Question

Refer below code

     public void acquire(){
        synchronized(a){
            print("acquire()");
            try{
                //Thread.sleep(5000);
                synchronized(this){
                    wait(5000);
                }
                print("I have awoken");
                print("" + a);
            }catch(Exception e){
                e.printStackTrace();
            }
        }
        print("Leaving acquire()");
    }


    public void modify(int n){
        print("Entered in modfy");
        synchronized(a){
            try{
                //Thread.sleep(5000);
                synchronized(this){
                    wait(5000);
                }
                this.a=n;
                print("new value" + a);
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }

And

        final SynchoTest ttObj = new SynchoTest();
        Thread A = new Thread(new Runnable(){
            public void run() {
                ttObj.acquire();    
            }
        },"A");

        Thread B = new Thread(new Runnable(){
            public void run() {
                ttObj.modify(97);   
            }
        },"B");
        A.start();
        B.start();

As i know about wait(n), it pauses a thread until notify()/notifyAll() get called or the specified time n is over.

But…

• In above methods if I directly use wait(n) as I used Thread.sleep(n),
  I get runtime exception.
• If I synchronize both methods instead of surrounding wait(n) with
  synchronized block then I am not getting any exception but both
  threads get blocked forever.
• But if I do like I attached ie wait(n) surrounding with synchronized
  block, it is working fine.

Please tell me why? Also tell me why is it not behaving different on positioning synchronized block if I use sleep(n) instead of wait(n)?

My question is about various result of wait(n) on various position of synchronized keyword.

@Gray

notify() or notifyAll(), and wait() must be in a synchronized block
for the object you are waiting on

explained me why I was getting run time exception by positioning synchronized block on various position.

Now please explain me, why

public void method(){
    synchronized(a){
        synchronized(this){
            wait(n);
        }
    }
}

is working fine. But

public synchronized void method(){
    synchronized(a){
        wait(n);
    }
}

is blocking my thread forever.

Answer 1

It is nothing about positioning synchronized keyword. You are facing problem since you locking other object and try to wait for another. Well @Gray has already been explained it, so not repeating it.

For your another problem, regarding why both threads are getting blocked;

Thread A: locks this [A: Runnable]

Thread A: locks a [A: Runnable]

Thread B: waiting for this [A: Runnable, B:BLOCKED]

Thread A: release this (meets wait) [A: TIMED WAITING, B:BLOCKED]

Thread B: lock this [A: TIMED WAITING, B: Runnable]

Thread B: waiting for a which is already locked by thread A [A: TIMED WAITING, B:BLOCKED]

Thread A: waiting for this which is locked by thread B [A: BLOCKED, B:BLOCKED]