Synchronized block working in java in multithreading

Question

I have one question regarding the post Synchronized block not working, the following code is printing “Hello Java” …. 20 times for obj1 and obj2. This code is similar to that one given in the post.

As per the explanation, shouldn't the following code also have varying output? Can someone please help me to understand the difference between the two?

class ThreadDemo implements Runnable 
{ 
    String x, y; 
    public void run() 
    { 
        for(int i = 0; i < 10; i++) 
            synchronized(this) 
            { 
                x = "Hello"; 
                y = "Java"; 
              System.out.print(x + " " + y + " ");
            } 
    }

    public static void main(String args[]) 
    { 
        ThreadDemo run = new ThreadDemo (); 
        Thread obj1 = new Thread(run); 
        Thread obj2 = new Thread(run); 
        obj1.start(); 
        obj2.start(); 
    } 
}

What do you mean by `shouldn't the following code also have varying output`. Also which post are you referring to? — akortex, Jan 02 '20 at 16:06
It will print "Hello Java" 20 times only, are you talking about the order or something? — Ajinkya Taranekar, Jan 02 '20 at 16:10

score 2 · Accepted Answer · answered Jan 02 '20 at 16:16

You are printing only x and y which are in synchronized block so it's printing the same value. Add i which is the outside synchronized block ,in print and you would see varying output.

class ThreadDemo implements Runnable 
{ 
    String x, y; 
    public void run() 
    {
        for(int i = 0; i < 10; i++) 
            synchronized(this)
            {
                x = "Hello";
                y = "Java";
              System.out.println(x + " " + y + " "+i);
            }
    }
    public static void main(String args[]) 
    { 
         ThreadDemo run = new ThreadDemo (); 
        Thread obj1 = new Thread(run); 
        Thread obj2 = new Thread(run); 
        obj1.start(); 
        obj2.start(); 
    } 
}

score 0 · Answer 2 · answered Jan 02 '20 at 16:15

0

You're obtaining a lock on the instance of ThreadDemo on which that run() method is being called. Since both threads are using the same object the Synchronized block is working here.

answered Jan 02 '20 at 16:15

Arun Kumar

16
1

Didn't downvote, but that does explain how the program works, not what the differences are compared to the other post. – Maarten Bodewes Jan 02 '20 at 16:18

Synchronized block working in java in multithreading

2 Answers2