Synchronized thread method in java

I have one question about thread. I have following Thread class and creating 2 thread objects.

public class MyThread extends Thread{
 
    String name="";
 
    public MyThread(String string) {
        name=string;
    }   
 
    @Override
    public void run() {
        callMe();
    }        
    synchronized private void callMe() {
        System.out.println("Started");
        for (int i = 1; i <= 5; i++) {
            System.out.println(name+" = "+i);
        }          
    }
 
 
    public static void main(String[] args) {                    MyThread a = new MyThread("A");
        MyThread b = new MyThread("B");
 
        a.start();
        b.start();
    }       }

Output:

Started

Started

B = 1

B = 2

A = 1

A = 2

B = 3

A = 3

B = 4

A = 4

B = 5

A = 5

my question is: why loop is NOT executed one after other? I have used synchronized keyword.

Last updated:10/29/2014 1:58:05 AM

1 Answers

marcel ethan
marcel ethan

Your synchronized method is effectively:

private void callMe() {
    synchronized(this) {
        System.out.println('Started');
        for (int i = 1; i <= 5; i++) {
            System.out.println(name+' = '+i);
        }
    }
}

Now you've created two different instances, so this will be different for each thread... so they're not synchronizing against each other. If you want to see two threads working with the same monitor, you could rewrite your code like this:

public final class DemoRunnable implements Runnable {
    @Override
    public synchronized void run() {
        System.out.println('Started');
        for (int i = 1; i <= 5; i++) {
            System.out.println(Thread.currentThread().getName() + ' = ' + i);
        }
    }
 
    public static void main(String[] args) {
        Runnable runnable = new DemoRunnable();
        Thread a = new Thread(runnable, 'A');
        Thread b = new Thread(runnable, 'B');
        a.start();
        b.start();
    }
}

Output:

Started

A = 1

A = 2

A = 3

A = 4

A = 5

Started

B = 1

B = 2

B = 3

B = 4

B = 5

Answer