Synchronized block in run() method itself

[Nuwan Arambage]

Hi all,

I have written a sample code which has synchronized code block inside run() method.By looking at the output I wouldn't see any synchronization occurs.
Can somebody explain me what went wrong in my code.

public class MyThread implements Runnable{
	Object obj=new Object();
	 public void run() {  		
		synchronized(obj){
			for(int i=0;i<20;i++){
			System.out.println("Thread Name : "+ Thread.currentThread().getName() +" i " +i); 
			}
		}    
     }
}

Main Method

public class Main{
	public static void main(String []args){
		MyThread thread1=new MyThread();
		MyThread thread2=new MyThread();
		
		Thread t1=new Thread(thread1,"Thread 1");
		Thread t2=new Thread(thread2,"Thread 2");
		
		t1.start();
		t2.start();
	
	}
}

This is output what I have got once I execute the main method

C:\Users\Nuwan Arambage\Desktop\HelloWorld\Threads\Runnable>java Main

Thread Name : Thread 2 i 0
Thread Name : Thread 2 i 1
Thread Name : Thread 2 i 2
Thread Name : Thread 2 i 3
Thread Name : Thread 2 i 4
Thread Name : Thread 2 i 5
Thread Name : Thread 2 i 6
Thread Name : Thread 2 i 7
Thread Name : Thread 2 i 8
Thread Name : Thread 2 i 9
Thread Name : Thread 2 i 10
Thread Name : Thread 2 i 11
Thread Name : Thread 2 i 12
Thread Name : Thread 1 i 0
Thread Name : Thread 2 i 13
Thread Name : Thread 1 i 1
Thread Name : Thread 1 i 2
Thread Name : Thread 2 i 14
Thread Name : Thread 1 i 3
Thread Name : Thread 2 i 15
Thread Name : Thread 1 i 4
Thread Name : Thread 2 i 16
Thread Name : Thread 1 i 5
Thread Name : Thread 2 i 17
Thread Name : Thread 2 i 18
Thread Name : Thread 2 i 19
Thread Name : Thread 1 i 6
Thread Name : Thread 1 i 7
Thread Name : Thread 1 i 8
Thread Name : Thread 1 i 9
Thread Name : Thread 1 i 10
Thread Name : Thread 1 i 11
Thread Name : Thread 1 i 12
Thread Name : Thread 1 i 13
Thread Name : Thread 1 i 14
Thread Name : Thread 1 i 15
Thread Name : Thread 1 i 16
Thread Name : Thread 1 i 17
Thread Name : Thread 1 i 18
Thread Name : Thread 1 i 19

C:\Users\Nuwan Arambage\Desktop\HelloWorld\Threads\Runnable>

Thanks & Regards,

Nuwan Arambage

[Steve Luke]

Nuwan Arambage wrote:

public class MyThread implements Runnable{
	Object obj=new Object();
	 public void run() {  		
		synchronized(obj){
			for(int i=0;i<20;i++){
			System.out.println("Thread Name : "+ Thread.currentThread().getName() +" i " +i); 
			}
		}    
     }
}

In this code, eacj new MyThread instance has its own obj Object. When you synchronize you get the lock on the obj Object that is local to just that one instance of MyThread. So when you have two instances there is no 'synchronization' because they are both gaining locks from different Objects. To synchronize two threads they need to synchronize on the same Object (the same instance).

Besides that, I am not sure the code you have there would show synchronization anyway. You are not protecting any shared data, and aren't performing any actions which would stand out as being synchronizing happening or not. Try to add some code that would make it easy to tell if there is some inter-leaving of processes. Something like this:

	 public void run() {  		
		synchronized(obj){
			for(int i=0;i<20;i++){
			    System.out.println("Thread Name : "+ Thread.currentThread().getName() +" is about to do some work.");
                            doSomeWork();
                            System.out.println("Thread Name : " + Thread.currentThread().getName() + " is done doing work."); 
			}
		}    
     }

    private void doSomeWork() {
        try {
            Thread.sleep(100);
        } catch (InterruptedException ie) {
            //Don't care.  p.s. don't ignore exceptions like this in real code!
        }
    }

With this code, when you have synchronization you should always see an 'is about to do some work' statement immediately followed by an 'is done doing work' from the same Thread name. When synchronization is not working you should see at least some interleaving between the 'is about to do some work' statements from one thread and statements from the other thread.

-- Edit --

Steve Luke wrote:Besides that, I am not sure the code you have there would show synchronization anyway.

Okay, actually it will because the synchronization is on the outside of the for loop...

[Nuwan Arambage]

Hi Steve,

I understood the mistake I have made in my code. I got your point
Thanks

Nuwan Arambage

[rahul sab]

Having MyThread.class instead of obj would have worked for you..

synchronized(MyThread.class)

[Rakesh K. Cherukuri]

Hey rahul, isn't that post age old and poster more or less got his question answered? Welcome to ranch by the way.