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JEP
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Resolution: Unresolved
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P4
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Jim Laskey & Gavin Bierman
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Feature
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Open
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SE
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494
Summary
Enhance the Java programming language with the ability to succinctly import all of the packages exported by a module. This simplifies the reuse of modular libraries, but does not require the importing code to be in a module itself. This is a preview language feature.
History
Module import declarations were first proposed as a preview feature by JEP 476 (JDK 23). We here propose to preview them for a second time to gain more experience and feedback, with two additions:
Lift the restriction that no module is able to declare a transitive dependence on the
java.base
module, and revise the declaration of thejava.se
module to transitively require thejava.base
module. With these changes, importing thejava.se
module will import the entire Java SE API on demand.Allow type-import-on-demand declarations to shadow module import declarations.
Goals
Simplify the reuse of modular libraries by allowing entire modules to be imported at once.
Avoid the noise of multiple type-import-on-demand declarations (e.g.,
import com.foo.bar.*
) when using diverse parts of the API exported by a module.Allow beginners to more easily use third-party libraries and fundamental Java classes without having to learn where they are located in a package hierarchy.
Ensure that module import declarations work smoothly alongside existing import declarations.
Do not require developers who use the module import feature to modularize their own code.
Motivation
Classes and interfaces in the java.lang
package, such as Object
, String
,
and Comparable
, are essential to every Java program. For this reason, the Java
compiler automatically imports, on demand, all of the classes and interfaces in
the java.lang
package, as if
import java.lang.*;
appears at the beginning of every source file.
As the Java Platform has evolved, classes and interfaces such as List
, Map
,
Stream
, and Path
have become almost as essential. However, none of these are
in java.lang
, so they are not automatically imported; rather, developers have
to keep the compiler happy by writing a plethora of import
declarations at the
beginning of every source file. For example, the following code converts an
array of strings into a map from capital letters to strings, but the imports
take almost as many lines as the code:
import java.util.Map; // or import java.util.*;
import java.util.function.Function; // or import java.util.function.*;
import java.util.stream.Collectors; // or import java.util.stream.*;
import java.util.stream.Stream; // (can be removed)
String[] fruits = new String[] { "apple", "berry", "citrus" };
Map<String, String> m =
Stream.of(fruits)
.collect(Collectors.toMap(s -> s.toUpperCase().substring(0,1),
Function.identity()));
Developers have diverse views as to whether to prefer single-type-import or type-import-on-demand declarations. Many prefer single-type imports in large, mature codebases where clarity is paramount. However, in early-stage situations where convenience trumps clarity, developers often prefer on-demand imports; for example,
When prototyping code and
;When exploring a new API in JShell, such as Stream Gatherers or the
; orWhen learning to program with new features that work in concert with new APIs, such as
.
Since Java 9, modules have allowed a set of packages to be grouped together for reuse under a single name. The exported packages of a module are intended to form a cohesive and coherent API, so it would be convenient if developers could import on-demand from the entire module, that is, from all of the packages exported by the module. It would be as if all the exported packages are imported in one go.
For example, importing the java.base
module on-demand would give immediate
access to List
, Map
, Stream
, and Path
, without having to manually import
java.util
on-demand, and java.util.stream
on-demand, and java.nio.file
on-demand.
The ability to import at the level of modules would be especially helpful when
APIs in one module have a close relationship with APIs in another module. This
is common in large multi-module libraries such as the JDK. For example, the
<code class="prettyprint" data-shared-secret="1731389207507-0.8802033004726278">java.sql</code>
module provides database access via its java.sql
and javax.sql
packages, but
one of its interfaces,
<code class="prettyprint" data-shared-secret="1731389207507-0.8802033004726278">java.sql.SQLXML</code>,
declares public
methods whose signatures use interfaces from the
javax.xml.transform
package in the
<code class="prettyprint" data-shared-secret="1731389207507-0.8802033004726278">java.xml</code>
module. Developers who call these methods in java.sql.SQLXML
typically import
both the java.sql
package and the javax.xml.transform
package. To
facilitate this extra import, the java.sql
module depends on the java.xml
module transitively, so
that a program which depends on the java.sql
module depends automatically on
the java.xml
module. In this scenario, it would be convenient if importing the
java.sql
module on-demand would also automatically import the java.xml
module on-demand. Automatically importing on-demand from transitive dependencies
would be a further convenience when prototyping and exploring.
Description
A module import declaration has the form
import module M;
It imports, on demand, all of the public
top-level classes and interfaces in
The packages exported by the module
M
to the current module, andThe packages exported by the modules that are read by the current module due to reading the module
M
.
The second clause allows a program to use the API of a module, which might refer to classes and interfaces from other modules, without having to import all of those other modules.
For example:
import module java.base
has the same effect as 54 on-demand package imports, one for each of the packages exported by thejava.base
module. It is as if the source file containsimport java.io.*
andimport java.util.*
and so on.
