JAR Guide

What is JAR?

JAR stands for Java ARchive. It's a file format based on the popular ZIP file format and is used for aggregating many files into one. Although JAR can be used as a general archiving tool, the primary motivation for its development was so that Java applets and their requisite components (.class files, images and sounds) can be downloaded to a browser in a single HTTP transaction, rather than opening a new connection for each piece. This greatly improves the speed with which an applet can be loaded onto a web page and begin functioning. The JAR format also supports compression, which reduces the size of the file and improves download time still further. Additionally, individual entries in a JAR file may be digitally signed by the applet author to authenticate their origin.

JAR is:

the only archive format that is cross-platform
the only format that handles audio and image files as well as class files
backward-compatible with existing applet code
an open standard, fully extendable, and written in java
the preferred way to bundle the pieces of a java applet

JAR consists of a zip archive, as defined by PKWARE, containing a manifest file and potentially signature files, as defined in the Manifest and Signature specification.

The APPLET tag

Changing the APPLET tag in your HTML page to accomodate a JAR file is simple. The JAR file on the server is identified by the ARCHIVE parameter, where the directory location of the jar file should be relative to the location of the html page:

    <applet code=Animator.class 
      archive="jars/animator.jar"
      width=460 height=160>
      <param name=foo value="bar">
    </applet>

Note that the familiar CODE=myApplet.class parameter must still be present. The CODE parameter, as always, identifies the name of the applet where execution begins. However, the class file for the applet and all of its helper classes are loaded from the JAR file.

Th ARCHIVE attribute describes one or more JAR files containing classes and other resources that will be "preloaded". The classes are loaded using an instance of an AppletClassLoader with the given CODEBASE. It takes the form archive = archiveList. The archives in archiveList are separated by ",". NB: in JDK1.1, multiple APPLET tags with the same CODEBASE share the same instance of a ClassLoader. This is used by some client code to implement inter-applet communication. Future JDKs may provide other mechanisms for inter-applet communication. For security reasons, the applet's class loader can read only from the same codebase from which the applet was started. This means that archives in archiveList must be in the same directory as, or in a subdirectory of, the codebase. Entries in archiveList of the form ../a/b.jar will not work unless explicitly allowed for in the security policy file (except in the case of an http codebase, where archives in archiveList must be from the same host as the codebase, but can have ".."'s in their paths.)

Once the archive file is identified, it is downloaded and separated into its components. During the execution of the applet, when a new class, image or audio clip is requested by the applet, it is searched for first in the archives associated with the applet. If the file is not found amongst the archives that were downloaded, it is searched for on the applet's server, relative to the CODEBASE (that is, it is searched for as in JDK1.0.2).

The archive tag may specify multiple JAR files. Each JAR file must be separated by "," (comma). Each file is downloaded in turn:

    <applet code=Animator.class 
      archive="classes.jar ,  images.jar ,  sounds.jar"
      width=460 height=160>
      <param name=foo value="bar">
    </applet>

There can be any amount of white space between entries within the archive parameter. In addition, the archive tag itself is case-insensitive; it can be lower-case, upper-case, or any combination of lower- and upper-case letters, such as ArCHiVe.

Executable JAR Files

On Win32 systems the Java 2 runtime environment's installation program will register a default association for JAR files so that double-clicking a JAR file on the desktop will automatically run it with javaw -jar. Dependent extensions bundled with the application will also be loaded automatically. This feature makes the end-user runtime environment easier to use on Win32 systems.

The Solaris 2.6 kernel has already been extended to recognize the special "magic" number that identifies a JAR file, and to invoke java -jar on such a JAR file as if it were a native Solaris executable. A application packaged in a JAR file can thus be executed directly from the command line or by clicking an icon on the CDE desktop.