jar-The Java Archive Tool

Combines multiple files into a single JAR archive file.

SYNOPSIS

jar [ options ] [manifest] destination input-file [input-files]

DESCRIPTION

The jar tool combines multiple files into a single JAR archive file. jar is a general-purpose archiving and compression tool, based on ZIP and the ZLIB compression format. However, jar was designed mainly to facilitate the packaging of java applets or applications into a single archive. When the components of an applet or application (.class files, images and sounds) are combined into a single archive, they may be downloaded by a java agent (like a browser) in a single HTTP transaction, rather than requiring a new connection for each piece. This dramatically improves download times. jar also compresses files and so further improves download time. In addition, it allows individual entries in a file to be signed by the applet author so that their origin can be authenticated. The syntax for the jar tool is almost identical to the syntax for the tar command. A jar archive can be use as a class path entry, whether it is compressed or not.

The 3 types of input files for the jar tool are

Typical usage is 
        % jar cf myjarfile *.class
In this example, all the class files in the current directory are placed into the file named "myjarfile". A manifest file is automatically generated by the jar tool and is always the first entry in the jar file. By default, it is named META-INF/MANIFEST.MF. The manifest file is the place where any meta-information about the archive is stored. Refer to the JAR file specification for details about how meta-information is stored in the manifest file.

If you have a pre-existing manifest file that you want the jar tool to use for the new jar archive, you can specify it using the -m option: 

        % jar cmf myManifestFile myJarFile *.class
Be sure that any pre-existing manifest file that you use ends with a new line. The last line of a manifest file will not be parsed if it doesn't end with a new line character. Note that when you specify "cfm" instead of "cmf" (i.e., you invert the order of the "m" and "f" options), you need to specify the name of the jar archive first, followed by the name of the manifest file: 
        % jar cfm myJarFile myManifestFile *.class
The manifest uses RFC822 ascii format, so it is easy to view and process manifest-file contents.

Beginning with version 1.3 of the Java 2 SDK, the jar utility supports JarIndex, which allows application class loaders to load classes efficiently from jar files. If an application or applet is bundled into multiple jar files,  only the necessary jar files will be downloaded and opened to load classes. This performance optimization is enabled by running jar with the new -i option. It will generate package location information for the specified main jar file and all the jar files it depends on, which need to be specified in the Class-Path attribute of the main jar file's manifest. 

% jar -i main.jar

In this example, and INDEX.LIST file is inserted into the META-INF directory of main.jar's manifest.
The application class loader will use the information stored in this file for efficient class loading.  Refer to the JarIndex specification for details about how location information is stored in the index file.

Examples of using the Jar tool to operate on Jar files and Jar file manifests are provided below and in the Jar trail of the Java Tutorial.

OPTIONS

c
Creates a new or empty archive on the standard output.
t
Lists the table of contents from standard output.
x file
Extracts all files, or just the named files, from standard input. If file is omitted, then all files are extracted; otherwise, only the specified file or files are extracted.
f
The second argument specifies a jar file to process. In the case of creation, this refers to the name of the jar file to be created (instead of on stdout). For table or xtract, the second argument identifies the jar file to be listed or extracted.
v
Generates verbose output on stderr.
m
Includes manifest information from specified pre-existing manifest file. Example use:
jar cmf myManifestFile myJarFile *.class
You can add special-purpose name-value attribute headers to the manifest file that aren't contained in the default manifest. Examples of such headers would be those for vendor information, version information, package sealing, and headers to make JAR-bundled applications executable. See the JAR Files trail in the Java Tutorial and the Notes for Developers web page for examples of using the m option.
0
Store only, without using ZIP compression.
M
Do not create a manifest file for the entries.
u
Update an existing JAR file by adding files or changing the manifest. For example,
jar uf foo.jar foo.class
would add the file foo.class to the existing JAR file foo.jar, and 
jar umf manifest foo.jar
would update foo.jar's manifest with the information in manifest.
i
Generate index information for the specified jar file and its dependent jar files. For example, 
jar -i foo.jar

would generate an INDEX.LIST file in foo.jar which contains location information for each package in foo.jar and all the jar files specified in foo.jar's Class-Path attribute.

