Using JAR Files: The Basics






Using JAR Files: The Basics

JAR files are packaged with the ZIP file format, so you can use them for tasks such as lossless data compression, archiving, decompression, and archive unpacking. These tasks are among the most common uses of JAR files, and you can realize many JAR file benefits using only these basic features.

Even if you want to take advantage of advanced functionality provided by the JAR file format, such as electronic signing, you will first need to become familiar with the fundamental operations.

To perform basic tasks with JAR files, you use the Java Archive Tool provided as part of the Java Development Kit. Because the Java Archive Tool is invoked by using the jar command, this tutorial refers to it as "the Jar tool."

As a synopsis and preview of some of the topics to be covered in this section, Figure summarizes common JAR file operations.

Common JAR File Operations

Operation

Command

To create a JAR file

jar cf jar-file input-file(s)

To view the contents of a JAR file

jar tf jar-file

To extract the contents of a JAR file

jar xf jar-file

To extract specific files from a JAR file

jar xf jar-file archived-file(s)

To run an application packaged as a JAR file (requires the Main-class[*] manifest header)

java -jar app.jar

To invoke an applet packaged as a JAR file

<applet code=AppletClassName.class
  archive="JarFileName.jar"
  width=width height= height>
</applet>


[*] tutorial/deployment/jar/appman.html

This section shows you how to perform the most common JAR-file operations, with examples for each of the basic features.

The documentation for the Java Development Kit includes reference pages for the Jar tool:

  • Jar tool reference for Windows platform[5]

    [5] docs/technotes/tools/windows/jar.html

  • Jar tool reference for Solaris platform[6]

    [6] docs/technotes/tools/solaris/jar.html

Creating a JAR File

The basic format of the command for creating a JAR file is:

jar cf jar-file input-file(s)

The options and arguments used in this command are:

  • The c option indicates that you want to create a JAR file.

  • The f option indicates that you want the output to go to a file rather than to stdout.

  • jar-file is the name that you want the resulting JAR file to have. You can use any filename for a JAR file. By convention, JAR filenames are given a .jar extension, though this is not required.

  • The input-file(s) argument is a space-separated list of one or more files that you want to include in your JAR file. The input-file(s) argument can contain the wildcard * symbol. If any of the "input-files" are directories, the contents of those directories are added to the JAR archive recursively.

The c and f options can appear in either order, but there must not be any space between them.

This command will generate a compressed JAR file and place it in the current directory. The command will also generate a default manifest file for the JAR archive (see the Understanding the Default Manifest section, page 500).

Note

The metadata in the JAR file, such as the entry names, comments, and contents of the manifest, must be encoded in UTF8.


You can add any of the additional options to the cf options of the basic command shown in Figure.

JAR Command Options

Option

Description

v

Produces verbose output on stdout while the JAR file is being built. The verbose output tells you the name of each file as it's added to the JAR file.

0 (zero)

Indicates that you don't want the JAR file to be compressed.

M

Indicates that the default manifest file should not be produced.

m

Used to include manifest information from an existing manifest file. The format for using this option is:

jar cmf existing-manifest jar-file input-file(s)

See the Modifying a Manifest File section (page 501) for more information about this option.

Warning

The manifest must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.


-C

To change directories during execution of the command. See the An Example section (page 491).


Note

When you create a JAR file, the time of creation is stored in the JAR file. Therefore, even if the contents of the JAR file do not change, when you create a JAR file multiple times, the resulting files are not exactly identical. You should be aware of this when you are using JAR files in a build environment. It is recommended that you use versioning information in the manifest file, rather than creation time, to control versions of a JAR file; see the Setting Package Version Information section (page 504).


An Example

Let us look at an example, a simple TicTacToe applet. This demo contains a bytecode class file, audio files, and images having the structure shown in Figure.

1. TicTacToe folder hierarchy.


The audio and images subdirectories contain sound files and GIF images used by the applet.

