum bytecode
size of a trivial method is set to 6 bytes:
-XX:MaxTrivialSize=6
-XX:MaxNodeLimit=nodesSets the maximum number of nodes to be used during single method compilation. By default the value depends on the features enabled. In the following example the maximum number of nodes is set to 100,000:
-XX:MaxNodeLimit=100000
-XX:NonNMethodCodeHeapSize=sizeSets the size in bytes of the code segment containing nonmethod code.
A nonmethod code segment containing nonmethod code, such as compiler
buffers and the bytecode interpreter. This code type stays in the code
cache forever. This flag is used only if
-XX:SegmentedCodeCache is enabled.
-XX:NonProfiledCodeHeapSize=size-XX:SegmentedCodeCache
is enabled.
-XX:+OptimizeStringConcatString concatenation
operations. This option is enabled by default. To disable the
optimization of String concatenation operations, specify
-XX:-OptimizeStringConcat.
-XX:+PrintAssemblyEnables printing of assembly code for bytecoded and native methods by
using the external hsdis-<arch>.so or
.dll library. For 64-bit VM on Windows, it’s
hsdis-amd64.dll. This lets you to see the generated code,
which may help you to diagnose performance issues.
By default, this option is disabled and assembly code isn’t printed.
The -XX:+PrintAssembly option has to be used together with
the -XX:UnlockDiagnosticVMOptions option that unlocks
diagnostic JVM options.
-XX:ProfiledCodeHeapSize=size-XX:SegmentedCodeCache is
enabled.
-XX:+PrintCompilationEnables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled. This lets you to see which methods actually get compiled. By default, this option is disabled and diagnostic output isn’t printed.
You can also log compilation activity to a file by using the
-XX:+LogCompilation option.
-XX:+PrintInliningEnables printing of inlining decisions. This let’s you see which methods are getting inlined.
By default, this option is disabled and inlining information isn’t
printed. The -XX:+PrintInlining option has to be used
together with the -XX:+UnlockDiagnosticVMOptions option
that unlocks diagnostic JVM options.
-XX:ReservedCodeCacheSize=sizek or K to indicate
kilobytes, m or M to indicate megabytes, or
g or G to indicate gigabytes. The default
maximum code cache size is 240 MB; if you disable tiered compilation
with the option -XX:-TieredCompilation, then the default
size is 48 MB. This option has a limit of 2 GB; otherwise, an error is
generated. The maximum code cache size shouldn’t be less than the
initial code cache size; see the option
-XX:InitialCodeCacheSize.
-XX:RTMAbortRatio=abort_ratio-XX:+UseRTMDeopt option is enabled.
The default value of this option is 50. This means that the compiled
code is deoptimized if 50% of all transactions are aborted.
-XX:RTMRetryCount=number_of_retries-XX:UseRTMLocking option must be enabled.
-XX:+SegmentedCodeCacheEnables segmentation of the code cache, without which the code cache
consists of one large segment. With
-XX:+SegmentedCodeCache, separate segments will be used for
non-method, profiled method, and non-profiled method code. The segments
are not resized at runtime. The advantages are better control of the
memory footprint, reduced code fragmentation, and better CPU iTLB
(instruction translation lookaside buffer) and instruction cache
behavior due to improved locality.
The feature is enabled by default if tiered compilation is enabled
(-XX:+TieredCompilation ) and the reserved code cache size
(-XX:ReservedCodeCacheSize) is at least 240 MB.
-XX:StartAggressiveSweepingAt=percent-XX:-TieredCompilation-XX:UseSSE=version-XX:UseAVX=version-XX:+UseAES-XX:+UseAES is used in conjunction with
UseAESIntrinsics. Flags that control intrinsics now require
the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseAESIntrinsicsEnables AES intrinsics. Specifying -XX:+UseAESIntrinsics
is equivalent to also enabling -XX:+UseAES. To disable
hardware-based AES intrinsics, specify
-XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable
hardware AES, use the following flags:
-XX:+UseAES -XX:+UseAESIntrinsics
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseAESCTRIntrinsics-XX:+UseAESIntrinsics enables AES/CTR
intrinsics.
-XX:+UseGHASHIntrinsics-XX:+UnlockDiagnosticVMOptions.
-XX:+UseChaCha20Intrinsics-XX:-UseChaCha20Intrinsics. Flags that control intrinsics
now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UsePoly1305Intrinsics-XX:-UsePoly1305Intrinsics. Flags that control intrinsics
now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseBASE64Intrinsicsjava.util.Base64. Enabled by default on platforms that
support it. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseAdler32Intrinsicsjava.util.zip.Adler32. Enabled by default on platforms that
support it. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32Intrinsicsjava.util.zip.CRC32. Enabled by default on platforms that
support it. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32CIntrinsicsjava.util.zip.CRC32C. Enabled by default on platforms that
support it. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHAEnables hardware-based intrinsics for SHA crypto hash functions for
some hardware. The UseSHA option is used in conjunction
with the UseSHA1Intrinsics,
UseSHA256Intrinsics, and UseSHA512Intrinsics
options.
The UseSHA and UseSHA*Intrinsics flags are
enabled by default on machines that support the corresponding
instructions.
This feature is applicable only when using the
sun.security.provider.Sun provider for SHA operations.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
To disable all hardware-based SHA intrinsics, specify the
-XX:-UseSHA. To disable only a particular SHA intrinsic,
use the appropriate corresponding option. For example:
-XX:-UseSHA256Intrinsics.
-XX:+UseSHA1Intrinsics-XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA256Intrinsics-XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA512Intrinsics-XX:+UnlockDiagnosticVMOptions.
-XX:+UseMathExactIntrinsicsjava.lang.Math.*Exact()
functions. Enabled by default. Flags that control intrinsics now require
the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMultiplyToLenIntrinsicBigInteger.multiplyToLen().
Enabled by default on platforms that support it. Flags that control
intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
BigInteger.squareToLen().
Enabled by default on platforms that support it. Flags that control
intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
BigInteger.mulAdd(). Enabled by
default on platforms that support it. Flags that control intrinsics now
require the option -XX:+UnlockDiagnosticVMOptions.
BigInteger.montgomeryMultiply(). Enabled by default on
platforms that support it. Flags that control intrinsics now require the
option -XX:+UnlockDiagnosticVMOptions.
BigInteger.montgomerySquare().
Enabled by default on platforms that support it. Flags that control
intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseCMoveUnconditionally-XX:+UseCodeCacheFlushing-XX:-UseCodeCacheFlushing.
-XX:+UseCondCardMark-XX:+UseCountedLoopSafepoints-XX:LoopStripMiningIter=number_of_iterations-XX:LoopStripMiningIterShortLoop=number_of_iterations-XX:LoopStripMiningIter.
-XX:+UseFMAjava.lang.Math.fma(a,
b, c) methods that calculate
the value of ( a * b
+ c ) expressions.
-XX:+UseRTMDeopt-XX:RTMAbortRatio option. If the number of
aborted transactions exceeds the abort ratio, then the method containing
the lock is deoptimized and recompiled with all locks as normal locks.
This option is disabled by default. The -XX:+UseRTMLocking
option must be enabled.
-XX:+UseRTMLockingGenerates Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. This option is disabled by default. Options related to RTM are available only on x86 CPUs that support Transactional Synchronization Extensions (TSX).
