Notify method notifying all the waiting threads

[G Batra]

Hi Everyone,
My first post on coderanch.

The problem which I am facing is : When notify() gets called in the below code, all the threads which are waiting gets notified and all starts execution. I am sure this could be a very basic problem but I just couldn't get it. Tried searching on the web, but found no answers unfortunately. Please help me.

public class ITC3 extends Thread {
	private ITC4 it;
	
	public ITC3(ITC4 it){
		this.it = it;
	}
	public static void main(String[] args) {
		ITC4 itr = new ITC4();
		ITC3 i = new ITC3(itr);
		ITC3 ii = new ITC3(itr);
				
		i.start(); ii.start();
		//iii.start();
		try{
			Thread.sleep(1000);
		}catch(InterruptedException ie){}
		itr.start();
	}

	public void run(){
		synchronized (it){
			try{
				
				System.out.println("Waiting - " + Thread.currentThread().getName());
				it.wait();
				System.out.println("Notified " + Thread.currentThread().getName());
			}catch (InterruptedException ie){}
		}
	}
}

class ITC4 extends Thread{
	public void run(){
		try{
			System.out.println("Sleeping : " + this);
			Thread.sleep(3000);
			synchronized (this){
				this.notify();
			}
		}catch(InterruptedException ie){}
	}
}

Output:
Waiting - Thread-1
Waiting - Thread-2
Sleeping : Thread[Thread-0,5,main]
Notified Thread-1
Notified Thread-2

[Ben Pheonix]

Dear Batra,

Welcome to Javaranch.

You are synchronizing on instance of thread. If we check the JoinMethodDocumentation, you will see that Thread instance
gets notified when run() method completes. Check the documentation of the join() method.So basically, you execute one explicit notify call
(the one we see in the code), and another implicit notifyAll when the thread is done. We shouldn't synchronize on the Thread object.
Thread toString() methods returns names of all threads in the ThreadGroup, not only name of current Thread. You could even remove your explicit notify
and the behavior wouldn't change due to the implicit notifyAll at the end. Check the modified code below.

package chapter9;

/**
 * Created with IntelliJ IDEA.
 * User: Ben
 * Date: 8/9/13
 * Time: 2:52 PM
 * To change this template use File | Settings | File Templates.
 */
public class ITC3 extends Thread {
  private ITC4 it;

  public ITC3(ITC4 it){
    this.it = it;
  }
  public static void main(String[] args) {
    ITC4 itr = new ITC4();
    System.out.println("name is:" + itr.getName());
    ITC3 i = new ITC3(itr);
    ITC3 ii = new ITC3(itr);


    i.start(); ii.start();
    //iii.start();
    try{
      Thread.sleep(1000);
    }catch(InterruptedException ie){}
    itr.start();
  }

  public void run(){
    synchronized (it){
      try{

        System.out.println("Waiting - " + Thread.currentThread().getName());
        it.wait();
        System.out.println("Notified " + Thread.currentThread().getName());
      }catch (InterruptedException ie){}
    }
  }
}

class ITC4 extends Thread{
  public void run(){
    try{
      System.out.println("Sleeping : " + this);
      Thread.sleep(1000);
      synchronized (this){
   //     this.notify();
      }
    }catch(InterruptedException ie){}
  }

  public String toString(){
    return Thread.currentThread().getName();
  }
}

HTH,
Ben

[G Batra]

Thanks Ben. Actually, the java 6 documentation is not very clear as java 7 documentation. It seems that there's no change in the join implementation as I removed the notify() call, the implicit notifyall() gets called.
http://docs.oracle.com/javase/6/docs/api/java/lang/Thread.html#join%28%29

Also, I found an old post on this very same issue.
https://coderanch.com/t/461261/java-programmer-SCJP/certification/notify-notifyAll-code

Thanks again Ben.