You can obtain all these files from jar/examples directory.[7] To package this demo into a single JAR file named TicTacToe.jar, you would run this command from inside the TicTacToe directory:

[7] tutorial/deployment/jar/examples/TicTacToe.java

jar cvf TicTacToe.jar TicTacToe.class audio images

The audio and images arguments represent directories, so the Jar tool will recursively place them and their contents in the JAR file. The generated JAR file TicTacToe.jar will be placed in the current directory. Because the command used the v option for verbose output, you would see something similar to this output when you run the command:

adding: TicTacToe.class (in=3825) (out=2222) (deflated 41%)
adding: audio/ (in=0) (out=0) (stored 0%)
adding: audio/beep.au (in=4032) (out=3572) (deflated 11%)
adding: audio/ding.au (in=2566) (out=2055) (deflated 19%)
adding: audio/return.au (in=6558) (out=4401) (deflated 32%)
adding: audio/yahoo1.au (in=7834) (out=6985) (deflated 10%)
adding: audio/yahoo2.au (in=7463) (out=4607) (deflated 38%)
adding: images/ (in=0) (out=0) (stored 0%)
adding: images/cross.gif (in=157) (out=160) (deflated -1%)
adding: images/not.gif (in=158) (out=161) (deflated -1%)

You can see from this output that the JAR file TicTacToe.jar is compressed. The Jar tool compresses files by default. You can turn off the compression feature by using the 0 (zero) option, so that the command would look like:

jar cvf0 TicTacToe.jar TicTacToe.class audio images

You might want to avoid compression, for example, to increase the speed with which a JAR file could be loaded by a browser. Uncompressed JAR files can generally be loaded more quickly than compressed files because the need to decompress the files during loading is eliminated. However, there is a tradeoff in that download time over a network may be longer for larger, uncompressed files.

The Jar tool will accept arguments that use the wildcard * symbol. As long as there weren't any unwanted files in the TicTacToe directory, you could have used this alternative command to construct the JAR file:

jar cvf TicTacToe.jar *

Though the verbose output doesn't indicate it, the Jar tool automatically adds a manifest file to the JAR archive with path name META-INF/MANIFEST.MF. See the Working with Manifest Files: The Basics section (page 500) for information about manifest files.

In the previous example, the files in the archive retained their relative path names and directory structure. The Jar tool provides the -C option that you can use to create a JAR file in which the relative paths of the archived files are not preserved. It's modeled after TAR's -C option.

As an example, suppose you wanted to put audio files and gif images used by the TicTacToe demo into a JAR file, and that you wanted all the files to be on the top level, with no directory hierarchy. You could accomplish that by issuing this command from the parent directory of the images and audio directories:

jar cf ImageAudio.jar -C images . -C audio .

The -C images part of this command directs the Jar tool to go to the images directory, and the . following -C images directs the Jar tool to archive all the contents of that directory. The -C audio . part of the command then does the same with the audio directory. The resulting JAR file would have this table of contents:

META-INF/MANIFEST.MF
cross.gif
not.gif
beep.au
ding.au
return.au
yahoo1.au
yahoo2.au

By contrast, suppose that you used a command that did not employ the -C option:

jar cf ImageAudio.jar images audio

The resulting JAR file would have this table of contents:

META-INF/MANIFEST.MF
images/cross.gif
images/not.gif
audio/beep.au
audio/ding.au
audio/return.au
audio/yahoo1.au
audio/yahoo2.au

Viewing the Contents of a JAR File

The basic format of the command for viewing the contents of a JAR file is:

jar tf jar-file

Let's look at the options and argument used in this command:

  • The t option indicates that you want to view the table of contents of the JAR file.

  • The f option indicates that the JAR file whose contents are to be viewed is specified on the command line.

  • The jar-file argument is the path and name of the JAR file whose contents you want to view.

The t and f options can appear in either order, but there must not be any space between them.

This command will display the JAR file's table of contents to stdout.

You can optionally add the verbose option, v, to produce additional information about file sizes and last-modified dates in the output.

