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Setting an Application's Entry Point

If you have an application bundled in a JAR file, you need some way to indicate which class within the JAR file is your application's entry point. You provide this information with the Main-Class header in the manifest, which has the general form:

Main-Class: classname

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

Recall that the entry point is a class having a method with signature public static void main(String[] args).

After you have set the Main-Class header in the manifest, you then run the JAR file using the following form of the java command:

java -jar JAR-name

The main method of the class specified in the Main-Class header is executed.

An Example

We want to execute the main method in the class MyClass in the package MyPackage when we run the JAR file.

We first create a text file named Manifest.txt with the following contents:

Main-Class: MyPackage.MyClass

---

Warning: The text file must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.

---

We then create a JAR file named MyJar.jar by entering the following command:

jar cmf MyJar.jar Manifest.txt MyPackage/*.class

This creates the JAR file with a manifest with the following contents:

Manifest-Version: 1.0
Created-By: 1.5.0_01 (Sun Microsystems Inc.)
Main-Class: MyPackage.MyClass

When you run the JAR file with the following command, the main method of MyClass executes:

java -jar MyJar.jar

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