-C
Temporaily changes directories during execution of jar command while processing the next argument. Its operation is intended to be similar to the -C option of the UNIX tar utility. For example, 
jar uf foo.jar -C classes bar.class
would change to the classes directory and add the bar.class from that directory to foo.jar. The following command, 
jar uf foo.jar -C classes . -C bin xyz.class
would change to the classes directory and add to foo.jar all files within the classes directory, but not the classes directory itself, and then change to the bin directory and add xyz.class to foo.jar.
If any of "files" is a directory, then that directory is processed recursively.

EXAMPLES

To add all the files in a particular directory to an archive: 
$ ls
0.au            3.au            6.au            9.au            at_work.gif
1.au            4.au            7.au            Animator.class  monkey.jpg
2.au            5.au            8.au            Wave.class      spacemusic.au
$ jar cvf bundle.jar *
adding: 0.au
adding: 1.au
adding: 2.au
adding: 3.au
adding: 4.au
adding: 5.au
adding: 6.au
adding: 7.au
adding: 8.au
adding: 9.au
adding: Animator.class
adding: Wave.class
adding: at_work.gif
adding: monkey.jpg
adding: spacemusic.au
$
If you already have subdirectories for images, audio files and classes in your html directory, I might jar up each directory into a single jar file: 
$ ls
audio classes images
$ jar cvf bundle.jar audio classes images
adding: audio/1.au
adding: audio/2.au
adding: audio/3.au
adding: audio/spacemusic.au
adding: classes/Animator.class
adding: classes/Wave.class
adding: images/monkey.jpg
adding: images/at_work.gif
$ ls -l
total 142
drwxr-xr-x   2 brown    green        512 Aug  1 22:33 audio
-rw-r--r--   1 brown    green      68677 Aug  1 22:36 bundle.jar
drwxr-xr-x   2 brown    green        512 Aug  1 22:26 classes
drwxr-xr-x   2 brown    green        512 Aug  1 22:25 images
$
You can then see the entry names in the jarfile using the jar tool and the "t" option: 
$ ls
audio bundle.jar classes images
$ jar tf bundle.jar
META-INF/MANIFEST.MF
audio/1.au
audio/2.au
audio/3.au
audio/spacemusic.au
classes/Animator.class
classes/Wave.class
images/monkey.jpg
images/at_work.gif
$
Enumerating verbosely (with the "v" option) will tell you more information about the files in the archive, such as their size and last modified date: 
$ jar tvf bundle.jar
   145 Thu Aug 01 22:27:00 PDT 1996 META-INF/MANIFEST.MF
   946 Thu Aug 01 22:24:22 PDT 1996 audio/1.au
  1039 Thu Aug 01 22:24:22 PDT 1996 audio/2.au
   993 Thu Aug 01 22:24:22 PDT 1996 audio/3.au
 48072 Thu Aug 01 22:24:23 PDT 1996 audio/spacemusic.au
 16711 Thu Aug 01 22:25:50 PDT 1996 classes/Animator.class
  3368 Thu Aug 01 22:26:02 PDT 1996 classes/Wave.class
 12809 Thu Aug 01 22:24:48 PDT 1996 images/monkey.jpg
   527 Thu Aug 01 22:25:20 PDT 1996 images/at_work.gif
$

If you bundled a stock trade application(applet) into the following jar files: 

main.jar buy.jar sell.jar other.jar
and you specified in the Class-Path attribute in main.jar's manifest as: 
Class-Path: buy.jar sell.jar other.jar
Then you can use the -i option to speed up your application's class loading time: 
$ jar -i main.jar
an INDEX.LIST file is inserted to the META-INF directory which will enable the application class loader to download the right jar files when it is searching for classes or resources.

SEE ALSO

The Jar Overview
The Jar File Specification
The JarIndex Spec
Java Tutorial on the Java Software web site.