two threads entering the two different synchrnozied method at same time on instance level lock

Question

I was ask in an interview that with respect to multi threading , Suppose you have 2 threads (Thread-1 and Thread-2) on same object. Thread-1 is in synchronized method1(), can Thread-2 enter synchronized method2() at same time in java by any way .

I replied no No, here when Thread-1 is in synchronized method1() it must be holding lock on object’s monitor and will release lock on object’s monitor only when it exits synchronized method1(). So, Thread-2 will have to wait for Thread-1 to release lock on object’s monitor so that it could enter synchronized method2().

but still please advise is there any way by which Thread-2 ,enter synchronized method2() at same time in java by any way is there any hack if to achieve this thing

below is my program , rite now I have changed the implementation now please advise on this as the output of the below program is

inside M1()
t1--->RUNNABLE
inside M2()
t2--->RUNNABLE

below is my updated code

public class Test {

     private final Object lockA = new Object();
     private final Object lockB = new Object();

    public  void m1() {
         synchronized(lockA) {
        try {
            System.out.println("inside M1()");
            Thread.sleep(100);         
        }
        catch (InterruptedException ie) 
        {}
         }
    }

    public  void m2() {
        synchronized(lockB) {
        try { 
            System.out.println("inside M2()");
            Thread.sleep(100); }
        catch (InterruptedException ie) {}
    }
    }

    public static void main(String[] args) throws InterruptedException {
        final Test t = new Test();
        Thread t1 = new Thread()
        { public void run() { t.m1(); } };
        Thread t2 = new Thread()
        { public void run() { t.m2(); } };

        t1.start();
        //Thread.sleep(500);

        t2.start();
      //  Thread.sleep(500);

        System.out.println("t1--->"+t1.getState());
        System.out.println("t2--->"+t2.getState());
    }
}

duplicate with [Do two synchronized methods execute simultaneously](http://stackoverflow.com/questions/3160023/do-two-synchronized-methods-execute-simultaneously) — nail fei, Dec 18 '16 at 05:47
There is still no evidence here that both threads have acquired the same lock at the same time. What you are suggesting is basically inconceivable. — user207421, Dec 18 '16 at 08:49
@EJP , Thnaks but please advise how you can simply prove that which lock is acquired is there any way to see the lock name please advise — tuntun82828282 hdjscd, Dec 18 '16 at 09:06
@Ejp Please check tis solution too http://stackoverflow.com/a/15438792/5254989 — tuntun82828282 hdjscd, Dec 18 '16 at 12:24

score 1 · Answer 1 · answered Dec 18 '16 at 06:23

1

Notice that in your example threads are not competing on any thing even if MyRunnable1 class had a single method1() and your run() function was calling on this method in loop. Because thread1 and thread2 are different instances and each one has it's own copy of method1() so the locks used by synchronized are different and not one single lock.

answered Dec 18 '16 at 06:23

urag

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I disagree with you because he uses single runnable, which uses same lock. The method is also shared, no other copy. – Tony Dec 18 '16 at 07:21
You are correct i did not notice that sorry my mistake – urag Dec 18 '16 at 07:40

score 0 · Answer 2 · answered Dec 18 '16 at 06:31

0

Two threads cannot hold the same lock at the same time. However, just because a thread is inside a synchronized method for an object, it does not follow that it holds the lock. For example, it could call await on a condition variable and release the lock while it's waiting. That allows other threads to acquire the lock by entering synchronized methods -- after all, that's what you're waiting for.

answered Dec 18 '16 at 06:31

David Schwartz

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The only correct answer in this thread and the only one with a down vote... – Voo Dec 18 '16 at 08:21

score 0 · Answer 3 · answered Dec 18 '16 at 07:23

0

You code is like the following:

void method1(){
      synchronized(this) {
           //...
      }
}

synchronized void method2(){
      synchronized(this) {
           //...
      }
}

And In your code, two threads can't enter two method at the same time for the lock contention. You can only do it on different instances of runnable i.e. different this.

answered Dec 18 '16 at 07:23

Tony

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This is not correct. See my answer -- one of those `//...` could contain code that releases the lock and lets another thread enter a `synchronized` block. – David Schwartz Dec 18 '16 at 18:00
@DavidSchwartz I say in his old code sample, he can't do it except use different instance. :) – Tony Dec 19 '16 at 02:10

score 0 · Answer 4 · answered Mar 19 '17 at 18:31

You have got the understanding correct, that every object has lock associated with it, and once Thread1 takes the lock to access synchronized method1, then Thread2 cannot access synchronized method2.

Now,coming to special case, you can always do a simple hack. Imagine Thread1 has the lock, and is accessing synchronized method1. Inside method1, if Thread1 calls wait on a variable, then it releases the lock on the object and goes to wait state. In this scenario, Thread2 can acquire the lock and enter synchronized method2. Hence, both the threads have access to different synchronized methods on same obj.

P.S. This is just for example, I would not recommend have another wait inside a synchronized block, it will lead to unnecessary complication. If any code has to do this, then it means there is flaw in logic and design.

two threads entering the two different synchrnozied method at same time on instance level lock

4 Answers4