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'll 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, the following table summarizes common JAR file 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, the following table summarizes 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>
|
Java ARchive file (JAR)
Creating a JAR File
Viewing the Contents of a JAR File
Extracting the Contents of a JAR File
Updating a JAR File
Understanding the Default Manifest
Working with Manifest Files
Java ARchive file (JAR)
Creating a JAR File
Viewing the Contents of a JAR File
Extracting the Contents of a JAR File
Updating a JAR File
Understanding the Default Manifest
Working with Manifest Files
Modifying a Manifest File
Adding Classes to the JAR File's Classpath
|
|
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.
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 these additional options to the cf options of the basic command:
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 Modifying a Manifest File for more information about his 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 below for an example. |
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.
An Example
Let us look at an example. A simple TicTacToe applet. You can see the source code of this Applet atTicTacToe.java. This demo contains a bytecode class file, audio files, and images having this structure:
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 when you download the entire Tutorial online. To package this demo into a single JAR file named TicTacToe.jar, you would run this command from inside the TicTacToedirectory:
jar cvf TicTacToe.jar TicTacToe.class audio imagesThe 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 imagesYou 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 theTicTacToe 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 for information about manifest files.
In the above 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.MFcross.gifnot.gifbeep.auding.aureturn.auyahoo1.auyahoo2.auBy contrast, suppose that you used a command that did not employ the -C option:
jar cf ImageAudio.jar images audioThe resulting JAR file would have this table of contents:
META-INF/MANIFEST.MFimages/cross.gifimages/not.gifaudio/beep.auaudio/ding.auaudio/return.auaudio/yahoo1.auaudio/yahoo2.auViewing the Contents of a JAR File
The basic format of the command for viewing the contents of a JAR file is:
jar tf jar-fileLet'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.jarThis command displays the contents of the JAR file to stdout:
META-INF/MANIFEST.MF
TicTacToe.classp.au
a
audio/audio/beeudio/ding.auu
audio/yahoo1.
audio/return.aauaudio/yahoo2.auimages/gif
images/cross.gifimages/not.
The JAR file contains the TicTacToe class file and the audio and images directory, as expected. The output also shows that 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.
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.jarFor 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.classApr 20 16:39:32 PDT 2005 audio/beep.au
2
0 Wed Apr 20 16:39:32 PDT 2005 audio/4032 Wed 566 Wed Apr 20 16:39:32 PDT 2005 audio/ding.auu
7834 Wed Apr 20 16:39:32 PDT 2005 audio/yahoo1.
6558 Wed Apr 20 16:39:32 PDT 2005 audio/return.aau7463 Wed Apr 20 16:39:32 PDT 2005 audio/yahoo2.au 0 Wed Apr 20 16:39:44 PDT 2005 images/gif
157 Wed Apr 20 16:39:44 PDT 2005 images/cross.gif 158 Wed Apr 20 16:39:44 PDT 2005 images/not.
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 options 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 ofTicTacToe.jar are:
META-INF/MANIFEST.MFTicTacToe.classaudio/audio/beep.auaudio/ding.auaudio/return.auaudio/yahoo1.auaudio/yahoo2.auimages/images/cross.gifimages/not.gifSuppose 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.gifThis 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.jarUpdating a JAR File
The Jar tool provides a u option which 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.
