Summary

Add an option to the jar tool to not overwriting existing file on extraction.

Problem

The JDK ships with the jar tool which can be used to extract the contents of a jar file. In its current form, the jar tool extracts the contents to the target directory, and always overwrite existing files. There are situations where user may want to prevent overwriting, in particular, if a malformed jar file crafted to append a jar file entry with the same name to overwrite existing file, prevent overwriting could be used as a defensive mechanism.

Solution

An optional -k or --keep-old-files option will be allowed to be used with the -x main operation. The -k option will prevent jar extraction to overwrite the existing file.

Specification

Optional -k option is added to the -x main operation of the jar tool. With -k option, if an entry with the same name already exist on the target directory, then the existing file will not be overwritten. Otherwise, the file is replaced with the extracted copy, which is current behavior.

This command extracts the test.jar file to the current working directory, create the directory if necessary. This is current operation mode of jar.

 $ jar xvf test.jar                                                                                                                                               
   created: META-INF/
  inflated: META-INF/MANIFEST.MF
  inflated: testfile1
  inflated: testfile2

This command extracts the test.jar file again to the current working directory, and keep current existing files.

$ jar xvkf test.jar                                                                                                                                              
  created: META-INF/
  skipped: META-INF/MANIFEST.MF exists
  skipped: testfile1 exists
  skipped: testfile2 exists

Same as above, but if we remove the testfile1 and testfile2 from the current working directory, the command output will be

$ jar xvkf test.jar
  created: META-INF/
  skipped: META-INF/MANIFEST.MF exists
 inflated: testfile1
 inflated: testfile2

Now if we remove META-INF folder, and use the gnu style options,

$ jar -x --keep-old-files -v -f test.jar                                                                                                                         
  created: META-INF/
 inflated: META-INF/MANIFEST.MF
  skipped: testfile1 exists
  skipped: testfile2 exists

jar --help has been updated to update the -x option description and introduce -k, --keep-old-files option in the help text:

  -x, --extract              Extract named (or all) files from the archive.
                             If a file with the same name appears more than once in
                             the archive, each copy will be extracted, with later copies
                             overwriting (replacing) earlier copies unless -k is specified.
...

Operation modifiers valid only in extract mode:

  -k, --keep-old-files       Do not overwrite existing files.
                             If a file with the same name exists in the target
                             directory, skip the extraction of that file.
                             As a result, if a file appears more than once in an
                             archive, later copies will not overwrite earlier copies.
                             Also note that some file system can be case insensitive.

man page updates for jar tool follows:

diff --git a/closed/src/jdk.jartool/share/man/jar.md b/closed/src/jdk.jartool/share/man/jar.md
index 8c20e1f920..3e0d17ced4 100644
--- a/closed/src/jdk.jartool/share/man/jar.md
+++ b/closed/src/jdk.jartool/share/man/jar.md
@@ -1,5 +1,5 @@
 ---
-# Copyright (c) 1997, 2023, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 1997, 2024, Oracle and/or its affiliates. All rights reserved.
 #

 title: 'JAR(1) JDK @@VERSION_SHORT@@ | JDK Commands'
@@ -80,6 +80,9 @@ argument is the first argument specified on the command line.

 `-x` or `--extract`
 :   Extracts the named (or all) files from the archive.
+    If a file with the same name appears more than once in
+    the archive, each copy will be extracted, with later copies
+    overwriting (replacing) earlier copies unless -k is specified.

 `-d` or `--describe-module`
 :   Prints the module descriptor or automatic module name.
@@ -159,6 +162,16 @@ or `--create`) the update (`-u` or `--update` ) and the generate-index (`-i` or
     format, to use for the timestamp of the entries,
     e.g. "2022-02-12T12:30:00-05:00".

+## Operation Modifiers Valid Only in Extract Mode
+
+`-k` or `--keep-old-files`
+:   Do not overwrite existing files.
+    If a Jar file entry with the same name exists in the target directory, the
+    existing file will not be overwritten.
+    As a result, if a file appears more than once in an archive, later copies will not overwrite
+    earlier copies.
+    Also note that some file system can be case insensitive.
+
 ## Other Options

 The following options are recognized by the `jar` command and not used with