RTM is part of Intel’s TSX, which is an x86 instruction set extension
and facilitates the creation of multithreaded applications. RTM
introduces the new instructions XBEGIN,
XABORT, XEND, and XTEST. The
XBEGIN and XEND instructions enclose a set of
instructions to run as a transaction. If no conflict is found when
running the transaction, then the memory and register modifications are
committed together at the XEND instruction. The
XABORT instruction can be used to explicitly abort a
transaction and the XTEST instruction checks if a set of
instructions is being run in a transaction.
A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking the thread that didn’t originally request access to the transaction. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails. An RTM lock is a lock that has been delegated to the TSX’s system.
RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently). RTM also improves the performance of coarse-grain locking, which typically doesn’t perform well in multithreaded applications. (Coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible.) Also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong. This occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line. As a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache.
-XX:+UseSuperWord-XX:-UseSuperWord.
These java options provide the ability to gather system
information and perform extensive debugging.
-XX:+DisableAttachMechanismDisables the mechanism that lets tools attach to the JVM. By default,
this option is disabled, meaning that the attach mechanism is enabled
and you can use diagnostics and troubleshooting tools such as
jcmd, jstack, jmap, and
jinfo.
Note: The tools such as jcmd, jinfo, jmap, and jstack shipped with the JDK aren’t supported when using the tools from one JDK version to troubleshoot a different JDK version.
-XX:+DTraceAllocProbesdtrace tool probes
for object allocation.
-XX:+DTraceMethodProbesdtrace tool probes
for method-entry and method-exit.
-XX:+DTraceMonitorProbesdtrace tool probes
for monitor events.
-XX:+HeapDumpOnOutOfMemoryErrorjava.lang.OutOfMemoryError exception is thrown. You can
explicitly set the heap dump file path and name using the
-XX:HeapDumpPath option. By default, this option is
disabled and the heap isn’t dumped when an OutOfMemoryError
exception is thrown.
-XX:HeapDumpPath=pathSets the path and file name for writing the heap dump provided by the
heap profiler (HPROF) when the
-XX:+HeapDumpOnOutOfMemoryError option is set. By default,
the file is created in the current working directory, and it’s named
java_pid<pid>.hprof where <pid> is
the identifier of the process that caused the error. The following
example shows how to set the default file explicitly (%p
represents the current process identifier):
-XX:HeapDumpPath=./java_pid%p.hprof
Non-Windows: The following example shows how to
set the heap dump file to
/var/log/java/java_heapdump.hprof:
-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
Windows: The following example shows how to set
the heap dump file to C:/log/java/java_heapdump.log:
-XX:HeapDumpPath=C:/log/java/java_heapdump.log
-XX:LogFile=pathSets the path and file name to where log data is written. By default,
the file is created in the current working directory, and it’s named
hotspot.log.
Non-Windows: The following example shows how to
set the log file to /var/log/java/hotspot.log:
-XX:LogFile=/var/log/java/hotspot.log
Windows: The following example shows how to set
the log file to C:/log/java/hotspot.log:
-XX:LogFile=C:/log/java/hotspot.log
-XX:+PrintClassHistogramEnables printing of a class instance histogram after one of the following events:
Non-Windows: Control+\
(SIGQUIT)
Windows: Control+C
(SIGTERM)
By default, this option is disabled.
Setting this option is equivalent to running the
jmap -histo command, or the jcmd pid
GC.class_histogram command, where pid is the
current Java process identifier.
-XX:+PrintConcurrentLocksEnables printing of java.util.concurrent locks after one
of the following events:
Non-Windows: Control+\
(SIGQUIT)
Windows: Control+C
(SIGTERM)
By default, this option is disabled.
Setting this option is equivalent to running the
jstack -l command or the jcmd pid
Thread.print -l command, where pid is the current
Java process identifier.
-XX:+PrintFlagsRanges-XX:+PerfDataSaveToFileIf enabled, saves jstat binary data when the
Java application exits. This binary data is saved in a file named
hsperfdata_pid, where pid is the process
identifier of the Java application that you ran. Use the
jstat command to display the performance data contained in
this file as follows:
jstat -class file:///path/hsperfdata_pid
jstat -gc file:///path/hsperfdata_pid
-XX:+UsePerfDataperfdata feature. This option is enabled by
default to allow JVM monitoring and performance testing. Disabling it
suppresses the creation of the hsperfdata_userid
directories. To disable the perfdata feature, specify
-XX:-UsePerfData.
These java options control how garbage collection (GC)
is performed by the Java HotSpot VM.
-XX:+AggressiveHeap-XX:+AlwaysPreTouch-XX:ConcGCThreads=threadsSets the number of threads used for concurrent GC. Sets
threads to approximately 1/4 of the number of
parallel garbage collection threads. The default value depends on the
number of CPUs available to the JVM.
For example, to set the number of threads for concurrent GC to 2, specify the following option:
-XX:ConcGCThreads=2
-XX:+DisableExplicitGCSystem.gc() method. This option is disabled by default,
meaning that calls to System.gc() are processed. If
processing of calls to System.gc() is disabled, then the
JVM still performs GC when necessary.
-XX:+ExplicitGCInvokesConcurrentSystem.gc()
request. This option is disabled by default and can be enabled only with
the -XX:+UseG1GC option.
-XX:G1AdaptiveIHOPNumInitialSamples=number-XX:UseAdaptiveIHOP is enabled, this option sets the
number of completed marking cycles used to gather samples until G1
adaptively determines the optimum value of
-XX:InitiatingHeapOccupancyPercent. Before, G1 uses the
value of -XX:InitiatingHeapOccupancyPercent directly for
this purpose. The default value is 3.
-XX:G1HeapRegionSize=sizeSets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value is a power of 2 and can range from 1 MB to 32 MB. The default region size is determined ergonomically based on the heap size with a goal of approximately 2048 regions.
The following example sets the size of the subdivisions to 16 MB:
-XX:G1HeapRegionSize=16m
-XX:G1HeapWastePercent=percent-XX:G1MaxNewSizePercent=percentSets the percentage of the heap size to use as the maximum for the young generation size. The default value is 60 percent of your Java heap.
This is an experimental flag. This setting replaces the
-XX:DefaultMaxNewGenPercent setting.
-XX:G1MixedGCCountTarget=numberG1MixedGCLIveThresholdPercent live data. The default is 8
mixed garbage collections. The goal for mixed collections is to be
within this target number.
-XX:G1MixedGCLiveThresholdPercent=percentSets the occupancy threshold for an old region to be included in a mixed garbage collection cycle. The default occupancy is 85 percent.
This is an experimental flag. This setting replaces the
-XX:G1OldCSetRegionLiveThresholdPercent setting.
-XX:G1NewSizePercent=percentSets the percentage of the heap to use as the minimum for the young generation size. The default value is 5 percent of your Java heap.
This is an experimental flag. This setting replaces the
-XX:DefaultMinNewGenPercent setting.
-XX:G1OldCSetRegionThresholdPercent=percent-XX:G1ReservePercent=percentSets the percentage of the heap (0 to 50) that’s reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. When you increase or decrease the percentage, ensure that you adjust the total Java heap by the same amount. By default, this option is set to 10%.