.import module java.sql
has the same effect asimport java.sql.*
andimport javax.sql.*
plus on-demand package imports for the
This is a preview language feature, disabled by default
To try the examples below in JDK 24, you must enable preview features:
Compile the program with
javac --release 24 --enable-preview Main.java
and run it withjava --enable-preview Main
; or,When using the source code launcher, run the program with
java --enable-preview Main.java
; or,When using <code class="prettyprint" data-shared-secret="1731389207507-0.8802033004726278">jshell</code>, start it with
jshell --enable-preview
.
Syntax and semantics
We extend the grammar of import declarations
(JLS §7.5)
to include import module
clauses:
ImportDeclaration:
SingleTypeImportDeclaration
TypeImportOnDemandDeclaration
SingleStaticImportDeclaration
StaticImportOnDemandDeclaration
ModuleImportDeclaration
ModuleImportDeclaration:
import module ModuleName;
import module
takes a module name, so it is not possible to import packages
from the unnamed module, i.e., from the class path. This aligns with requires
clauses in module declarations, i.e., module-info.java
files, which take
module names and cannot express a dependence on the unnamed module.
import module
can be used in any source file, whether or not that file is part
of the definition of an explicit module. For example, java.base
and java.sql
are part of the standard Java runtime, and can be imported by classes which are
only ever deployed on the class path. (For technical background, see
JEP 261.)
Within a source file that is part of the definition of an explicit module,
import module
can be used to conveniently import all of the packages that are
exported, without qualification, by that module. In such a source file, packages
in the module that are either not exported or exported with qualification must
continue to be imported in the traditional way. (In other words, import module
M
is not more powerful for code inside module M
than for code outside M
.)
A source file may import the same module more than once.
Resolving ambiguous imports
Importing a module has the effect of importing multiple packages, so it is possible to import classes with the same simple name from different packages. The simple name is ambiguous, so using it will cause a compile-time error.
For example, in this source file the simple name Element
is ambiguous:
import module java.desktop; // exports javax.swing.text,
// which has a public Element interface,
// and also exports javax.swing.text.html.parser,
// which has a public Element class
...
Element e = ... // Error - Ambiguous name!
...
As another example, in this source file the simple name List
is ambiguous:
import module java.base; // exports java.util, which has a public List interface
import module java.desktop; // exports java.awt, which a public List class
...
List l = ... // Error - Ambiguous name!
...
As a final example, in this source file the simple name Date
is ambiguous:
import module java.base; // exports java.util, which has a public Date class
import module java.sql; // exports java.sql, which has a public Date class
...
Date d = ... // Error - Ambiguous name!
...
Resolving ambiguities is straightforward: Use another import declaration. For
example, adding a single-type-import declaration resolves the ambiguous Date
of the previous example by shadowing the Date
classes imported by the
import module
declarations:
import module java.base; // exports java.util, which has a public Date class
import module java.sql; // exports java.sql, which has a public Date class
import java.sql.Date; // resolve the ambiguity of the simple name Date!
...
Date d = ... // Ok! Date is resolved to java.sql.Date
...
In other cases, it is more convenient to add an on-demand declaration to resolve ambiguities by shadowing all of the classes in a package:
import module java.base;
import module java.desktop;
import java.util.*;
import javax.swing.text.*;
...
Element e = ... // Element is resolved to javax.swing.text.Element
List l = ... // List is resolved to java.util.List
Document d = ... // Document is resolved to javax.swing.text.Document,
// regardless of any module imports
...
The shadowing behavior of import declarations matches their specificity. The most specific, i.e., single-type-import declarations, can shadow both on-demand and module import declarations, which are less specific. On-demand declarations can shadow module import declarations, which are less specific, but not single-type-import declarations, which are more specific.
Coalescing import declarations
You may be able to coalesce multiple on-demand declarations into a single module import declaration; for example:
import javax.xml.*;
import javax.xml.parsers.*;
import javax.xml.stream.*;
could be replaced with:
import module java.xml;
which is easier to read.
Grouping import declarations
If a source file has a mix of different kinds of import declarations then grouping them by kind may further improve readability; for example:
// Module imports
import module M1;
import module M2;
...
// Package imports
import P1.*;
import P2.*;
...
// Single-type imports
import P1.C1;
import P2.C2;
...
class Foo { ... }
The order of the groups reflects their shadowing behavior: The least-specific module import declarations are first, the most-specific single-type imports are last, and the on-demand imports are in between.
A worked example
Here is an example of how import module
works. Suppose the source file
C.java
is part of the definition of module M0
:
// C.java
package q;
import module M1; // What does this import?
class C { ... }
where module M0
has the following declaration:
module M0 { requires M1; }
The meaning of import module M1
depends on the exports of M1
and any modules
that M1
requires transitively.
module M1 {
exports p1;
exports p2 to M0;
exports p3 to M3;
requires transitive M4;
requires M5;
}
module M3 { ... }
module M4 { exports p10; }
module M5 { exports p11; }
The effect of import module M1
is to
Import the
public
top level classes and interfaces from packagep1
, sinceM1
exportsp1
to everyone;Import the
public
top level classes and interfaces from packagep2
, sinceM1
exportsp2
toM0
, the module with whichC.java
is associated; andImport the
public
top level classes and interfaces from packagep10
, sinceM1
requires transitivelyM4
, which exportsp10
.
