What is a JAR file?

The JAR file format is based on the popular ZIP file format, and is used for aggregating many files into one. Unlike ZIP files, JAR files are used not only for archiving and distribution, but also for deployment and encapsulation of libraries, components, and plug-ins, and are consumed directly by tools such as compilers and JVMs. Special files contained in the JAR, such as manifests and deployment descriptors, instruct tools how a particular JAR is to be treated.

A JAR file might be used:

• For distributing and using class libraries


• As building blocks for applications and extensions


• As deployment units for components, applets, or plug-ins


• For packaging auxiliary resources associated with components

The JAR file format provides many benefits and features, many of which are not provided with a traditional archive format such as ZIP or TAR. These include:

• Security. You can digitally sign the contents of a JAR file. Tools that recognize your signature can then optionally grant your software security privileges it wouldn't otherwise have, and detect if the code has been tampered with.


• Decreased download time. If an applet is bundled in a JAR file, the applet's class files and associated resources can be downloaded by a browser in a single HTTP transaction, instead of opening a new connection for each file.


• Compression. The JAR format allows you to compress your files for efficient storage.


• Transparent platform extension. The Java Extensions Framework provides a means by which you can add functionality to the Java core platform, which uses the JAR file for packaging of extensions. (Java 3D and JavaMail are examples of extensions developed by Sun.)


• Package sealing. Packages stored in JAR files can be optionally sealed to enforce version consistency and security. Sealing a package means that all classes defined in that package must be found in the same JAR file.


• Package versioning. A JAR file can hold data about the files it contains, such as vendor and version information.


• Portability. The mechanism for handling JAR files is a standard part of the Java platform's core API.

Compressed and uncompressed JARs

The jar tool (see The jar tool for details) compresses files by default. Uncompressed JAR files can generally be loaded more quickly than compressed JAR files, because the need to decompress the files during loading is eliminated, but download time over a network may be longer for uncompressed files.

The jar tool

To perform basic tasks with JAR files, you use the Java Archive Tool (jar tool) provided as part of the Java Development Kit. You invoke the jar tool with the jar command. Table 1 shows some common applications:

Common usages of the jar tool

Creating a JAR file from individual files

jar -cvMf jar-file input-file...

Creating a JAR file from a directory

jar -cvMf jar-file dir-name

Creating an uncompressed JAR file

jar -cvMf0 jar-file input-file...

Updating a JAR file

jar -uf jar-file input-file

Viewing the contents of a JAR file

jar -tf jar-file

Extracting the contents of a JAR file

jar -xf jar-file

Extracting specific files from a JAR file

jar -xf jar-file archived-file

Here is the usage for AIX 6.1:

Usage: jar {ctxu}[vfm0Mi] [jar-file] [manifest-file] [-C dir] files ...
Options:
-c create new archive
-t list table of contents for archive
-x extract named (or all) files from archive
-u update existing archive
-v generate verbose output on standard output
-f specify archive file name
-m include manifest information from specified manifest file
-0 store only; use no ZIP compression
-M do not create a manifest file for the entries
-i generate index information for the specified jar files
-C change to the specified directory and include the following file
If any file is a directory then it is processed recursively.
The manifest file name and the archive file name needs to be specified
in the same order the 'm' and 'f' flags are specified.

Example 1: to archive two class files into an archive called classes.jar:
jar cvf classes.jar Foo.class Bar.class
Example 2: use an existing manifest file 'mymanifest' and archive all the
files in the foo/ directory into 'classes.jar':
jar cvfm classes.jar mymanifest -C foo/ .