Little donuts in a glass jar

Yes this is in fact a jar (a delicious one at that), but it’s not the type of jar we’re going to be talking about. The JAR’s we’re looking for are in the programming language Java. JAR stands for J-ava AR-chive. and let you bundle up all of your application classes into a neat and compact file.

When do you need a JAR?

JAR files are based on the pkzip format, and can be used for compression, archiving, and unpacking. Most developers use JAR’s during the final stages of a project to consolidate their project files into a single executable that the client can run.

How is a JAR created?

In order for a JAR file to work properly you must create a file called “manifest.txt”. This manifest file tells the JVM which compressed and packaged file inside the JAR is your “entry point” and contains the main() method.

// inside manifest.txt
Main-Class: App  // <---- this is the name of the class where main() lives

Building a JAR works with the command line jar tool.

You can run jar -cvmf manifest.txt myJar.jar App.class from the command line, and the tool will grab your manifest file, add it to the JAR named “myJar.jar” along with the classes that are necessary to run your application.

 _________          _________          ____________
|         |        |         |        |            |
| classes | -----> |  myJar  | <----- |  manifest  |
|_________|        |_________|        |____________|

How can you run a JAR?

Once a JAR has been created you’ll probably want to tell your client / friend / anyone how to execute it, and get your application running.

If the person running the application happens to be in the classes directory, they can simply run java -jar myJar.jar otherwise you’ll have to give them the full path to the jars location (within the classes directory). This command tells the JVM that you’re giving it a “jar” file, and that it should look inside the manifest file for the main() method.

The JVM is smart enough to understand how to read JAR’s and will take care of the execution process.

Summary

JAR files are one of Java’s great tools to streamline the deployment process. Most local apps (non-web) are deployed as executable JAR’s so that developers don’t have to manually handoff every class required to run their app.