The following example sets the reserved heap to 20%:
-XX:G1ReservePercent=20
-XX:+G1UseAdaptiveIHOPControls adaptive calculation of the old generation occupancy to
start background work preparing for an old generation collection. If
enabled, G1 uses -XX:InitiatingHeapOccupancyPercent for the
first few times as specified by the value of
-XX:G1AdaptiveIHOPNumInitialSamples, and after that
adaptively calculates a new optimum value for the initiating occupancy
automatically. Otherwise, the old generation collection process always
starts at the old generation occupancy determined by
-XX:InitiatingHeapOccupancyPercent.
The default is enabled.
-XX:InitialHeapSize=sizeSets the initial size (in bytes) of the memory allocation pool. This
value must be either 0, or a multiple of 1024 and greater than 1 MB.
Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes, or
g or G to indicate gigabytes. The default
value is selected at run time based on the system configuration.
The following examples show how to set the size of allocated memory to 6 MB using various units:
-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6mIf you set this option to 0, then the initial size is set as the sum
of the sizes allocated for the old generation and the young generation.
The size of the heap for the young generation can be set using the
-XX:NewSize option. Note that the -Xms option
sets both the minimum and the initial heap size of the heap. If
-Xms appears after -XX:InitialHeapSize on the
command line, then the initial heap size gets set to the value specified
with -Xms.
-XX:InitialRAMPercentage=percentSets the initial amount of memory that the JVM will use for the Java
heap before applying ergonomics heuristics as a percentage of the
maximum amount determined as described in the -XX:MaxRAM
option. The default value is 1.5625 percent.
The following example shows how to set the percentage of the initial amount of memory used for the Java heap:
-XX:InitialRAMPercentage=5
-XX:InitialSurvivorRatio=ratioSets the initial survivor space ratio used by the throughput garbage
collector (which is enabled by the -XX:+UseParallelGC
option). Adaptive sizing is enabled by default with the throughput
garbage collector by using the -XX:+UseParallelGC option,
and the survivor space is resized according to the application behavior,
starting with the initial value. If adaptive sizing is disabled (using
the -XX:-UseAdaptiveSizePolicy option), then the
-XX:SurvivorRatio option should be used to set the size of
the survivor space for the entire execution of the application.
The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R):
S=Y/(R+2)
The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size.
By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), then the initial size of the survivor space is 0.2 MB.
The following example shows how to set the initial survivor space ratio to 4:
-XX:InitialSurvivorRatio=4
-XX:InitiatingHeapOccupancyPercent=percentSets the percentage of the old generation occupancy (0 to 100) at which to start the first few concurrent marking cycles for the G1 garbage collector.
By default, the initiating value is set to 45%. A value of 0 implies nonstop concurrent GC cycles from the beginning until G1 adaptively sets this value.
See also the -XX:G1UseAdaptiveIHOP and
-XX:G1AdaptiveIHOPNumInitialSamples options.
The following example shows how to set the initiating heap occupancy to 75%:
-XX:InitiatingHeapOccupancyPercent=75
-XX:MaxGCPauseMillis=timeSets a target for the maximum GC pause time (in milliseconds). This is a soft goal, and the JVM will make its best effort to achieve it. The specified value doesn’t adapt to your heap size. By default, for G1 the maximum pause time target is 200 milliseconds. The other generational collectors do not use a pause time goal by default.
The following example shows how to set the maximum target pause time to 500 ms:
-XX:MaxGCPauseMillis=500
-XX:MaxHeapSize=sizeSets the maximum size (in byes) of the memory allocation pool. This
value must be a multiple of 1024 and greater than 2 MB. Append the
letter k or K to indicate kilobytes,
m or M to indicate megabytes, or
g or G to indicate gigabytes. The default
value is selected at run time based on the system configuration. For
server deployments, the options -XX:InitialHeapSize and
-XX:MaxHeapSize are often set to the same value.
The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:
-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80mThe -XX:MaxHeapSize option is equivalent to
-Xmx.
-XX:MaxHeapFreeRatio=percentSets the maximum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space expands above this value, then the heap is shrunk. By default, this value is set to 70%.
Minimize the Java heap size by lowering the values of the parameters
MaxHeapFreeRatio (default value is 70%) and
MinHeapFreeRatio (default value is 40%) with the
command-line options -XX:MaxHeapFreeRatio and
-XX:MinHeapFreeRatio. Lowering
MaxHeapFreeRatio to as low as 10% and
MinHeapFreeRatio to 5% has successfully reduced the heap
size without too much performance regression; however, results may vary
greatly depending on your application. Try different values for these
parameters until they’re as low as possible yet still retain acceptable
performance.
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Customers trying to keep the heap small should also add the option
-XX:-ShrinkHeapInSteps. See Performance Tuning Examples for
a description of using this option to keep the Java heap small by
reducing the dynamic footprint for embedded applications.
-XX:MaxMetaspaceSize=sizeSets the maximum amount of native memory that can be allocated for class metadata. By default, the size isn’t limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system.
The following example shows how to set the maximum class metadata size to 256 MB:
-XX:MaxMetaspaceSize=256m
-XX:MaxNewSize=size-XX:MaxRAM=sizeSets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics. The default value is the maximum amount of available memory to the JVM process or 128 GB, whichever is lower.
The maximum amount of available memory to the JVM process is the minimum of the machine’s physical memory and any constraints set by the environment (e.g. container).
Specifying this option disables automatic use of compressed oops if
the combined result of this and other options influencing the maximum
amount of memory is larger than the range of memory addressable by
compressed oops. See -XX:UseCompressedOops for further
information about compressed oops.
The following example shows how to set the maximum amount of available memory for sizing the Java heap to 2 GB:
-XX:MaxRAM=2G
-XX:MaxRAMPercentage=percentSets the maximum amount of memory that the JVM may use for the Java
heap before applying ergonomics heuristics as a percentage of the
maximum amount determined as described in the -XX:MaxRAM
option. The default value is 25 percent.
Specifying this option disables automatic use of compressed oops if
the combined result of this and other options influencing the maximum
amount of memory is larger than the range of memory addressable by
compressed oops. See -XX:UseCompressedOops for further
information about compressed oops.
The following example shows how to set the percentage of the maximum amount of memory used for the Java heap:
-XX:MaxRAMPercentage=75
-XX:MinRAMPercentage=percentSets the maximum amount of memory that the JVM may use for the Java
heap before applying ergonomics heuristics as a percentage of the
maximum amount determined as described in the -XX:MaxRAM
option for small heaps. A small heap is a heap of approximately 125 MB.
The default value is 50 percent.
The following example shows how to set the percentage of the maximum amount of memory used for the Java heap for small heaps:
-XX:MinRAMPercentage=75
-XX:MaxTenuringThreshold=thresholdSets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector.
The following example shows how to set the maximum tenuring threshold to 10:
-XX:MaxTenuringThreshold=10
-XX:MetaspaceSize=size-XX:MinHeapFreeRatio=percentSets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap is expanded. By default, this value is set to 40%.
Minimize Java heap size by lowering the values of the parameters
MaxHeapFreeRatio (default value is 70%) and
MinHeapFreeRatio (default value is 40%) with the
command-line options -XX:MaxHeapFreeRatio and
-XX:MinHeapFreeRatio. Lowering
MaxHeapFreeRatio to as low as 10% and
MinHeapFreeRatio to 5% has successfully reduced the heap
size without too much performance regression; however, results may vary
greatly depending on your application. Try different values for these
parameters until they’re as low as possible, yet still retain acceptable
performance.
