Thread synchronisation - Using a string literal for locking

[O. Ziggy]

   
class RunnableThread implements Runnable{
	
	String str = "HELLO";
	
	public void run(){
		runMe();
	}
	
	public void runMe(){
		
		synchronized(str){
			for (int a : new int[]{1,2,3,4,5}){
				System.out.println(Thread.currentThread().getName());
			}
		}	
	}
}

 class ZiggyTest{
    
    	public static void main(String[] args){
    	
    		RunnableThread rt = new RunnableThread();
    		
    		Thread t1 = new Thread(rt);
    		Thread t2 = new Thread(rt);
    		Thread t3 = new Thread(rt);
    		
    		t1.start();
    		t2.start();
    		t3.setPriority(10);
    		t3.start();
    		
    		try{
    			t3.join();
    			t2.join();
    			t1.join();
    			
    			System.out.println("Joined");
    		}catch(InterruptedException e){System.out.println(e);}
    		
    		
    		Thread t4 = new Thread(new RunnableThread());
    		Thread t5 = new Thread(new RunnableThread());
    		Thread t6 = new Thread(new RunnableThread());
    		
    		t4.start();
    		t5.start();
    		t6.start();
    	}
    		
    }

The above test produces the following output

    Thread-0
    Thread-0
    Thread-0
    Thread-0
    Thread-0
    Thread-2
    Thread-2
    Thread-2
    Thread-2
    Thread-2
    Thread-1
    Thread-1
    Thread-1
    Thread-1
    Thread-1
    Joined
    Thread-3
    Thread-3
    Thread-4
    Thread-4
    Thread-3
    Thread-4
    Thread-5
    Thread-4
    Thread-3
    Thread-4
    Thread-5
    Thread-3
    Thread-5
    Thread-5
    Thread-5

I don't understand why the last three thread are not using the String object as a shared lock like the first three threads. Even though they last three threads are using a different instance of 'RunnableThread', shouldnt they be synchronised because there is only once copy for 'str' in the string constant pool?

Thanks

[O. Ziggy]

Ill answer my own question:
The code is not actually synchronising on the str object..