A Java™ ARchive (JAR) file is a file format that combines many files into one. The Java environment differs from other programming environments in that the Java compiler does not generate machine code for a hardware-specific instruction set. Instead, the Java compiler converts Java source code into Java virtual machine instructions, which Java class files store. You can use JAR files to store class files. The class file does not target a specific hardware platform, but instead targets the Java virtual machine architecture.
You can use JAR as a general archiving tool and also to distribute Java programs of all types, including applets. Java applets download into a browser in a single Hypertext Transfer Protocol (HTTP) transaction rather than by opening a new connection for each piece. This method of downloading improves the speed at which an applet loads on a Web page and begins functioning.
JAR is the only archive format that is cross-platform. JAR is also the only format that handles audio files and image files, as well as class files. JAR is an open standard, fully extendable format that is written in Java.
The JAR format also supports compression, which reduces the size of the file and decreases download time. Additionally, an applet author may digitally sign individual entries in a JAR file to authenticate their origin.
To update classes in JAR files, see the Java jar tool.
Java class files are stream files that are produced when a source file is compiled by the Java compiler. The class file contains tables that describe each field and method of the class. The file also contains the bytecodes for each method, static data, and descriptions that are used to represent Java objects.