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Customers trying to keep the heap small should also add the option
-XX:-ShrinkHeapInSteps. See Performance Tuning Examples for
a description of using this option to keep the Java heap small by
reducing the dynamic footprint for embedded applications.
-XX:MinHeapSize=sizeSets the minimum size (in bytes) of the memory allocation pool. This
value must be either 0, or a multiple of 1024 and greater than 1 MB.
Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes, or
g or G to indicate gigabytes. The default
value is selected at run time based on the system configuration.
The following examples show how to set the minimum size of allocated memory to 6 MB using various units:
-XX:MinHeapSize=6291456
-XX:MinHeapSize=6144k
-XX:MinHeapSize=6mIf you set this option to 0, then the minimum size is set to the same value as the initial size.
-XX:NewRatio=ratioSets the ratio between young and old generation sizes. By default, this option is set to 2. The following example shows how to set the young-to-old ratio to 1:
-XX:NewRatio=1
-XX:NewSize=sizeSets the initial size (in bytes) of the heap for the young generation
(nursery). Append the letter k or K to
indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate
gigabytes.
The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs are performed. If the size is too high, then only full GCs are performed, which can take a long time to complete. It is recommended that you keep the size for the young generation greater than 25% and less than 50% of the overall heap size.
The following examples show how to set the initial size of the young generation to 256 MB using various units:
-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456The -XX:NewSize option is equivalent to
-Xmn.
-XX:ParallelGCThreads=threadsSets the number of the stop-the-world (STW) worker threads. The default value depends on the number of CPUs available to the JVM and the garbage collector selected.
For example, to set the number of threads for G1 GC to 2, specify the following option:
-XX:ParallelGCThreads=2
-XX:+ParallelRefProcEnabled-XX:+PrintAdaptiveSizePolicy-XX:+ScavengeBeforeFullGC-XX:-ScavengeBeforeFullGC.
-XX:SoftRefLRUPolicyMSPerMB=timeSets the amount of time (in milliseconds) a softly reachable object
is kept active on the heap after the last time it was referenced. The
default value is one second of lifetime per free megabyte in the heap.
The -XX:SoftRefLRUPolicyMSPerMB option accepts integer
values representing milliseconds per one megabyte of the current heap
size (for Java HotSpot Client VM) or the maximum possible heap size (for
Java HotSpot Server VM). This difference means that the Client VM tends
to flush soft references rather than grow the heap, whereas the Server
VM tends to grow the heap rather than flush soft references. In the
latter case, the value of the -Xmx option has a significant
effect on how quickly soft references are garbage collected.
The following example shows how to set the value to 2.5 seconds:
-XX:SoftRefLRUPolicyMSPerMB=2500
-XX:-ShrinkHeapInStepsIncrementally reduces the Java heap to the target size, specified by
the option -XX:MaxHeapFreeRatio. This option is enabled by
default. If disabled, then it immediately reduces the Java heap to the
target size instead of requiring multiple garbage collection cycles.
Disable this option if you want to minimize the Java heap size. You will
likely encounter performance degradation when this option is
disabled.
See Performance Tuning
Examples for a description of using the
MaxHeapFreeRatio option to keep the Java heap small by
reducing the dynamic footprint for embedded applications.
-XX:StringDeduplicationAgeThreshold=thresholdIdentifies String objects reaching the specified age
that are considered candidates for deduplication. An object’s age is a
measure of how many times it has survived garbage collection. This is
sometimes referred to as tenuring.
Note:
Stringobjects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. The default value for this option is3. See the-XX:+UseStringDeduplicationoption.
-XX:SurvivorRatio=ratioSets the ratio between eden space size and survivor space size. By default, this option is set to 8. The following example shows how to set the eden/survivor space ratio to 4:
-XX:SurvivorRatio=4
-XX:TargetSurvivorRatio=percentSets the desired percentage of survivor space (0 to 100) used after young garbage collection. By default, this option is set to 50%.
The following example shows how to set the target survivor space ratio to 30%:
-XX:TargetSurvivorRatio=30
-XX:TLABSize=sizeSets the initial size (in bytes) of a thread-local allocation buffer
(TLAB). Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes, or
g or G to indicate gigabytes. If this option
is set to 0, then the JVM selects the initial size automatically.
The following example shows how to set the initial TLAB size to 512 KB:
-XX:TLABSize=512k
-XX:+UseAdaptiveSizePolicy-XX:-UseAdaptiveSizePolicy and set the size of the memory
allocation pool explicitly. See the -XX:SurvivorRatio
option.
-XX:+UseG1GC-XX:+UseGCOverheadLimitOutOfMemoryError exception is
thrown. This option is enabled, by default, and the parallel GC will
throw an OutOfMemoryError if more than 98% of the total
time is spent on garbage collection and less than 2% of the heap is
recovered. When the heap is small, this feature can be used to prevent
applications from running for long periods of time with little or no
progress. To disable this option, specify the option
-XX:-UseGCOverheadLimit.
-XX:+UseNUMA-XX:+UseParallelGC).
-XX:+UseParallelGCEnables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors.
By default, this option is disabled and the default collector is used.
-XX:+UseSerialGC-XX:+UseStringDeduplicationEnables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector.
String deduplication reduces the memory footprint of
String objects on the Java heap by taking advantage of the
fact that many String objects are identical. Instead of
each String object pointing to its own character array,
identical String objects can point to and share the same
character array.
-XX:+UseTLAB-XX:-UseTLAB.
-XX:+UseZGC-XX:ZAllocationSpikeTolerance=factor-XX:ZCollectionInterval=seconds-XX:ZFragmentationLimit=percent-XX:+ZProactive-XX:+ZUncommit-XX:ZUncommitDelay=secondsThese java options are deprecated and might be removed
in a future JDK release. They’re still accepted and acted upon, but a
warning is issued when they’re used.
-Xfuture-Xloggc:filenameSets the file to which verbose GC events information should be
redirected for logging. The -Xloggc option overrides
-verbose:gc if both are given with the same java command.
-Xloggc:filename is replaced by
-Xlog:gc:filename. See Enable Logging with the JVM
Unified Logging Framework.
Example:
-Xlog:gc:garbage-collection.log
-XX:+FlightRecorder-XX:InitialRAMFraction=ratioSets the initial amount of memory that the JVM may use for the Java
heap before applying ergonomics heuristics as a ratio of the maximum
amount determined as described in the -XX:MaxRAM option.
The default value is 64.
Use the option -XX:InitialRAMPercentage instead.
-XX:MaxRAMFraction=ratioSets the maximum amount of memory that the JVM may use for the Java
heap before applying ergonomics heuristics as a fraction of the maximum
amount determined as described in the -XX:MaxRAM option.
The default value is 4.
Specifying this option disables automatic use of compressed oops if
the combined result of this and other options influencing the maximum
amount of memory is larger than the range of memory addressable by
compressed oops. See -XX:UseCompressedOops for further
information about compressed oops.
Use the option -XX:MaxRAMPercentage instead.
-XX:MinRAMFraction=ratioSets the maximum amount of memory that the JVM may use for the Java
heap before applying ergonomics heuristics as a fraction of the maximum
amount determined as described in the -XX:MaxRAM option for
small heaps. A small heap is a heap of approximately 125 MB. The default
value is 2.
Use the option -XX:MinRAMPercentage instead.
These java options are still accepted but ignored, and a
warning is issued when they’re used.
