Why do I get "no main manifest attribute" when I double-click a JAR?

Windows runs the JAR with javaw -jar, which needs Main-Class just like java -jar. If the JAR is a mod or a library, there is nothing to run. Install or use it instead.

Can I fix it without rebuilding?

Yes, if you know the main class: jar ufe app.jar com.example.App adds it in place, or java -cp app.jar com.example.App runs it without touching the file.

Is a JAR with no Main-Class broken or malicious?

No. Most JARs are libraries and have no Main-Class. Missing Main-Class says nothing about safety. If you didn't expect the file, scan it. ---

Fix "no main manifest attribute, in app.jar"

Fix the Java error "no main manifest attribute": check MANIFEST.MF online, run the right class, or add Main-Class with Maven, Gradle, Spring Boot or jar.

Fix "no main manifest attribute, in app.jar"

Step 1: Look at the Manifest (No Java Needed)

Step 2: It's a Minecraft Mod — Don't Run It

Step 3: Run the Main Class Directly

Step 4: Add Main-Class When You Build

Step 5: Spring Boot — Run the Repackaged JAR

Check the Fix

FAQ

Summary