Two threads executing synchronized block simultaneously

somnathchakrabarti :

Below is the code where a Thread enters a synchronized block, waits for 5 seconds and then exits. I have started two Thread instances simultaneously.

The expectation was one of the threads will own the lock on the synchronized object, and the other will wait. After 5 seconds, when the lock owner exits, the waiting thread will execute.

But, in actual, both the threads are executing the synchronized block simultaneously and also exiting at the same time.

Expected Output:

Thread-X <timeX> received the lock.
Thread-X <timeX+5s> exiting...
Thread-Y <timeY> received the lock.
Thread-Y <timeY+5s> exiting...

Actual Output:

Thread-X <time> received the lock.
Thread-Y <time> received the lock.
Thread-X <time+5s> exiting...
Thread-Y <time+5s> exiting...

Am I missing something here?

import java.text.SimpleDateFormat;
import java.util.Date;

public class Test2 {
public static void main(String[] args) {
    MyRunnable m = new MyRunnable();
    Thread t = new Thread(m);
    Thread t1 = new Thread(m);
    t.start();
    t1.start();
    }
}

class MyRunnable implements Runnable {
    @Override
    public void run() {
        synchronized (this) {
            try {
                SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
                Date date = new Date(System.currentTimeMillis());
                System.out.println(Thread.currentThread().getName() + " " + formatter.format(date) + " received the lock.");
                wait(5000);
                date = new Date(System.currentTimeMillis());
                System.out.println(Thread.currentThread().getName() + " " + formatter.format(date) + " exiting...");
            } catch(InterruptedException ie) {}
        }
    }
}

Markus :

The answer lies in java.lang.Object.wait(long) whose documentation says:

[...] This method causes the current thread (call it T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. [...]