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Java archive(file format, filename extension)
(jar) A compressed archive file containing Java class files, filename extension: ".jar". The Java Development Kit contains a tool called "jar" for creating .jar files, similar to the standard Unix tar command. As well as archiving and compressing the Java class files, it also inserts a "manifest" file which can contain information about the class files, such as a digital signature. Combining class files into a single archive file makes it possible to download them in a single HTTP transaction. This, and the compression, speeds up execution of Java programs delivered via the Internet.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)