--illegal-access=parameter-XX:+UseHugeTLBFS-XX:+UseLargePages. This option is disabled by default.
This option pre-allocates all large pages up-front, when memory is
reserved; consequently the JVM can’t dynamically grow or shrink large
pages memory areas; see -XX:UseTransparentHugePages if you
want this behavior.
-XX:+UseSHMNo documented java options have been removed in JDK @@VERSION_SPECIFICATION@@.
For the lists and descriptions of options removed in previous releases see the Removed Java Options section in:
You can shorten or simplify the java command by using
@ argument files to specify one or more text files that
contain arguments, such as options and class names, which are passed to
the java command. This let’s you to create
java commands of any length on any operating system.
In the command line, use the at sign (@) prefix to
identify an argument file that contains java options and
class names. When the java command encounters a file
beginning with the at sign (@), it expands the contents of
that file into an argument list just as they would be specified on the
command line.
The java launcher expands the argument file contents
until it encounters the --disable-@files option. You can
use the --disable-@files option anywhere on the command
line, including in an argument file, to stop @ argument
files expansion.
The following items describe the syntax of java argument
files:
The argument file must contain only ASCII characters or characters in system default encoding that’s ASCII friendly, such as UTF-8.
The argument file size must not exceed MAXINT (2,147,483,647) bytes.
The launcher doesn’t expand wildcards that are present within an argument file.
Use white space or new line characters to separate arguments included in the file.
White space includes a white space character, \t,
\n, \r, and \f.
For example, it is possible to have a path with a space, such as
c:\Program Files that can be specified as either
"c:\\Program Files" or, to avoid an escape,
c:\Program" "Files.
Any option that contains spaces, such as a path component, must be within quotation marks using quotation (‘"’) characters in its entirety.
A string within quotation marks may contain the characters
\n, \r, \t, and \f.
They are converted to their respective ASCII codes.
If a file name contains embedded spaces, then put the whole file name in double quotation marks.
File names in an argument file are relative to the current directory, not to the location of the argument file.
Use the number sign # in the argument file to
identify comments. All characters following the # are
ignored until the end of line.
Additional at sign @ prefixes to @
prefixed options act as an escape, (the first @ is removed
and the rest of the arguments are presented to the launcher
literally).
Lines may be continued using the continuation character
(\) at the end-of-line. The two lines are concatenated with
the leading white spaces trimmed. To prevent trimming the leading white
spaces, a continuation character (\) may be placed at the
first column.
Because backslash (\) is an escape character, a backslash character must be escaped with another backslash character.
Partial quote is allowed and is closed by an end-of-file.
An open quote stops at end-of-line unless \ is the
last character, which then joins the next line by removing all leading
white space characters.
Wildcards (*) aren’t allowed in these lists (such as specifying
*.java).
Use of the at sign (@) to recursively interpret
files isn’t supported.
In the argument file,
-cp "lib/
cool/
app/
jarsthis is interpreted as:
-cp lib/cool/app/jars
To output the following:
-cp c:\Program Files (x86)\Java\jre\lib\ext;c:\Program Files\Java\jre9\lib\ext
The backslash character must be specified in the argument file as:
-cp "c:\\Program Files (x86)\\Java\\jre\\lib\\ext;c:\\Program Files\\Java\\jre9\\lib\\ext"
In the argument file,
-cp "/lib/cool app/jars:\
/lib/another app/jars"This is interpreted as:
-cp /lib/cool app/jars:/lib/another app/jars
In the argument file,
-cp "/lib/cool\
\app/jars"This is interpreted as:
-cp /lib/cool app/jars
You can use a single argument file, such as
myargumentfile in the following example, to hold all
required java arguments:
java @myargumentfile
You can include relative paths in argument files; however, they’re
relative to the current working directory and not to the paths of the
argument files themselves. In the following example,
path1/options and path2/options represent
argument files with different paths. Any relative paths that they
contain are relative to the current working directory and not to the
argument files:
java @path1/options @path2/classes
There are occasions when having insight into the current state of the JVM code heap would be helpful to answer questions such as:
Why was the JIT turned off and then on again and again?
Where has all the code heap space gone?
Why is the method sweeper not working effectively?
To provide this insight, a code heap state analytics feature has been implemented that enables on-the-fly analysis of the code heap. The analytics process is divided into two parts. The first part examines the entire code heap and aggregates all information that is believed to be useful or important. The second part consists of several independent steps that print the collected information with an emphasis on different aspects of the data. Data collection and printing are done on an “on request” basis.
Requests for real-time, on-the-fly analysis can be issued with the following command:
jcmdpidCompiler.CodeHeap_Analytics[function] [granularity]
If you are only interested in how the code heap looks like after running a sample workload, you can use the command line option:
-Xlog:codecache=Trace
To see the code heap state when a “CodeCache full” condition exists, start the VM with the command line option:
-Xlog:codecache=Debug
See CodeHeap State Analytics (OpenJDK) for a detailed description of the code heap state analytics feature, the supported functions, and the granularity options.
You use the -Xlog option to configure or enable logging
with the Java Virtual Machine (JVM) unified logging framework.
-Xlog[:[what][:[output][:[decorators][:output-options[,…]]]]]
-Xlog:directive
+tag2…][*][=level][,…].
Unless the wildcard (*) is specified, only log messages
tagged with exactly the tags specified are matched. See -Xlog Tags and Levels.
stdout. See -Xlog Output.
uptime, level,
and tags. See Decorations.
-Xlog logging output options.
The Java Virtual Machine (JVM) unified logging framework provides a common logging system for all components of the JVM. GC logging for the JVM has been changed to use the new logging framework. The mapping of old GC flags to the corresponding new Xlog configuration is described in Convert GC Logging Flags to Xlog. In addition, runtime logging has also been changed to use the JVM unified logging framework. The mapping of legacy runtime logging flags to the corresponding new Xlog configuration is described in Convert Runtime Logging Flags to Xlog.
The following provides quick reference to the -Xlog
command and syntax for options:
-Xloginfo level.
-Xlog:help-Xlog usage syntax and available tags, levels, and
decorators along with example command lines with explanations.
-Xlog:disable-Xlog[:option]Applies multiple arguments in the order that they appear on the
command line. Multiple -Xlog arguments for the same output
override each other in their given order.
The option is set as:
[tag-selection][
:[output][:[decorators][:output-options]]]
Omitting the tag-selection defaults to a tag-set of
all and a level of info.
tag[
+…]all
The all tag is a meta tag consisting of all tag-sets
available. The asterisk * in a tag set definition denotes a
wildcard tag match. Matching with a wildcard selects all tag sets that
contain at least the specified tags. Without the wildcard, only
exact matches of the specified tag sets are selected.
output-options is
filecount=file-countfilesize=file size with optional K, M or G suffixfoldmultilines=<true|false>
When foldmultilines is true, a log event that consists
of multiple lines will be folded into a single line by replacing newline
characters with the sequence '\' and 'n' in
the output. Existing single backslash characters will also be replaced
with a sequence of two backslashes so that the conversion can be
reversed. This option is safe to use with UTF-8 character encodings, but
other encodings may not work. For example, it may incorrectly convert
multi-byte sequences in Shift JIS and BIG5.
