Java™ Application Development on Linux - Dator

160Chapter 5The Sun Microsystems Java Software <strong>Development</strong> Kit45 invokevirtual #26 48 iinc 1 151 iload_152 aload_053 arraylength54 if_icmplt 557 returnA JAR file packages a subdirectory and its descendants into a single file. A JavaCLASSPATH specification may contain a JAR filename everywhere it mightcontain a directory name. Let’s say you use the GPL’ed Java personal financeprogram called jgnash and you’ve compiled it from source, so you have a directoryoff your home directory called jgnash/bin. Suppose you run the programby directly invoking java to run the class jgnashMain and you have$HOME/jgnash/bin on your CLASSPATH. You could clean up the mess on yourhard drive by using the jar command to squash all the files in jgnash/bintogether into a single JAR file, as shown in Example 5.20.Example 5.20 Making a JAR file$ cd ; mkdir jars$ jar cvf jars/jgnash.jar jgnash/binYou could then replace the $HOME/jgnash/bin entry in your CLASSPATHwith $HOME/jars/jgnash.jar. After that you would still run jgnash withexactly the same java command you always did, but now you got rid of thecluttered pile of files.This is only the most basic purpose of jar, however. Its uses extend wellbeyond merely concatenating and compressing collections of .class files.5.11.1 Deploying <strong>Application</strong>sOne of the best uses of jar is to package applications for distribution. You canput a large Java application into a single file with jar, and by using a manifest(which we are about to discuss) you can nominate the main class to run in thatJAR file. You can then provide a shell script (and a batch file, if you are alsodeploying to Microsoft Windows) that will set the CLASSPATH to point to the