JAR File Overview
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 consists of a zip archive, as defined by PKWARE, containing a manifest file and potentially signature files, as defined in the JAR File Specification.
<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 ",".
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.
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.
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