Wait() / notify() synchronization

Question

I\'m trying to check how wait/notify works in java.

Code:

public class Tester {
    public static void main(String[] args) {
                


        

Answer 1

Object monitor locks need to be performed a single reference of the same lock...

In your example you are waiting on an instance of the Thread, but using notify from the Runnable. Instead, you should use a single, common lock object...for example

public class Tester {

    public static final Object LOCK = new Object();

    public static void main(String[] args) {
        MyRunnable r = new MyRunnable();
        Thread t = new Thread(r);
        t.start();
        synchronized (LOCK) {
            try {
                System.out.println("wating for t to complete");
                LOCK.wait();
                System.out.println("wait over");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static class MyRunnable implements Runnable {

        public void run() {
            System.out.println("entering run method");
            synchronized (LOCK) {
                System.out.println("entering syncronised block");
                LOCK.notify();
                try {
                    Thread.currentThread().sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("leaving syncronized block");
            }
            System.out.println("leaving run method");
        }
    }
}

Output...

wating for t to complete
entering run method
entering syncronised block
leaving syncronized block
wait over
leaving run method

wait over and leaving run method could change positions depending on the thread scheduling.

You could try putting the sleep out side the synchronized block. This will release the monitor lock allowing the wait section to continue running (as it can't start until the lock is released)

    public static class MyRunnable implements Runnable {

        public void run() {
            System.out.println("entering run method");
            synchronized (LOCK) {
                System.out.println("entering syncronised block");
                LOCK.notify();
                System.out.println("leaving syncronized block");
            }
            try {
                Thread.currentThread().sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("leaving run method");
        }
    }