Introduction to Java : Constructing .Jar Files
Building a Jar File
After you have created your ZIP and CAB archives, you can then create a JAR (Java ARchive) file that will contain your compressed and archived Applet for use with users who have the most recent versions of Netscape Navigator and HotJava.
Building JAR files is very similar to building TAR files, something with which most UNIX developers are already familiar. Unfortunately, though, as with CABARC, there is only a command line interface, so you need to build your JAR files in the DOS shell using the JAR program provided with the JDK1.1.
The JAR program should be located in the “Bin” directory under your main JDK directory (this is also where javac and appletviewer are located, as you recall from earlier chapters).
JAR follows the generic format:
1 | jar [command][options] jarfile [manifest file] [files] |
| Commands | |
|---|---|
| Parameter | Comments |
| t | Used to list the contents of a jar file |
| c | Used to create a jar file |
| x | Used to extract single or multiple files from a jar file |
| v | Generates verbose output to standard error |
| f | Specifies the file name of the archive |
| m | Used to include manifest information from the given manifest file |
| O | Used for storage only. No compression is applied and the file may be stored in your CLASSPATH |
| M | Specifies that no manifest file should be created |
Creating a Jar File
To create a JAR file, use the basic form:
1 | jar cvf JARFILENAME listoffiles |
The “c” comand tells JAR that we are creating an archive, the “v” option tells JAR to produce verbose information about its successes and errors, and the “f” option specifies that we will be providing a name for the JAR file. Unlike with CABARC, JAR will automatically recursively add any directories which are named.
JAR, like CABARC, has the problem of adding too many files to the archive, but unlike CABARC, we do not have an easy way to restrict what is added without specifying every detail of the archive. Thus, to excise the useless files from the archive, we would have to use the following command:
1 | jar cvf MyApplet.jar *.class Images/*.jpg Images/*.gif Utility/*.class |
Listing Files In and Extracting Files From a Jar Archive
To list the contents of a JAR file, you will use the “t” command instead of the “c” command. For example, the following command will list the files that we just added to the MyApplet.jar file in the previous section:
1 | jar tvf MyApplet.jar |
Similarly, you can easily extract the entire contents of a JAR file using the “x” command. For example, to extract the contents of the MyApplet.jar file created above, you would use the following command:
1 | jar xvf MyApplet.jar |
Jar Manifests
The manifest file is a list of files within an archive that are to be signed. You can find out more information about them at http://www.javasoft.com/products/jdk/1.1/docs/guide/jar/manifest.html.
Distributing Jar Archives
Once you have created your JAR file, you can transfer it to your web server and reference it in the HTML file by using the ARCHIVE attribute of the <APPLET> tag as follows:
1 2 3 4 | <APPLET CODE = "appname.class" ARCHIVE = "jarfilename.jar" WIDTH = "x" HEIGHT = "y"> </APPLET> |
Another interesting trick with JAR files is that you can also specify several jar files in a comma-separated list as follows:
1 2 3 4 | <APPET CODE = "my.class" ARCHIVE = "classes.jar, graphics.jar" WIDTH = "x" HEIGHT = "y"> </APPLET> |
As with CAB and ZIP files, the CODE attribute must still be present, even though you would jar up the class file with the rest.