When the -Xlog option and nothing else is specified on
the command line, the default configuration is used. The default
configuration logs all messages with a level that matches either warning
or error regardless of what tags the message is associated with. The
default configuration is equivalent to entering the following on the
command line:
-Xlog:all=warning:stdout:uptime,level,tags
Logging can also be controlled at run time through Diagnostic
Commands (with the jcmd utility). Everything
that can be specified on the command line can also be specified
dynamically with the VM.log command. As the diagnostic
commands are automatically exposed as MBeans, you can use JMX to change
logging configuration at run time.
Each log message has a level and a tag set associated with it. The
level of the message corresponds to its details, and the tag set
corresponds to what the message contains or which JVM component it
involves (such as, gc, jit, or
os). Mapping GC flags to the Xlog configuration is
described in Convert GC
Logging Flags to Xlog. Mapping legacy runtime logging flags to the
corresponding Xlog configuration is described in Convert Runtime Logging
Flags to Xlog.
Available log levels:
offtracedebuginfowarningerrorAvailable log tags:
There are literally dozens of log tags, which in the right
combinations, will enable a range of logging output. The full set of
available log tags can be seen using -Xlog:help. Specifying
all instead of a tag combination matches all tag
combinations.
The -Xlog option supports the following types of
outputs:
stdout — Sends output to stdoutstderr — Sends output to stderrfile=filename — Sends output to text
file(s).When using file=filename, specifying
%p and/or %t in the file name expands to the
JVM’s PID and startup timestamp, respectively. You can also configure
text files to handle file rotation based on file size and a number of
files to rotate. For example, to rotate the log file every 10 MB and
keep 5 files in rotation, specify the options
filesize=10M, filecount=5. The target size of the files
isn’t guaranteed to be exact, it’s just an approximate value. Files are
rotated by default with up to 5 rotated files of target size 20 MB,
unless configured otherwise. Specifying filecount=0 means
that the log file shouldn’t be rotated. There’s a possibility of the
pre-existing log file getting overwritten.
By default logging messages are output synchronously - each log message is written to the designated output when the logging call is made. But you can instead use asynchronous logging mode by specifying:
-Xlog:asyncIn asynchronous logging mode, log sites enqueue all logging messages to an intermediate buffer and a standalone thread is responsible for flushing them to the corresponding outputs. The intermediate buffer is bounded and on buffer exhaustion the enqueuing message is discarded. Log entry write operations are guaranteed non-blocking.
The option -XX:AsyncLogBufferSize=N specifies the memory
budget in bytes for the intermediate buffer. The default value should be
big enough to cater for most cases. Users can provide a custom value to
trade memory overhead for log accuracy if they need to.
Logging messages are decorated with information about the message. You can configure each output to use a custom set of decorators. The order of the output is always the same as listed in the table. You can configure the decorations to be used at run time. Decorations are prepended to the log message. For example:
[6.567s][info][gc,old] Old collection completeOmitting decorators defaults to uptime,
level, and tags. The none
decorator is special and is used to turn off all decorations.
time (t), utctime
(utc), uptime (u),
timemillis (tm), uptimemillis
(um), timenanos (tn),
uptimenanos (un), hostname
(hn), pid (p), tid
(ti), level (l),
tags (tg) decorators can also be specified as
none for no decoration.
| Decorations | Description |
|---|---|
time or t |
Current time and date in ISO-8601 format. |
utctime or
utc |
Universal Time Coordinated or Coordinated Universal Time. |
uptime or u |
Time since the start of the JVM in seconds and milliseconds. For example, 6.567s. |
timemillis or
tm |
The same value as generated by
System.currentTimeMillis() |
uptimemillis or
um |
Milliseconds since the JVM started. |
timenanos or
tn |
The same value generated by
System.nanoTime(). |
uptimenanos or
un |
Nanoseconds since the JVM started. |
hostname or
hn |
The host name. |
pid or p |
The process identifier. |
tid or ti |
The thread identifier. |
level or l |
The level associated with the log message. |
tags or tg |
The tag-set associated with the log message. |
| Legacy Garbage Collection (GC) Flag | Xlog Configuration | Comment |
|---|---|---|
G1PrintHeapRegions |
-Xlog:gc+region=trace |
Not Applicable |
GCLogFileSize |
No configuration available | Log rotation is handled by the framework. |
NumberOfGCLogFiles |
Not Applicable | Log rotation is handled by the framework. |
PrintAdaptiveSizePolicy |
-Xlog:gc+ergo*=level |
Use a level of debug
for most of the information, or a level of trace
for all of what was logged for
PrintAdaptiveSizePolicy. |
PrintGC |
-Xlog:gc |
Not Applicable |
PrintGCApplicationConcurrentTime |
-Xlog:safepoint |
Note that
PrintGCApplicationConcurrentTime and
PrintGCApplicationStoppedTime are logged on the same tag
and aren’t separated in the new logging. |
PrintGCApplicationStoppedTime |
-Xlog:safepoint |
Note that
PrintGCApplicationConcurrentTime and
PrintGCApplicationStoppedTime are logged on the same tag
and not separated in the new logging. |
PrintGCCause |
Not Applicable | GC cause is now always logged. |
PrintGCDateStamps |
Not Applicable | Date stamps are logged by the framework. |
PrintGCDetails |
-Xlog:gc* |
Not Applicable |
PrintGCID |
Not Applicable | GC ID is now always logged. |
PrintGCTaskTimeStamps |
-Xlog:gc+task*=debug |
Not Applicable |
PrintGCTimeStamps |
Not Applicable | Time stamps are logged by the framework. |
PrintHeapAtGC |
-Xlog:gc+heap=trace |
Not Applicable |
PrintReferenceGC |
-Xlog:gc+ref*=debug |
Note that in the old logging,
PrintReferenceGC had an effect only if
PrintGCDetails was also enabled. |
PrintStringDeduplicationStatistics |
`-Xlog:gc+stringdedup*=debug | ` Not Applicable |
PrintTenuringDistribution |
-Xlog:gc+age*=level |
Use a level of debug
for the most relevant information, or a level of
trace for all of what was logged for
PrintTenuringDistribution. |
UseGCLogFileRotation |
Not Applicable | What was logged for
PrintTenuringDistribution. |
These legacy flags are no longer recognized and will cause an error if used directly. Use their unified logging equivalent instead.
| Legacy Runtime Flag | Xlog Configuration | Comment |
|---|---|---|
TraceExceptions |
-Xlog:exceptions=info |
Not Applicable |
TraceClassLoading |
-Xlog:class+load=level |
Use level=info for
regular information, or level=debug for additional
information. In Unified Logging syntax, -verbose:class
equals -Xlog:class+load=info,class+unload=info. |
TraceClassLoadingPreorder |
-Xlog:class+preorder=debug |
Not Applicable |
TraceClassUnloading |
-Xlog:class+unload=level |
Use level=info for
regular information, or level=trace for additional
information. In Unified Logging syntax, -verbose:class
equals -Xlog:class+load=info,class+unload=info. |
VerboseVerification |
-Xlog:verification=info |
Not Applicable |
TraceClassPaths |
-Xlog:class+path=info |
Not Applicable |
TraceClassResolution |
-Xlog:class+resolve=debug |
Not Applicable |
TraceClassInitialization |
-Xlog:class+init=info |
Not Applicable |
TraceLoaderConstraints |
-Xlog:class+loader+constraints=info |
Not Applicable |
TraceClassLoaderData |
-Xlog:class+loader+data=level |
Use level=debug for
regular information or level=trace for additional
information. |
TraceSafepointCleanupTime |
-Xlog:safepoint+cleanup=info |
Not Applicable |
TraceSafepoint |
-Xlog:safepoint=debug |
Not Applicable |
TraceMonitorInflation |
-Xlog:monitorinflation=debug |
Not Applicable |
TraceRedefineClasses |
-Xlog:redefine+class*=level |
level=info,
debug, and trace provide increasing amounts of
information. |
The following are -Xlog examples.
