Running JAR-Packaged Software

Now that you have learned how to create JAR files, how do you actually run the code you packaged? Consider these scenarios:

  • Your JAR file contains an applet that is to be run inside a browser.
  • Your JAR file contains an application that is to be started from the command line.
  • Your JAR file contains code that you want to use as an extension.

This section will cover the first two situations. A separate trail in the tutorial on the extension mechanism covers the use of JAR files as extensions.

Applets Packaged in JAR Files

To start any applet from an HTML file for running inside a browser, you use the applet tag. For more information, see the Java Applets lesson. If the applet is bundled as a JAR file, the only thing you need to do differently is to use the archive parameter to specify the relative path to the JAR file.

As an example, use the TicTacToe demo applet. The applet tag in the HTML file that displays the applet can be marked up like this:

<applet code=TicTacToe.class 
        width="120" height="120">

If the TicTacToe demo was packaged in a JAR file named TicTacToe.jar, you can modify the applet tag with the addition of an archive parameter:

<applet code=TicTacToe.class 
        width="120" height="120">

The archive parameter specifies the relative path to the JAR file that contains TicTacToe.class. For this example it is assumed that the JAR file and the HTML file are in the same directory. If they are not, you must include the JAR file's relative path in the archive parameter's value. For example, if the JAR file was one directory below the HTML file in a directory called applets, the applet tag would look like this:

<applet code=TicTacToe.class 
        width="120" height="120">

JAR Files as Applications

You can run JAR packaged applications with the Java launcher (java command). The basic command is:

java -jar jar-file

The -jar flag tells the launcher that the application is packaged in the JAR file format. You can only specify one JAR file, which must contain all of the application-specific code.

Before you execute this command, make sure that the runtime environment has information about which class within the JAR file is the application's entry point.

To indicate which class is the application's entry point, you must add a Main-Class header to the JAR file's manifest. The header takes the form:

Main-Class: classname

The header's value, classname, is the name of the class that is the application's entry point.

For more information, see the Setting an Application's Entry Point section.

When the Main-Class is set in the manifest file, you can run the application from the command line:

java -jar app.jar

To run the application from the JAR file that is in another directory, you must specify the path of that directory: java -jar path/app.jar