In software, JAR (for Java Archive) is an archive file format typically used to aggregate many Java class files and associated metadata and resources (text, images and so on) into one file to distribute application software or libraries on the Java platform.
JAR files are built on the ZIP file format and have the .jar file extension. Computer users can create or extract JAR files using the jar command that comes with a JDK. They can also use zip tools to do so; however, the order of entries in the zip file headers is important when compressing, as the manifest often needs to be first.
A JAR file allows Java runtimes to efficiently deploy a set of classes and their associated resources. The elements in a JAR file can be compressed, which, together with the ability to download an entire application in a single request, makes downloading a JAR file much faster than separately downloading the many uncompressed files which would form a single Java Application. The package java.util.zip contains classes that read and write JAR files.
The Apache Ant build tool has its own package to read and write Zip and JAR archives, including support for the Unix filesystem extensions. The org.apache.tools.zip package is released under the Apache Software Foundation license and is designed to be usable outside Ant.