-XlogLogs all messages by using the info level to
stdout with uptime, levels, and
tags decorations. This is equivalent to using:
-Xlog:all=info:stdout:uptime,levels,tags
-Xlog:gcgc tag using
info level to stdout. The default
configuration for all other messages at level warning is in
effect.
-Xlog:gc,safepointgc or
safepoint tags, both using the info level, to
stdout, with default decorations. Messages tagged with both
gc and safepoint won’t be logged.
-Xlog:gc+ref=debuggc and ref
tags, using the debug level to stdout, with
default decorations. Messages tagged only with one of the two tags won’t
be logged.
-Xlog:gc=debug:file=gc.txt:nonegc tag using the
debug level to a file called gc.txt with no
decorations. The default configuration for all other messages at level
warning is still in effect.
-Xlog:gc=trace:file=gctrace.txt:uptimemillis,pids:filecount=5,filesize=1024Logs messages tagged with the gc tag using the
trace level to a rotating file set with 5 files with size 1
MB with the base name gctrace.txt and uses decorations
uptimemillis and pid.
The default configuration for all other messages at level
warning is still in effect.
-Xlog:gc::uptime,tidgc tag using the default
‘info’ level to default the output stdout and uses
decorations uptime and tid. The default
configuration for all other messages at level warning is
still in effect.
-Xlog:gc*=info,safepoint*=offgc using the
info level, but turns off logging of messages tagged with
safepoint. Messages tagged with both gc and
safepoint won’t be logged.
-Xlog:disable -Xlog:safepoint=trace:safepointtrace.txtsafepointusing tracelevel
to the file safepointtrace.txt. The default configuration
doesn’t apply, because the command line started with
-Xlog:disable.
The following describes a few complex examples of using the
-Xlog option.
-Xlog:gc+class*=debuggc and
class tags using the debug level to
stdout. The default configuration for all other messages at
the level warning is still in effect
-Xlog:gc+meta*=trace,class*=off:file=gcmetatrace.txtgc and
meta tags using the trace level to the file
metatrace.txt but turns off all messages tagged with
class. Messages tagged with gc,
meta, and class aren’t be logged as
class* is set to off. The default configuration for all
other messages at level warning is in effect except for
those that include class.
-Xlog:gc+meta=tracegc and
meta tags using the trace level to
stdout. The default configuration for all other messages at
level warning is still be in effect.
-Xlog:gc+class+heap*=debug,meta*=warning,threads*=offgc, class,
and heap tags using the trace level to
stdout but only log messages tagged with meta
with level. The default configuration for all other messages at the
level warning is in effect except for those that include
threads.
You use values provided to all Java Virtual Machine (JVM) command-line flags for validation and, if the input value is invalid or out-of-range, then an appropriate error message is displayed.
Whether they’re set ergonomically, in a command line, by an input
tool, or through the APIs (for example, classes contained in the package
java.lang.management) the values provided to all Java
Virtual Machine (JVM) command-line flags are validated. Ergonomics are
described in Java Platform, Standard Edition HotSpot Virtual Machine
Garbage Collection Tuning Guide.
Range and constraints are validated either when all flags have their
values set during JVM initialization or a flag’s value is changed during
runtime (for example using the jcmd tool). The JVM is
terminated if a value violates either the range or constraint check and
an appropriate error message is printed on the error stream.
For example, if a flag violates a range or a constraint check, then the JVM exits with an error:
java -XX:AllocatePrefetchStyle=5 -version
intx AllocatePrefetchStyle=5 is outside the allowed range [ 0 ... 3 ]
Improperly specified VM option 'AllocatePrefetchStyle=5'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.The flag -XX:+PrintFlagsRanges prints the range of all
the flags. This flag allows automatic testing of the flags by the values
provided by the ranges. For the flags that have the ranges specified,
the type, name, and the actual range is printed in the output.
For example,
intx ThreadStackSize [ 0 ... 9007199254740987 ] {pd product}For the flags that don’t have the range specified, the values aren’t displayed in the print out. For example:
size_t NewSize [ ... ] {product}This helps to identify the flags that need to be implemented. The automatic testing framework can skip those flags that don’t have values and aren’t implemented.
You use large pages, also known as huge pages, as memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system). Large pages optimize processor Translation-Lookaside Buffers.
A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations. A TLB is a scarce system resource. A TLB miss can be costly because the processor must then read from the hierarchical page table, which may require multiple memory accesses. By using a larger memory page size, a single TLB entry can represent a larger memory range. This results in less pressure on a TLB, and memory-intensive applications may have better performance.
However, using large pages can negatively affect system performance. For example, when a large amount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When this happens, either the OS or JVM reverts to using regular pages.
Linux and Windows support large pages.
Linux supports large pages since version 2.6. To check if your environment supports large pages, try the following:
# cat /proc/meminfo | grep Huge
HugePages_Total: 0
HugePages_Free: 0
...
Hugepagesize: 2048 kBIf the output contains items prefixed with “Huge”, then your system
supports large pages. The values may vary depending on environment. The
Hugepagesize field shows the default large page size in
your environment, and the other fields show details for large pages of
this size. Newer kernels have support for multiple large page sizes. To
list the supported page sizes, run this:
# ls /sys/kernel/mm/hugepages/
hugepages-1048576kB hugepages-2048kBThe above environment supports 2 MB and 1 GB large pages, but they
need to be configured so that the JVM can use them. When using large
pages and not enabling transparent huge pages (option
-XX:+UseTransparentHugePages), the number of large pages
must be pre-allocated. For example, to enable 8 GB of memory to be
backed by 2 MB large pages, login as root and run:
# echo 4096 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
It is always recommended to check the value of
nr_hugepages after the request to make sure the kernel was
able to allocate the requested number of large pages.
Note: The values contained in
/procand/sysreset after you reboot your system, so may want to set them in an initialization script (for example,rc.localorsysctl.conf).
If you configure the OS kernel parameters to enable use of large pages, the Java processes may allocate large pages for the Java heap as well as other internal areas, for example:
Consequently, if you configure the nr_hugepages
parameter to the size of the Java heap, then the JVM can still fail to
allocate the heap using large pages because other areas such as the code
cache might already have used some of the configured large pages.
To use large pages support on Windows, the administrator must first assign additional privileges to the user who is running the application:
Note that these steps are required even if it’s the administrator who’s running the application, because administrators by default don’t have the privilege to lock pages in memory.
Application Class Data Sharing (AppCDS) stores classes used by your applications in an archive file. Since these classes are stored in a format that can be loaded very quickly (compared to classes stored in a JAR file), AppCDS can improve the start-up time of your applications. In addition, AppCDS can reduce the runtime memory footprint by sharing parts of these classes across multiple processes.
Classes in the CDS archive are stored in an optimized format that’s about 2 to 5 times larger than classes stored in JAR files or the JDK runtime image. Therefore, it’s a good idea to archive only those classes that are actually used by your application. These usually are just a small portion of all available classes. For example, your application may use only a few APIs provided by a large library.
