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 JAR File Specification.