Parent Thread gets the same object lock has child threads? Confused!!

[V Pinto]

Hi guys,

I have the following code:

package pt.ptin.threads;

import java.util.HashSet;

public class Main {

	/**
	 * @param args
	 */
	public static void main(String[] args) {

		Classe1 c1 = new Classe1();
		System.out.println("Thread id: " + Thread.currentThread().getId() + " running main(), name " + Thread.currentThread().getName());
		t t1 = new t(c1,"b",  Thread.currentThread());
		System.out.println("First Thread id: " + t1.getId());
		t1.start();

		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		t2 t_2 = new t2(c1,"c", Thread.currentThread());
		System.out.println("Second Thread id: " + t_2.getId());
		t_2.start();


	}
}
class t extends Thread{

	Classe1 c1;
	String s;
	Thread t;


	public t(Classe1 c1, String s, Thread t) {
		super();
		this.c1 = c1;
		this.s=s;
		this.t = t;

	}


	public void run (){
		c1.add(s, c1, t);

	}
}

class t2 extends Thread{

	Classe1 c1;
	String s;
	Thread m;


	public t2(Classe1 c1, String s, Thread m) {
		super();
		this.c1 = c1;
		this.s=s;
		this.m=m;

	}


	public void run (){
		c1.add(s, c1, m);

	}
}

class Classe1{

	HashSet<String> hash = new HashSet<String>();

	void  add(String b, Classe1 c1, Thread mainT){

		synchronized (hash) {

			System.out.println("Inside add() Thread Main " + mainT.getId() + " has lock of c1? " + mainT.holdsLock(c1));
			System.out.println("Inside add() Thread Main " + mainT.getId() + " has lock of hash? " + mainT.holdsLock(hash));
			System.out.println("Inside add() Thread id " + Thread.currentThread().getId() + " has lock of hash? " + Thread.currentThread().holdsLock(hash));
			System.out.println("Inside add() Thread id " + Thread.currentThread().getId() + " has lock of c1? " + Thread.currentThread().holdsLock(c1));
			try {
				hash.add(b);
				add2(c1); //Calling thread has lock over c1 so can call any sync method on c1
				Thread.sleep(5000);
				System.out.println("b on hash? " + hash.contains("b"));
				System.out.println("c on hash ? " + hash.contains("c"));
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}

		}



	}
	synchronized void add2 (Classe1 c1){
		System.out.println("Inside add2() Thread id " + Thread.currentThread().getId() + " has lock of hash? " + Thread.currentThread().holdsLock(hash));
		System.out.println("Inside add2() Thread id " + Thread.currentThread().getId() + " has lock of c1? " + Thread.currentThread().holdsLock(c1));

	}
}

When I run this i get the following output

Thread id: 1 running main(), name main
First Thread id: 8
Inside add() Thread Main 1 has lock of c1? false
Inside add() Thread Main 1 has lock of hash? true
Inside add() Thread id 8 has lock of hash? true
Inside add() Thread id 8 has lock of c1? false
Inside add2() Thread id 8 has lock of hash? true
Inside add2() Thread id 8 has lock of c1? true
Second Thread id: 9
b on hash? true
c on hash ? false
Inside add() Thread Main 1 has lock of c1? false
Inside add() Thread Main 1 has lock of hash? true
Inside add() Thread id 9 has lock of hash? true
Inside add() Thread id 9 has lock of c1? false
Inside add2() Thread id 9 has lock of hash? true
Inside add2() Thread id 9 has lock of c1? true
b on hash? true
c on hash ? true

On lines 4 and 5 of the output the jvm is telling me that both the main thread and Thread 8 have the lock over the hash hashtable object?
How can two different threads have the lock on a same syncronized object??? I'm lost here.... Is it because thread 8 (and 9) were started by the Main thread (are child threads of main thread)?

Regards

[Alexander Kober]

The method Thread.holdsLock(Object) is static. That means it is a method associated with the class Thread that returns whether the current Thread is holding the lock. Unfortunately, java allows accessing static members via concrete instances (this is, however, discouraged by most IDEs and best practice guides). 'mainT.holdsLock(hash)' effectively resolves to 'Thread.holdsLock(hash)' - admittedly this is rather ugly and intuitive.

[V Pinto]

I totally missed that holdsLock() is a static method, so basically holdLock always refers to current Thread executing the add method and not to the main Thread as I was aiming for.
Thanks for the reply!