Learning Java

Java Archive (JAR) Files

Java archive files (JAR files) are Java’s suitcases. They are the standard and portable way to pack up all of the parts of your Java application into a compact bundle for distribution or installation. You can put whatever you want into a JAR file: Java class files, serialized objects, data files, images, sounds, etc. As we’ll see in Chapter 20, a JAR file can carry one or more digital signatures that attest to the integrity and authenticity of that data. A signature can be attached to the file as a whole or to individual items in the file.

The Java runtime system understands JAR files and can load class files directly from an archive. So you can pack your application’s classes in a JAR file and place it in your CLASSPATH. You can do the equivalent for applets by listing the JAR file in the ARCHIVE attribute of the HTML <APPLET> tag. Other types of files (data, images, etc.) contained in your JAR file can be retrieved using the getResource( ) method. (described in Chapter 10). Therefore, your code doesn’t have to know whether any resource is a plain file or a member of a JAR archive. Whether a given class or data file is an item in a JAR file, is an individual file on the class path, or is located on a remote applet server, you can always refer to it in a standard way, and let Java’s class loader resolve the location.

File Compression

Items stored in JAR files may be compressed with ZLIB^[10] compression. JAR files are completely compatible with the ZIP ...