Java Archive (JAR) Features

JAR (Java Archive) is a platform-independent file format that aggregates many files into one. Multiple Java applets and their requisite components (.class files, images and sounds) can be bundled in a JAR file and subsequently downloaded to a browser in a single HTTP transaction, greatly improving the download speed. The JAR format also supports compression, which reduces the file size, further improving the download time. In addition, the applet author can digitally sign individual entries in a JAR file to authenticate their origin. It is fully backward-compatible with existing applet code and is fully extensible, being written in Java.
This version of the Java platform extends the utility of the JAR format. Enhancements include added functionality for the command-line JAR tool for creating and updating signed JAR files. There are also new standard APIs for reading and writing JAR files. In addition, the Java Extensions Framework provides a mechanism for handling dependencies on extensions and other third-party libraries packaged as JAR files.

JAR Features

JAR Files and the Extensions Framework

The new extensions mechanism in the Java platform uses the JAR file format to package extension classes. New manifest attributes are available to support the extension mechanism and related features such as package sealing and package versioning. See the extensions specification for details.

Enhanced Jar Tool

The Jar tool has the new -C and u options for creating and updating JAR files. See the Jar tool reference pages:

Jar tool reference page for Solaris
Jar tool reference page for Windows

Reading and Writing JAR Files: API Specification

Package java.util.jar: classes for creating and reading JAR files
Class java.net.JarURLConnection: Abstract class for URL connections to JAR files using the jar protocol.

Tutorial

On the Java Software website:

The Java Archive (JAR) File Format trail in the Java Tutorial.

Java Software