Articles : JavaBoutique's Introduction to Java :
Constructing .Zip, .Cab, and .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:

jar [command][options] jarfile [manifest file] [files]

Creating a Jar File

To create a JAR file, use the basic form:

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:

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:

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:

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:

<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:

<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.

Selena Sol contributes to the JavaBoutique's Introduction to Java. Selena curently works for Barclays Capital in London, one of the leading global investment banks in Europe and has worked as a software developer for the National Center for Human Genome research, Microline Software, Neuron Data, and Electric Eye in Singapore. Selena is perhaps best-known for creating the Public Domain Web Script Archive (Extropia) and writing several books on Web Programming (Perl, CGI, Java).
Email: selena@extropia.com