Is there any way to distinguish the main Thread from any Threads that it spawns?

Question

I know that the getName() function on the main thread will return the String main, but this can be changed with setName().

Is

Answer 1

One possibility is to call Thread.currentThread() at the start of main(), and hold on to the reference.

Answer 2

It seems that the main thread has an id of 1 as indicated by Thread.getId():

class test{
    public static boolean isMainThread(){
        return Thread.currentThread().getId() == 1;
    }

    public static void main(String[]args){
        System.out.println(isMainThread());
        new Thread( new Runnable(){
            public void run(){
                System.out.println(isMainThread());
            }
        }).start();
    }
}    

I'm not sure if it is part of the specification or an implementation-specific feature.

A more portable way is this:

class test{

    static long mainThreadId = Thread.currentThread().getId();

    public static boolean isMainThread(){
        return Thread.currentThread().getId() == mainThreadId;
    }

    public static void main(String[]args){
        System.out.println(isMainThread());
        new Thread( new Runnable(){
            public void run(){
                System.out.println(isMainThread());
            }
        }).start();
    }
}    

with the caveat that mainThreadId has to be either in a class that is loaded by the main thread (e.g. the class containing the main method). For instance, this doesn't work:

class AnotherClass{
    static long mainThreadId = Thread.currentThread().getId();

    public static boolean isMainThread(){
        return Thread.currentThread().getId() == mainThreadId;
    }
}
class test{
    public static void main(String[]args){
        //System.out.println(isMainThread());
        new Thread( new Runnable(){
            public void run(){
                System.out.println(AnotherClass.isMainThread());
            }
        }).start();
    }
}    

Answer 3

From your question and your responses to comments I would suggest the following 2 approaches:

1. Place all requests to an event queue and main thread will take the requests from request queue to call the method you are talking about. In this case there must be a contract that any other method wanting to access the method you are talking about can do it only via the event queue (same idea as EDT).

2. Place an extra parameter in the method that you want to be called by main only to act as a token.Inside the method check if the token is correct (only main will have it/know it).If it is correct then proceed.Otherwise return false. Of course if you are allowed to make such a modification