File Extension JAR
File extension JAR is most commonly associated with a Java Archive, which is often used to package and distribute Java classes and metadata in development environments. JAR files may also be found to contain themes and add-ons for Mozilla's FireFox web browser or Thunderbird E-Mail client.
JAR files are standard Zip compressed archives which contain the classes and associated resources of a program as well as a manifest file which determines how the JAR file is handled. A manifest which defines a contained class as the main class will be treated as a standalone application, for example. The manifest can also contain links to other JAR files to be loaded upon execution, though this does rely on the paths to additional resources remaining the same as when the package was created. JAR files also support digital signing, whereby signature information is added to the manifest file and read when the file is opened. This prevents the contents being altered, as changing any of the enclosed files will result in a non-matching digital signature. It is also possible to digitally sign individual CLASS files, with any that do not have a valid signature being rejected by the Classloader of the Java Virtual Machine, preventing malicious code being injected into existing JAR packages.
JAR files are executed if the host machine has the Java Runtime Environment (JRE) installed, and should not be opened if the source is unknown as it is possible that the contained program is malicious.
JAR files can be opened by opening the file on a machine with the Java Runtime Environment installed, which is available on the Windows, Solaris, Mac and Linux platforms. JAR files may also be opened in web browsers with Java support. Alternatively, JAR packages can be opened using any archiving software with unzipping capabilities and the contents extracted, though most resources are already compiled and therefore not human-readable.