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:The value classname is the name of the class that is your application's entry point.Main-Class: classname
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:The main method of the class specified in the Main-Class header is executed.java -jar JAR-name
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.MyClassWe then create a JAR file named MyJar.jar by entering the following command:
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.This creates the JAR file with a manifest with the following contents:jar cfm MyJar.jar Manifest.txt MyPackage/*.classWhen you run the JAR file with the following command, the main method of MyClass executes:Manifest-Version: 1.0 Created-By: 1.6.0 (Sun Microsystems Inc.) Main-Class: MyPackage.MyClassjava -jar MyJar.jar
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.jarwhere 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.classis in a package called
foothe entry point can be specified in the following ways:jar -cfe Main.jar foo.Main foo/Main.class