By default, in most JDK distributions, unless
-Xshare:off is specified, the JVM starts up with a default
CDS archive, which is usually located in
JAVA_HOME/lib/server/classes.jsa (or
JAVA_HOME\bin\server\classes.jsa on Windows). This archive
contains about 1300 core library classes that are used by most
applications.
To use CDS for the exact set of classes used by your application, you
can use the -XX:SharedArchiveFile option, which has the
general form:
-XX:SharedArchiveFile=<static_archive>:<dynamic_archive>
<static_archive> overrides the default CDS
archive.<dynamic_archive> provides additional classes
that can be loaded on top of those in the
<static_archive>.: should be
replaced with ;(The names “static” and “dynamic” are used for historical reasons. The only significance is that the “static” archive is loaded first and the “dynamic” archive is loaded second).
The JVM can use up to two archives. To use only a single
<static_archive>, you can omit the
<dynamic_archive> portion:
-XX:SharedArchiveFile=<static_archive>
For convenience, the <dynamic_archive> records the
location of the <static_archive>. Therefore, you can
omit the <static_archive> by saying only:
-XX:SharedArchiveFile=<dynamic_archive>
CDS archives can be created manually using several methods:
-Xshare:dump-XX:ArchiveClassesAtExitjcmd VM.cdsOne common operation in all these methods is a “trial run”, where you run the application once to determine the classes that should be stored in the archive.
The following steps create a static CDS archive file that contains
all the classes used by the test.Hello application.
Create a list of all classes used by the test.Hello
application. The following command creates a file named
hello.classlist that contains a list of all classes used by
this application:
java -Xshare:off -XX:DumpLoadedClassList=hello.classlist -cp hello.jar test.Hello
The classpath specified by the -cp parameter must
contain only JAR files.
Create a static archive, named hello.jsa, that
contains all the classes in hello.classlist:
java -Xshare:dump -XX:SharedArchiveFile=hello.jsa -XX:SharedClassListFile=hello.classlist -cp hello.jar
Run the application test.Hello with the archive
hello.jsa:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
Optional Verify that the test.Hello
application is using the class contained in the hello.jsa
shared archive:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar -Xlog:class+load test.Hello
The output of this command should contain the following text:
[info][class,load] test.Hello source: shared objects file
By default, when the -Xshare:dump option is used, the
JVM runs in interpreter-only mode (as if the -Xint option
were specified). This is required for generating deterministic output in
the shared archive file. I.e., the exact same archive will be generated,
bit-for-bit, every time you dump it. However, if deterministic output is
not needed, and you have a large classlist, you can explicitly add
-Xmixed to the command-line to enable the JIT compiler.
This will speed up the archive creation.
Advantages of dynamic CDS archives are:
The following steps create a dynamic CDS archive file that contains
the classes that are used by the test.Hello application,
excluding those that are already in the default CDS archive.
Create a dynamic CDS archive, named hello.jsa, that
contains all the classes in hello.jar loaded by the
application test.Hello:
java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello
Run the application test.Hello with the shared
archive hello.jsa:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
Optional Repeat step 4 of the previous section
to verify that the test.Hello application is using the
class contained in the hello.jsa shared archive.
It’s also possible to create a dynamic CDS archive with a non-default static CDS archive. E.g.,
java -XX:SharedArchiveFile=base.jsa -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello
To run the application using this dynamic CDS archive:
java -XX:SharedArchiveFile=base.jsa:hello.jsa -cp hello.jar Hello
(On Windows, the above path delimiter : should be
replaced with ;)
As mention above, the name of the static archive can be skipped:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello
The previous two sections require you to modify the application’s start-up script in order to create a CDS archive. Sometimes this could be difficult, for example, if the application’s class path is set up by complex routines.
The jcmd VM.cds command provides a less intrusive way
for creating a CDS archive by connecting to a running JVM process. You
can create either a static:
jcmd <pid> VM.cds static_dump my_static_archive.jsa
or a dynamic archive:
jcmd <pid> VM.cds dynamic_dump my_dynamic_archive.jsa
To use the resulting archive file in a subsequent run of the application without modifying the application’s start-up script, you can use the following technique:
env JAVA_TOOL_OPTIONS=-XX:SharedArchiveFile=my_static_archive.jsa bash app_start.sh
Note: to use jcmd <pid> VM.cds dynamic_dump, the
JVM process identified by <pid> must be started with
-XX:+RecordDynamicDumpInfo, which can also be passed to the
application start-up script with the same technique:
env JAVA_TOOL_OPTIONS=-XX:+RecordDynamicDumpInfo bash app_start.sh
-XX:+AutoCreateSharedArchive is a more convenient way of
creating/using CDS archives. Unlike the methods of manual CDS archive
creation described in the previous section, with
-XX:+AutoCreateSharedArchive, it’s no longer necessary to
have a separate trial run. Instead, you can always run the application
with the same command-line and enjoy the benefits of CDS
automatically.
java -XX:+AutoCreateSharedArchive -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello
If the specified archive file exists and was created by the same version of the JDK, then it will be loaded as a dynamic archive; otherwise it is ignored at VM startup.
At VM exit, if the specified archive file does not exist, it will be created. If it exists but was created with a different (but post JDK 19) version of the JDK, then it will be replaced. In both cases the archive will be ready to be loaded the next time the JVM is launched with the same command line.
If the specified archive file exists but was created by a JDK version prior to JDK 19, then it will be ignored: neither loaded at startup, nor replaced at exit.
Developers should note that the contents of the CDS archive file are
specific to each build of the JDK. Therefore, if you switch to a
different JDK build, -XX:+AutoCreateSharedArchive will
automatically recreate the archive to match the JDK. If you intend to
use this feature with an existing archive, you should make sure that the
archive is created by at least version 19 of the JDK.
Neither the class path (-classpath and
-Xbootclasspath/a) nor the module path
(--module-path) can contain non-empty directories.
Only modular JAR files are supported in
--module-path. Exploded modules are not supported.
The class path used at archive creation time must be the same as (or a prefix of) the class path used at run time. (There’s no such requirement for the module path.)
The CDS archive cannot be loaded if any JAR files in the class path or module path are modified after the archive is generated.
If any of the VM options --upgrade-module-path,
--patch-module or --limit-modules are
specified, CDS is disabled. This means that the JVM will execute without
loading any CDS archives. In addition, if you try to create a CDS
archive with any of these 3 options specified, the JVM will report an
error.
You can use the Java advanced runtime options to optimize the performance of your applications.
Use the following commands and advanced options to achieve higher throughput performance for your application:
java -server -XX:+UseParallelGC -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g
Use the following commands and advanced options to achieve lower response times for your application:
java -XX:+UseG1GC -XX:MaxGCPauseMillis=100
Use the following advanced runtime options to keep the Java heap small and reduce the dynamic footprint of embedded applications:
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Note: The defaults for these two options are 70% and 40% respectively. Because performance sacrifices can occur when using these small settings, you should optimize for a small footprint by reducing these settings as much as possible without introducing unacceptable performance degradation.
The following exit values are typically returned by the launcher when
the launcher is called with the wrong arguments, serious errors, or
exceptions thrown by the JVM. However, a Java application may choose to
return any value by using the API call
System.exit(exitValue). The values are:
0: Successful completion
>0: An error occurred