Managing Source and Class Files
Many implementations of the Java platform rely on hierarchical file systems to manage source and class files, although The Java Language Specification does not require this. The strategy is as follows.
Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java . For example:
//in the Rectangle.java file package graphics; public class Rectangle
Then, put the source file in a directory whose name reflects the name of the package to which the type belongs:
The qualified name of the package member and the path name to the file are parallel, assuming the Microsoft Windows file name separator backslash (for UNIX, use the forward slash).
- class name – graphics.Rectangle
- pathname to file – graphics\Rectangle.java
As you should recall, by convention a company uses its reversed Internet domain name for its package names. The Example company, whose Internet domain name is example.com , would precede all its package names with com.example . Each component of the package name corresponds to a subdirectory. So, if the Example company had a com.example.graphics package that contained a Rectangle.java source file, it would be contained in a series of subdirectories like this:
. \com\example\graphics\Rectangle.java
When you compile a source file, the compiler creates a different output file for each type defined in it. The base name of the output file is the name of the type, and its extension is .class . For example, if the source file is like this
//in the Rectangle.java file package com.example.graphics; public class Rectangle < . . . >class Helper
then the compiled files will be located at:
\com\example\graphics\Rectangle.class \com\example\graphics\Helper.class
Like the .java source files, the compiled .class files should be in a series of directories that reflect the package name. However, the path to the .class files does not have to be the same as the path to the .java source files. You can arrange your source and class directories separately, as:
\sources\com\example\graphics\Rectangle.java \classes\com\example\graphics\Rectangle.class
By doing this, you can give the classes directory to other programmers without revealing your sources. You also need to manage source and class files in this manner so that the compiler and the Java Virtual Machine (JVM) can find all the types your program uses.
The full path to the classes directory, \classes , is called the class path, and is set with the CLASSPATH system variable. Both the compiler and the JVM construct the path to your .class files by adding the package name to the class path. For example, if
is your class path, and the package name is
then the compiler and JVM look for .class files in
\classes\com\example\graphics.
A class path may include several paths, separated by a semicolon (Windows) or colon (UNIX). By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in your class path.
Setting the CLASSPATH System Variable
To display the current CLASSPATH variable, use these commands in Windows and UNIX (Bourne shell):
In Windows: C:\> set CLASSPATH In UNIX: % echo $CLASSPATH
To delete the current contents of the CLASSPATH variable, use these commands:
In Windows: C:\> set CLASSPATH= In UNIX: % unset CLASSPATH; export CLASSPATH
To set the CLASSPATH variable, use these commands (for example):
In Windows: C:\> set CLASSPATH=C:\users\george\java\classes In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH
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.
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 cmf jar-file existing-manifest input-file(s)
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.
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 by downloading the JDK Demos and Samples bundle from Java SE Downloads. This demo contains class files, 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 TicTacToe directory:
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:
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.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