I am building an automation utility for a client's software development pipeline. Every time I try to run the executable JAR file from the command line, it fails. How do I resolve the "no main manifest attribute" error when trying to run a Java JAR file so it executes properly?
3 answers
To fix this error, you need to understand that the Java Virtual Machine requires a specific entry point to run your application. When you see this message, it means the MANIFEST.MF file inside your JAR's META-INF folder is either completely missing or does not specify which class contains the public static void main method. To resolve this, ensure your build tools like Maven or Gradle are explicitly configured to add the Main-Class attribute. For Maven, you should include the maven-jar-plugin configuration block inside your pom file to automatically inject your main class path during compilation.
I tried unpacked the archive and manually adding the class path string to the text file, but now it says the ZIP archive is corrupted when I recompress it. Does your build plugin approach handle the compression headers safely without corrupting the file?
You can also bypass the manifest file entirely at runtime by specifying the class path directly in your terminal command using the standard classpath flag.
That is a solid alternative, Charles. Just keep in mind that running it with the explicit class path means users have to type a much longer command every time. It is great for quick local testing though!
Yes, Jeffrey, using build tools completely avoids file corruption. When you configure the plugin in Maven, it packages the archive natively using proper ZIP compression standards. If you must do it manually, make sure to add a blank line at the end of your manifest file and use the standard jar tool command line flags instead of a basic commercial archiving program to zip it back up.