Question

With respect to C/C++ main() must always return an integer(zero to indicate success and non-zero to indicate failure). I can understand this as the program is run it becomes a process and every process should have an exit status, which we obtain by doing echo $? from shell after the process gets over.

Now I don’t understand why is the main method does not return anything in Java? Has it got anything to do with the fact that the program is run on JVM and the JVM process is reposnsible for the returning of exit status?

Please clarify.

Thanks,
Roger

Answer 1

If the main method of a single threaded java application terminates, the application will terminate with exit code 0. If you need another exit code, maybe to indicate an error, you can place

System.exit(yourNumberHere);

anywhere in the code (especially outside of the main method).

This is different for multi-threaded applications , where you either have to use System.exit from the inside of kill -9 from the outside to stop the JVM.

Here’s a quick example where termination of main doesn’t stop the application (a typical service or daemon behaviour):

public static void main(String args[]) {  
  Thread iWillSurvive = new Thread(new Runnable() {
    public void run() {
      while(true) {
        // heat the CPU
      }
    }
  });
  iWillSurvive.start();
}

Remark: Sure, a thread will terminate when it’s run method (or the main method in case of the main thread) terminates. And in this case, when all threads have terminated, the JVM will terminate with exit code 0 (which brings us back to the initial question). Hope everybody is happy now.