Resolve “no main manifest attribute” Error in Executable JAR Files

The infamous “no main manifest attribute” error halts Java apps when the JAR lacks a defined entry point. Here’s how to fix it—whether you’re the developer or user—and ensure your JARs run seamlessly.

Why Does This Error Occur?

Java requires an executable JAR to specify the Main-Class in its MANIFEST.MF file. Without it, the JVM can’t find the starting point (main method). Common scenarios:

• The JAR wasn’t built as executable.
• The MANIFEST.MF is missing or misconfigured.
• You’re running a non-executable JAR with java -jar.

Fix 1: Specify the Main Class When Running

If you can’t modify the JAR, bypass the manifest by directly invoking the main class:

java -cp app.jar com.company.MainClass  

Replace com.company.MainClass with the actual class containing public static void main.

How to Find the Main Class:

1. Check the app’s documentation.
2. Extract the JAR and search for main methods: jar tf app.jar | grep .class # List classes javap -cp app.jar com.company.MainClass | grep "public static void main"

Fix 2: Add the Manifest Yourself

If you have the source code, rebuild the JAR with a valid manifest.

Using Maven:

<plugin>  
  <groupId>org.apache.maven.plugins</groupId>  
  <artifactId>maven-jar-plugin</artifactId>  
  <version>3.3.0</version>  
  <configuration>  
    <archive>  
      <manifest>  
        <mainClass>com.company.MainClass</mainClass>  
      </manifest>  
    </archive>  
  </configuration>  
</plugin>  

Run mvn clean package.

Using Gradle:

plugins { id 'application' }  
application { mainClass = 'com.company.MainClass' }  

Run gradle build.

Via Command Line:

1. Create MANIFEST.MF: Main-Class: com.company.MainClass
2. Build the JAR: jar cfm app.jar MANIFEST.MF -C target/classes .

Fix 3: Extract and Rebuild the JAR

For third-party JARs with missing manifests:

1. Extract the JAR: mkdir temp && cd temp jar xf ../app.jar
2. Create/edit META-INF/MANIFEST.MF: Main-Class: com.company.MainClass
3. Repackage: jar cmvf META-INF/MANIFEST.MF ../new-app.jar .

Prevent the Error: Best Practices

• Use Build Tools: Maven/Gradle automate manifest generation.
• Test Your JAR: Always validate with java -jar before distribution.
• IDE Setup: In Eclipse/IntelliJ, ensure “export as runnable JAR” is selected.

Troubleshooting Tips

• Invalid Main Class: Double-check package + class name casing.
• Corrupted JAR: Rebuild from source.
• Dependency Issues: Use maven-assembly-plugin for uber-JARs.

FAQs

Q: Can I add a manifest without rebuilding?
Yes—extract the JAR, add the manifest, and repackage (as shown in Fix 3).

Q: Why does it work on other machines?
They might run it via a script invoking the main class directly, not using java -jar.

Q: How to view a JAR’s manifest?

unzip -p app.jar META-INF/MANIFEST.MF  

Final Thoughts

The “no main manifest attribute” error is a rite of passage for Java devs. 🔑 Master your build tools, validate manifests, and never let a missing Main-Class block your JAR again!

(Keywords: no main manifest attribute, executable JAR error, Main-Class manifest, Java JAR troubleshooting, fix JAR manifest, Maven Gradle JAR setup)

Build right, run smooth—happy coding! 🚀