Nothing from packages p3
or p11
is imported by C.java
.
Importing modules in simple source files
This JEP is co-developed with the JEP
, which specifies that everypublic
top level class and interface in every package exported by the java.base
module is automatically imported on-demand in a simple source file. In other
words, it is as if import module java.base
appears at the beginning of every
such file. A simple source file may import other modules, e.g., java.desktop
,
and may explicitly import the java.base
module even though doing so is
redundant.
The JShell tool automatically imports ten
packages on-demand. The list of packages is ad-hoc. We therefore propose to
change JShell to automatically import module java.base
.
Importing aggregator modules
It is sometimes useful to import an aggregator module, i.e., a module which
does not export any packages itself but does export the packages exported by the
modules it requires. For example, the
<code class="prettyprint" data-shared-secret="1731389207507-0.8802033004726278">java.se</code>
module does not export any packages, but it requires nineteen other modules
transitively, so the
effect of import module java.se
is to import the packages exported by those
modules, and so on, recursively — specifically, the 123 packages listed as the
indirect exports
of the java.se
module.
In earlier previews of this feature, developers were surprised to see that
importing the java.se
module did not have the effect of importing the
java.base
module. They thus had to either import the java.base module as well,
or else import specific packages from java.base
, e.g., import java.util.*
.
Importing the java.se
module did not import the java.base
module because the
Java Language Specification
java.base
module.
This restriction was sensible in the original design of the modules feature,
since every module has an implicit dependence on java.base
. With the module
import feature, however, which uses module declarations to derive a set of
packages to be imported, the ability to require java.base
transitively is
useful.
We therefore propose to lift this language restriction. We will also revise the
declaration of the java.se
module so that it transitively requires the
java.base
module. Thus a single import module java.se
is all that is needed
to use the entire standard Java API, no matter how many modules take part in
exporting the API.
Only aggregator modules in the Java Platform should use requires transitive
java.base
. The clients of such aggregators expect all java.*
modules to be
imported, including java.base
. Modules in the Java Platform that have both
direct exports and indirect exports are not, strictly speaking, aggregators.
Hence they should not use requires transitive java.base
because it may pollute
the client's namespace. For example, the java.sql
module exports its own
packages as well as packages from java.xml
and others, but a client that says
import module java.sql
is not necessarily interested in importing everything
from java.base
.
The directive import module java.se
works only in source files that are part
of the definition of an explicit module that already requires java.se
. In a
source file outside of a module definition, and in particular in a simple source
file which implicitly declares a class, using import module java.se
fails
because java.se
is not in the default set of root modules for the unnamed
module. In a source file that is part of the definition of an automatic module,
import module java.se
will work if some other resolved explicit module
requires java.se
.
Alternatives
An alternative to
import module ...
is to automatically import more packages than justjava.lang
. This would bring more classes into scope, i.e., usable by their simple names, and delay the need for beginners to learn about imports of any kind. But, which additional packages should we import automatically?Every reader will have suggestions for which packages to auto-import from the omnipresent
java.base
module:java.io
andjava.util
would be near-universal suggestions;java.util.stream
andjava.util.function
would be common; andjava.math
,java.net
, andjava.time
would each have supporters. For the JShell tool, we managed to find tenjava.*
packages which are broadly useful when experimenting with one-off Java code, but it is difficult to see which subset ofjava.*
packages deserves to be permanently and automatically imported into every Java program. The list would, moreover, change change as the Java Platform evolves; e.g.,java.util.stream
andjava.util.function
were introduced only in Java 8. Developers would likely become reliant on IDEs to remind them of which automatic imports are in effect — an undesirable outcome.An important use case for this feature is to automatically import on-demand from the
java.base
module in implicitly declared classes. This could alternatively be achieved by automatically importing the 54 packages exported byjava.base
. However, when an implicit class is migrated to an ordinary explicit class, which is the expected lifecycle, the developer would either have to write 54 on-demand package imports, or else figure out which imports are necessary.
Risks and Assumptions
Using one or more module import declarations leads to a risk of name ambiguity due to different packages declaring members with the same simple name. This ambiguity is not detected until the ambiguous simple name is used in a program, when a compile-time error will occur. The ambiguity can be resolved by adding a single-type-import declaration, but managing and resolving such name ambiguities could be burdensome and lead to code that is brittle and difficult to read and maintain.
- relates to
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JDK-8315129 JEP 476: Module Import Declarations (Preview)
- Closed
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JDK-8335984 JEP 495: Simple Source Files and Instance Main Methods (Fourth Preview)
- Proposed to Target