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

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();
    }
}