May 31, 2008 at 7:37 PM
JAR file is a file format based on the popular ZIP file format and is used for aggregating many files into one. A JAR file is essentially a zip file that contains an optional META-INF directory. A JAR file can be created by the command-line jar tool, or by using the java.util.jar API in the Java platform. There is no restriction on the name of a JAR file, it can be any legal file name on a particular platform.
In many cases, JAR files are not just simple archives of java classes files and/or resources. They are used as building blocks for applications and extensions. The META-INF directory, if it exists, is used to store package and extension configuration data, including security, versioning, extension and services.