help regarding synchronized methods

[Venu Chakravorty]

Hi,
I can't understand the output of the following code:

  class MyThread implements Runnable {
    String currth;
    public void run() {
	currth = Thread.currentThread().getName();
	System.out.println(currth + ": in run");
	meth();
    }
    synchronized void meth() {
	System.out.println(currth + ": about to sleep");
	try {
	    Thread.sleep(5000);
	} catch (InterruptedException ie) {}
	System.out.println(currth + ": dying");
    }
}

class A {
    public static void main(String[] args) {
	MyThread mt1 = new MyThread();
	Thread t1 = new Thread(mt1);
	t1.setName("t1");
	MyThread mt2 = new MyThread();
	Thread t2 = new Thread(mt2);
	t2.setName("t2");
	t1.start();
	t2.start();
    }
}  

Output (on my machine):
t1: in run
t1: about to sleep
t2: in run
t2: about to sleep
t1: dying
t2: dying


I thought (and still think) the output would be:
t1: in run
t1: about to sleep
t2: in run
t1: dying
t2: about to sleep
t2: dying

How is "t2: about to sleep" printed before "t1: dying"? Isn't "t1" supposed to die before "t2" can get a lock on "synchronized void meth()"?

regards, venu

[Miklos Szeles]

Hi venu,

There's no connection between mt1's and mt2's meth method. It's not a static method. synchronized void meth() guarantess only that the meth calls for the given instance will be synchronized.

[sujith Acharya]

synchronized method doesnt allow another thread enter to the method through the same object. In your case t1 and t2 enters the method meth() through different objects mt1 and mt2. So the lock doesnt prevent t2 entering meth() while t1 executing meth() or vice versa.

   

  

class MyThread implements Runnable {  
	      String currth;  
	      public void run() {  
	      currth = Thread.currentThread().getName();  
	      System.out.println(currth + ": in run");  
	      meth();  
	      }  
	      synchronized void meth() {  
	    	  currth = Thread.currentThread().getName();  
	      System.out.println(currth + ": about to sleep");  
	      try {  
	          Thread.sleep(5000);  
	      } catch (InterruptedException ie) {}  
	      System.out.println(currth + ": dying");  
	      }  
	  }  

         class A{
                    public static void main(String[] args) {
                    MyThread mt1 = new MyThread();  
		     Thread t1 = new Thread(mt1);  
		     t1.setName("t1");  
		     MyThread mt2 = new MyThread();  
		     Thread t2 = new Thread(mt1);  
		     t2.setName("t2");  
		     t1.start();  
		     t2.start(); 
                    }
          }


 

[Venu Chakravorty]

thanks mates.