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:
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 -jar JAR-name
main method of the class specified in the
Main-Class header is executed.
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:
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: 1.7.0_06 (Oracle Corporation) Main-Class: MyPackage.MyClass
When you run the JAR file with the following command, the
main method of
java -jar MyJar.jar
Setting an Entry Point with the JAR Tool
The 'e' flag (for 'entrypoint') 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
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