The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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.
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
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 MyClass
executes:
java -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.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