Examples
Recall that TicTacToe.jar has these contents:
META-INF/MANIFEST.MFTicTacToe.classaudio/audio/beep.auaudio/ding.auaudio/return.auaudio/yahoo1.auaudio/yahoo2.auimages/images/cross.gifimages/not.gifSuppose 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.gifThe revised JAR file would have this table of contents:
META-INF/MANIFEST.MFTicTacToe.classaudio/audio/beep.auaudio/ding.auaudio/return.auaudio/yahoo1.auaudio/yahoo2.auimages/images/cross.gifimages/not.gifimages/new.gifYou can use the -C option to "change directories" during execution of the command. For example:
jar uf TicTacToe.jar -C images new.gifThis 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.MFTicTacToe.classaudio/audio/beep.auaudio/ding.auaudio/return.auaudio/yahoo1.auaudio/yahoo2.auimages/images/cross.gifimages/not.gifnew.gifWorking with Manifest Files: The Basics
JAR files support a wide range of functionality, including electronic signing, version control, package sealing, and others. What gives a JAR file this versatility? The answer is the JAR file's manifest.The manifest is a special file that can contain information about the files packaged in a JAR file. By tailoring this "meta" information that the manifest contains, you enable the JAR file to serve a variety of purposes. This lesson will explain the contents of the manifest file and show you how to work with it, with examples for the basic features:
Understanding the Default Manifest
When you create a JAR file, it automatically receives a default manifest file. There can be only one manifest file in an archive, and it always has the pathname
META-INF/MANIFEST.MFWhen you create a JAR file, the default manifest file simply contains the following:
Manifest-Version: 1.0Created-By: 1.6.0 (Sun Microsystems Inc.)These lines show that a manifest's entries take the form of "header: value" pairs. The name of a header is separated from its value by a colon. The default manifest conforms to version 1.0 of the manifest specification and was created by the 1.6.0 version of the JDK. The manifest can also contain information about the other files that are packaged in the archive. Exactly what file information should be recorded in the manifest depends on how you intend to use the JAR file. The default manifest makes no assumptions about what information it should record about other files.
Modifying a Manifest File
You use the m command-line option to add custom information to the manifest during creation of a JAR file. This section describes the m option.The Jar tool automatically puts a default manifest with the pathname META-INF/MANIFEST.MF into any JAR file you create. You can enable special JAR file functionality, such as package sealing, by modifying the default manifest. Typically, modifying the default manifest involves adding special-purpose headers to the manifest that allow the JAR file to perform a particular desired function. To modify the manifest, you must first prepare a text file containing the information you wish to add to the manifest. You then use the Jar tool's m option to add the information in your file to the manifest.
Warning: The text file from which you are creating 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.
The basic command has this format:jar cfm jar-file manifest-addition input-file(s)Let's look at the options and arguments used in this command:· The c option indicates that you want to create a JAR file.
· The m option indicates that you want to merge information from an existing file into the manifest file of the JAR file you're creating.
· The f option indicates that you want the output to go to a file (the JAR file you're creating) rather than to standard output.
· manifest-addition is the name (or path and name) of the existing text file whose contents you want to add to the contents of JAR file's manifest.
· jar-file is the name that you want the resulting JAR file to have.
· The input-file(s) argument is a space-separated list of one or more files that you want to be placed in your JAR file.
The m and f options must be in the same order as the corresponding arguments.Adding Classes to the JAR File's Classpath
You may need to reference classes in other JAR files from within a JAR file.For example, in a typical situation an applet is bundled in a JAR file whose manifest references a different JAR file (or several different JAR files) that serves as utilities for the purposes of that applet. You specify classes to include in the Class-Path header field in the manifest file of an applet or application. TheClass-Path header takes the following form:
Class-Path: jar1-name jar2-name directory-name/jar3-nameBy using the Class-Path header in the manifest, you can avoid having to specify a long -classpath flag when invoking Java to run the your application.
Note: The Class-Path header points to classes or JAR files on the local network, not JAR files within the JAR file or classes accessible over internet protocols. To load classes in JAR files within a JAR file into the class path, you must write custom code to load those classes. For example, if MyJar.jar contains another JAR file called MyUtils.jar, you cannot use the Class-Path header in MyJar.jar's manifest to load classes in MyUtils.jar into the class path.
An Example
We want to load classes in MyUtils.jar into the class path for use in MyJar.jar. These two JAR files are in the same directory.We first create a text file named Manifest.txt with the following contents:
Class-Path: MyUtils.jar
Warning: The text file 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.
We then create a JAR file named MyJar.jar by entering the following command:
jar cfm MyJar.jar Manifest.txt MyPackage/*.classThis creates the JAR file with a manifest with the following contents:
Manifest-Version: 1.0Class-Path: MyUtils.jarCreated-By: 1.6.0 (Sun Microsystems Inc.)The classes in MyUtils.jar are now loaded into the class path when you run MyJar.jar.