An Example

Let's use the Jar tool to list the contents of the TicTacToe.jar file we created in the previous section:

jar tf TicTacToe.jar

This command displays the contents of the JAR file to stdout:

META-INF/MANIFEST.MF
TicTacToe.class
audio/
audio/beep.au
audio/ding.au
audio/return.au
audio/yahoo1.au
audio/yahoo2.au
images/
images/cross.gif
images/not.gif

The JAR file contains the TicTacToe class file and the audio and images directory, as expected. The output also shows that the JAR file contains a default manifest file, META-INF/MANIFEST.MF, which was automatically placed in the archive by the JAR tool. For more information, see the Understanding the Default Manifest section (page 500).

All pathnames are displayed with forward slashes, regardless of the platform or operating system you're using. Paths in JAR files are always relative; you'll never see a path beginning with C:, for example.

The JAR tool will display additional information if you use the v option:

jar tvf TicTacToe.jar

For example, the verbose output for the TicTacToe JAR file would look similar to this:

 256 Mon Apr 18 10:50:28 PDT 2005 META-INF/MANIFEST.MF
3885 Mon Apr 18 10:49:50 PDT 2005 TicTacToe.class
   0 Wed Apr 20 16:39:32 PDT 2005 audio/
4032 Wed Apr 20 16:39:32 PDT 2005 audio/beep.au
2566 Wed Apr 20 16:39:32 PDT 2005 audio/ding.au
6558 Wed Apr 20 16:39:32 PDT 2005 audio/return.au
7834 Wed Apr 20 16:39:32 PDT 2005 audio/yahoo1.au
7463 Wed Apr 20 16:39:32 PDT 2005 audio/yahoo2.au
   0 Wed Apr 20 16:39:44 PDT 2005 images/
 157 Wed Apr 20 16:39:44 PDT 2005 images/cross.gif
 158 Wed Apr 20 16:39:44 PDT 2005 images/not.gif

Extracting the Contents of a JAR File

The basic command to use for extracting the contents of a JAR file is:

jar xf jar-file [archived-file(s)]

Let's look at the options and arguments in this command:

  • The x option indicates that you want to extract files from the JAR archive.

  • The f option indicates that the JAR file from which files are to be extracted is specified on the command line, rather than through stdin.

  • The jar-file argument is the filename (or path and filename) of the JAR file from which to extract files.

  • archived-file(s) is an optional argument consisting of a space-separated list of the files to be extracted from the archive. If this argument is not present, the Jar tool will extract all the files in the archive.

As usual, the order in which the x and f options appear in the command doesn't matter, but there must not be a space between them.

When extracting files, the Jar tool makes copies of the desired files and writes them to the current directory, reproducing the directory structure that the files have in the archive. The original JAR file remains unchanged.

Caution

When it extracts files, the Jar tool will overwrite any existing files having the same pathname as the extracted files.


An Example

Let's extract some files from the TicTacToe JAR file we've been using in previous sections. Recall that the contents of TicTacToe.jar are:

META-INF/MANIFEST.MF
TicTacToe.class
audio/
audio/beep.au
audio/ding.au
audio/return.au
audio/yahoo1.au
audio/yahoo2.au
images/
images/cross.gif
images/not.gif

Suppose you want to extract the TicTacToe class file and the cross.gif image file. To do so, you can use this command:

jar xf TicTacToe.jar TicTacToe.class images/cross.gif

This command does two things:

  • It places a copy of TicTacToe.class in the current directory.

  • It creates the directory images, if it doesn't already exist, and places a copy of cross.gif within it.

The original TicTacToe JAR file remains unchanged.

As many files as desired can be extracted from the JAR file in the same way. When the command doesn't specify which files to extract, the Jar tool extracts all files in the archive. For example, you can extract all the files in the TicTacToe archive by using this command:

jar xf TicTacToe.jar

Updating a JAR File

The Jar tool provides a u option that you can use to update the contents of an existing JAR file by modifying its manifest or by adding files.

The basic command for adding files has this format:

jar uf jar-file input-file(s)

In this command:

  • The u option indicates that you want to update an existing JAR file.

  • The f option indicates that the JAR file to update is specified on the command line.

