How to check whether sleep() method in Java holds lock or not?

Question

class MyThread extends Thread{
   public void run(){
       print();
   }
   synchronized public void print() {
       // for(int i = 0; i< )
       System.out.println("Thread : " + Thread.currentThread().getName());
       try{
           Thread.sleep(5000);}
           catch(InterruptedException e ){
               
           }
   }
}



public class MyClass {
   public static void main(String args[]) throws InterruptedException {
     
     MyThread t1 = new MyThread();
     MyThread t2 = new MyThread();
     t1.setName("A");
     t2.setName("B");
     t1.start();
     t2.start();
     t1.join();
     t2.join();
     
     System.out.println("EOM");
     
   }
}

Output of the Program -

ThreadA
ThreadB

Immediately prints both line After 5 seconds

EOM

According to my understanding, one of the thread should go in print() and acquire lock and only releases it after 5 seconds but here both the threads executed immediately and then "EOM" got printed after 5 seconds.

You should use synchronized on static method if you want lock class. — Gawain, Sep 12 '20 at 03:04

score 2 · Answer 1 · answered Sep 11 '20 at 16:12

2

The synchronized is in an instance method, and each instance doesn't interfere with each other.

If the method were static then it would be shared between the instances.

answered Sep 11 '20 at 16:12

Jonas Fagundes

1,519
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score 0 · Accepted Answer · answered Sep 11 '20 at 16:36

0

You wrote:

synchronized public void print() {
    ...
}

That's just a short-cut way of writing:

public void print() {
    synchronized(this) {
        ...
    }
}

Each of your two threads is operating on a different instance of your MyClass class, therefore, each of your two threads is synchronizing on a different object (different this reference).

When you use synchronized(o) blocks, that only prevents two different threads from synchronizing on the same object o at the same time.

answered Sep 11 '20 at 16:36

Solomon Slow

25,130
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I am little confused. How can we create two threads thats work on the same object? I mean, when we create thread instance we are creating two objects, right? Can you please explain with an example where two threads acts on the same object? – Abhishek Chaudhary Sep 11 '20 at 16:57
@AbhishekChaudhary, That's a separate question. Fortunately, it already has been answered: (see https://stackoverflow.com/q/877096/801894 .) Having two or more threads "work on" the same object is pretty much an essential feature of any multi-threaded program. Working with shared objects is how threads communicate with one another. – Solomon Slow Sep 11 '20 at 17:50

How to check whether sleep() method in Java holds lock or not?

2 Answers2