Question

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

Is there any way to always determine the main thread of an application?

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();
    }
}