  • jar-file is the existing JAR file that's to be updated.

  • input-file(s) is a space-deliminated list of one or more files that you want to add to the Jar file.

Any files already in the archive having the same pathname as a file being added will be overwritten.

When creating a new JAR file, you can optionally use the -C option to indicate a change of directory. For more information, see the Creating a JAR File section (page 490).

Examples

Recall that TicTacToe.jar has these contents:

META-INF/MANIFEST.MF
TicTacToe.class
audio/
audio/beep.au
audio/ding.au
audio/return.au
audio/yahoo1.au
audio/yahoo2.au
images/
images/cross.gif
images/not.gif

Suppose that you want to add the file images/new.gif to the JAR file. You could accomplish that by issuing this command from the parent directory of the images directory:

jar uf TicTacToe.jar images/new.gif

The revised JAR file would have this table of contents:

META-INF/MANIFEST.MF
TicTacToe.class
audio/
audio/beep.au
audio/ding.au
audio/return.au
audio/yahoo1.au
audio/yahoo2.au
images/
images/cross.gif
images/not.gif
images/new.gif

You can use the -C option to "change directories" during execution of the command. For example:

jar uf TicTacToe.jar -C images new.gif

This command would change to the images directory before adding new.gif to the JAR file. The images directory would not be included in the pathname of new.gif when it's added to the archive, resulting in a table of contents that looks like this:

META-INF/MANIFEST.MF
TicTacToe.class
audio/
audio/beep.au
audio/ding.au
audio/return.au
audio/yahoo1.au
audio/yahoo2.au
images/
images/cross.gif
images/not.gif
new.gif

Running JAR-Packaged Software

Now that you've learned how to create JAR files, how do you actually run the code that you've packaged? Consider these three scenarios:

  • Your JAR file contains an applet that is to be run inside a browser.

  • Your JAR file contains an application that is to be invoked from the command line.

  • Your JAR file contains code that you want to use as an extension.

This section will cover the first two situations. The Extension Mechanism by Alan Sommerer[8] covers the use of JAR files as extensions.

[8] tutorial/ext/index.html

Applets Packaged in JAR Files

To invoke any applet from an HTML file for running inside a browser, you need to use the APPLET tag. For more information, see Chapter 18. If the applet is bundled as a JAR file, the only thing you need to do differently is to use the ARCHIVE parameter to specify the relative path to the JAR file.

As an example, let's use (again!) the TicTacToe demo applet that ships with the Java Development Kit. The APPLET tag in the HTML file that calls the demo looks like this:

<applet code=TicTacToe.class
  width=120 height=120>
</applet>

If the TicTacToe demo were packaged in a JAR file named TicTacToe.jar, you could modify the APPLET tag with the simple addition of an ARCHIVE parameter:

<applet code=TicTacToe.class
  archive="TicTacToe.jar"
  width=120 height=120>
</applet>

The ARCHIVE parameter specifies the relative path to the JAR file that contains TicTacToe.class. This example assumes that the JAR file and the HTML file are in the same directory. If they're not, you would need to include the JAR file's relative path in the ARCHIVE parameter's value. For example, if the JAR file was one directory below the HTML file in a directory called applets, the APPLET tag would look like this:

<applet code=TicTacToe.class
  archive="applets/TicTacToe.jar"
  width=120 height=120>
</applet>

JAR Files as Applications

You can run JAR-packaged applications with the Java interpreter. The basic command is:

java -jar jar-file

The -jar flag tells the interpreter that the application is packaged in the JAR file format. You can only specify one JAR file, which must contain all the application-specific code.

Before you execute this command, make sure the runtime environment has information as to which class within the JAR file is the application's entry point.

To indicate which class is the application's entry point, you must add a Main-Class header to the JAR file's manifest. The header takes the form:

Main-Class: classname

The header's value, classname, is the name of the class that's the application's entry point.

For more information, see the Setting an Application's Entry Point section (page 502).

When the Main-Class is set in the manifest file, you can run the application from the command line:

java -jar app.jar



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