How to specify the main class in the manifest file for an executable JAR

Introduction

This tutorial will guide you through the process of specifying the main class in the manifest file for an executable JAR in Java. By the end of this article, you will understand how to properly configure the manifest file and create a self-contained Java application that can be easily executed.

Skills Graph

Understanding Executable JAR Files

In the world of Java development, executable JAR (Java Archive) files play a crucial role in packaging and distributing Java applications. An executable JAR file is a special type of JAR file that can be run directly from the command line, without the need to specify the main class manually.

What is an Executable JAR?

An executable JAR file is a JAR file that has been configured with a special manifest file. The manifest file contains metadata about the JAR file, including the main class that should be executed when the JAR file is run.

Advantages of Executable JAR Files

Using executable JAR files offers several advantages:

1. Portability: Executable JAR files can be run on any system with a Java Runtime Environment (JRE) installed, making your application more portable.
2. Simplicity: Users can run your application by simply double-clicking the JAR file or executing it from the command line, without needing to know the specific main class.
3. Distribution: Executable JAR files can be easily distributed and shared with others, as they contain all the necessary components to run the application.

Anatomy of an Executable JAR

An executable JAR file consists of the following key components:

1. JAR File: The JAR file itself, which contains the compiled Java classes, resources, and other supporting files.
2. Manifest File: A special file named MANIFEST.MF that is located in the META-INF directory of the JAR file. This file contains metadata about the JAR file, including the main class to be executed.

By understanding the structure and components of an executable JAR file, you'll be better equipped to configure the manifest file and create your own executable JAR files.

Configuring the Manifest File

The key to creating an executable JAR file is properly configuring the manifest file. The manifest file is a special file located in the META-INF directory of the JAR file, and it contains metadata about the JAR file, including the main class that should be executed.

Manifest File Structure

The manifest file follows a specific format, with each entry separated by a newline and a colon (:) to separate the key and value. Here's an example of a basic manifest file:

Manifest-Version: 1.0
Main-Class: com.labex.example.Main

In this example, the Main-Class attribute specifies the fully qualified name of the main class that should be executed when the JAR file is run.

Configuring the Manifest File

To configure the manifest file, you can use a text editor or a tool like the jar command-line tool. Here's an example of how to create a manifest file using the jar command on an Ubuntu 22.04 system:

## Create a new directory for the project
mkdir my-project
cd my-project

## Compile the Java classes
javac com/labex/example/Main.java

## Create the manifest file
echo "Main-Class: com.labex.example.Main" > MANIFEST.MF

## Create the JAR file with the manifest
jar cvfm my-app.jar MANIFEST.MF com/

In this example, we first create a new directory for the project and compile the Java classes. Then, we create the manifest file with the Main-Class attribute set to the fully qualified name of the main class. Finally, we use the jar command to create the executable JAR file, specifying the manifest file and the compiled Java classes.

By properly configuring the manifest file, you can ensure that your JAR file is executable and can be easily distributed and run by users.

Building and Running the Executable JAR

Now that you've configured the manifest file, you're ready to build and run the executable JAR file. The process involves compiling the Java classes, creating the JAR file, and then executing the JAR file.

Building the Executable JAR

To build the executable JAR file, you can use the jar command-line tool. Here's an example of how to do this on an Ubuntu 22.04 system:

## Compile the Java classes
javac com/labex/example/Main.java

## Create the manifest file
echo "Main-Class: com.labex.example.Main" > MANIFEST.MF

## Create the JAR file with the manifest
jar cvfm my-app.jar MANIFEST.MF com/

In this example, we first compile the Java classes, then create the manifest file with the Main-Class attribute, and finally use the jar command to create the executable JAR file, specifying the manifest file and the compiled Java classes.

Running the Executable JAR

Once you've built the executable JAR file, you can run it using the java command. Here's an example:

java -jar my-app.jar

This command will execute the main class specified in the manifest file, and run your Java application.

You can also double-click the JAR file in a file manager, or use the open command on macOS, to run the executable JAR file. The operating system will automatically recognize the JAR file as an executable and launch the Java application.

By following these steps, you can easily build and run your own executable JAR files, making it easier to distribute and run your Java applications.

Summary

In this Java tutorial, you have learned how to configure the manifest file to specify the main class for an executable JAR. By understanding the structure of the manifest file and the necessary steps to build and run the executable JAR, you can now create self-contained Java applications that can be easily distributed and executed on different systems.