JAR file format is based on the popular ZIP file format. Different from ZIP files, JAR files are not only used for compression and publishing, but also for deploying and encapsulating libraries, components and plug-ins, which can be directly used by compilers, JVMs and other tools. JAR contains some special files, such as manifest and deployment descriptor, which are used to instruct tools how to deal with a specific JAR.
JAR files can be used to:
□ Used for publishing and using class libraries.
□ As a building block for applications and extensions
□ As a deployment unit of a component, applet or plug-in.
□ Used for packaging auxiliary resources related to components.
JAR file format provides many advantages and functions, many of which are not available in traditional compression formats such as ZIP or TAR. They include:
Safe. You can digitally sign the contents of the JAR file. In this way, the tool that can recognize the signature can selectively grant you software security rights, which other files can't do, and it can also detect whether the code has been tampered with.
Reduce download time. If an applet is bundled into a JAR file, the browser can download the applet's class files and related resources in an HTTP transaction instead of opening a new connection for each file.
Compression. JAR format allows you to compress files to improve storage efficiency.
Transmission platform extension. Java Extensions Framework provides a way to add functions to Java core platform, and these extensions are packaged with JAR files (Java 3D and JavaMail are examples of extensions developed by Sun).
The package is sealed. Packages stored in JAR files can be sealed to enhance the consistency and security of versions. Sealing a package means that all classes in the package must be found in the same JAR file.
Package version control. A JAR file can contain data about the files it contains, such as vendor and version information.
Portable. The mechanism of processing JAR files is a standard part of the core API of Java platform.