Can't execute JAR file: no main manifest attribute

Abdul D.

—December 15, 2024

The ProblemThe Problem

I get a “no main manifest attribute” error when I try to execute a JAR file in Java.

no main manifest attribute, in /Users/User/test/app.jar

The SolutionThe Solution

This error occurs when you try to execute a JAR file without a properly specified Main-Class attribute in its manifest file. The Main-Class attribute tells the Java runtime which class contains the main() method used to start execution.

Specify the Main-Class in the Manifest FileSpecify the Main-Class in the Manifest File

To resolve the error, you need to create a manifest file that indicates which class is the main class of your application.

We’ll demonstrate this process using the example of a simple Java application with the following Main class:

package com.example;

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

The project structure for this example looks as follows:

src
  └── com
      └── example
          └── Main.java

We’ll use the following steps to create an executable JAR file with a manifest file specifying this class.

1. In a terminal, navigate to the src directory and compile the Java code:

   Click to Copy

   Click to Copy

   javac -d bin com/example/Main.java
   

2. In the src directory, create a manifest.txt manifest file with the following content:

   Click to Copy

   Click to Copy

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

   Replace com.example.Main with the fully qualified name of your main class. You can identify the main class as the class containing the public static void main(String[] args) method.

3. Create the JAR file and include the manifest with a command similar to the one below:

   Click to Copy

   Click to Copy

   jar cfm MyApp.jar manifest.txt -C bin/ .
   

   This command uses c to create a new JAR file,f to specify the output JAR file name (MyApp.jar), m to include the manifest file (manifest.txt), and -C bin/ . to add the compiled classes from the bin directory.

4. Test the execution of the JAR file by running the following command:

   Click to Copy

   Click to Copy

   java -jar MyApp.jar
   

   In this example, a correctly defined manifest will output the following message:

   Click to Copy

   Click to Copy

   Hello, World!
   

Alternatively Use an IDE to Generate the JAR FileAlternatively Use an IDE to Generate the JAR File

If you’re using an IDE like IntelliJ IDEA or Eclipse, you can use the built-in export tools to automatically generate a JAR file with the correct manifest. Be sure to specify the main class during the export process.