Now that you've learned how to create JAR files, how do you actually run the code that you've packaged? Consider these three 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 invoked 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 invoke any applet from an HTML file for running inside a browser, you need to use the APPLET tag. For more information, see the 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, let's use (again!) the TicTacToe demo applet that ships with the Java^TM Development Kit. The APPLET tag in the HTML file that calls the demo looks like this:

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

If the TicTacToe demo were packaged in a JAR file named TicTacToe.jar, you could modify the APPLET tag with the simple addition of an ARCHIVE parameter:

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

The ARCHIVE parameter specifies the relative path to the JAR file that contains TicTacToe.class. This example assumes that the JAR file and the HTML file are in the same directory. If they're not, you would need to 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 
        archive="applets/TicTacToe.jar"
        width=120 height=120>
</applet>

JAR Files as Applications

You can run JAR-packaged applications with the Java interpreter. The basic command is:

java -jar jar-file

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

Before you execute this command make sure the runtime environment has an information of 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's 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