How to determine main class at runtime in threaded java application?

Question

I want to determine the class name where my application started, the one with the main() method, at runtime, but I'm in another thread and my stacktrace doesn't go all the way back to the original class.

I've searched System properties and everything that ClassLoader has to offer and come up with nothing. Is this information just not available?

Thanks.

Answer 1

Try using Thread.getAllStackTraces(). It returns a Map of the stack traces from all running threads, not just the current one.

Answer 2

How about something like:

Map<Thread,StackTraceElement[]> stackTraceMap = Thread.getAllStackTraces();
for (Thread t : stackTraceMap.keySet())
{
 if ("main".equals(t.getName()))
 {
  StackTraceElement[] mainStackTrace = stackTraceMap.get(t);
  for (StackTraceElement element : mainStackTrace)
  {
   System.out.println(element);
  }
 }
}

This will give you something like

java.lang.Object.wait(Native Method)
java.lang.Object.wait(Object.java:231)
java.lang.Thread.join(Thread.java:680)
com.mypackage.Runner.main(Runner.java:10)

The main thread is probably not guarenteed to be called "main" though - might be better to check for a stack trace element that contains (main

Edit if the main thread has exited, this is no good!

Answer 3

I figured it out. Can anyone tell me if this environment variable will always be around in other java implementations across operating systems?

public static String getMainClassName()
{
  for(final Map.Entry<String, String> entry : System.getenv().entrySet())
  {
    if(entry.getKey().startsWith("JAVA_MAIN_CLASS"))
      return entry.getValue();
  }
  throw new IllegalStateException("Cannot determine main class.")
}

Answer 4

Given the clarification, I suggest using "Parameterisation from Above". You have the information to start with, keep hold of it.

I did put in an RFE 4827318 (six years ago!) for something like this for use with test runners.

Answer 5

I suggest to put this information into a system property. This is usually simple to do when you start your application from a script.

If you can't do that, I suggest to set the property in the main() method of every application. The most simple way here would be to have every app derive it's "main class" from a common base class and run an init step in there. I often do this for command line processing:

public class Demo extends Main {
    main(String[] args) {
        Main._main(new Demo (), args);
    }

    // This gets called by Main._main()
    public void run (String[] args) {
    }
}