Setting an Application’s Entry Point

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 cfm MyJar.jar Manifest.txt MyPackage/*.class
This creates the JAR file with a manifest with the following contents:
Manifest-Version: 1.0
Created-By: 1.6.0 (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

Setting an Entry Point with the JAR Tool


The ‘e’ flag (for ‘entrypoint’), introduced in JDK 6, creates or overrides the manifest’s Main-Class attribute. It can be used while creating or updating a jar file. Use it to specify the application entry point without editing or creating the manifest file.
For example, this command creates app.jar where the Main-Class attribute value in the manifest is set to MyApp:
jar cfe app.jar MyApp MyApp.class

You can directly invoke this application by running the following command:


java -jar app.jar

If the entrypoint class name is in a package it may use a ‘.’ (dot) character as the delimiter. For example, if Main.class is in a package called foo the entry point can be specified in the following ways:


jar cfe Main.jar foo.Main foo/Main.class

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