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Trail: Deployment
Lesson: Packaging Programs in JAR Files
Section: Working with Manifest Files: The Basics
Packaging Programs in JAR Files
Using JAR Files: The Basics
Creating a JAR File
Viewing the Contents of a JAR File
Extracting the Contents of a JAR File
Updating a JAR File
Running JAR-Packaged Software
Working with Manifest Files: The Basics
Understanding the Default Manifest
Modifying a Manifest File
Setting an Application's Entry Point
Adding Classes to the JAR File's Classpath
Setting Package Version Information
Sealing Packages within a JAR File
Signing and Verifying JAR Files
Understanding Signing and Verification
Signing JAR Files
Verifying Signed JAR Files
Using JAR-related APIs
The JarClassLoader Class
The JarRunner Class
Home Page > Deployment > Packaging Programs in JAR Files
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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 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') 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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