IllegalMonitorStateException inside synchronized block

Question

Though I have written wait inside synchronized block. I am getting IllegalMonitorStateException. What's the reason then?

package trials;

public class WaitNotifyTrial {

    public static void main(String[] args){
        Generator g=new Generator();
        g.start();
        System.out.println("Start");
        synchronized (g) {
                try {
                    g.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    System.out.println("Printing exception");
                    e.printStackTrace();
                }
                System.out.println(g.value);
        }
    }
}

class Generator extends Thread{

    int value;

    public void run(){
        synchronized (this) {
            value=10;
        }
        notify();
        }
    }
}

Answer 1

These are some of the things wrong with your code:

• you call notify() outside of synchronized (this) (this is your immediate problem);
• you don't use the proper wait-loop idiom, thus risking nondeterministic behavior/deadlocks in the face of spurious wakeups and missed notifications;
• you use the wait-notify mechanism on a Thread instance, which is recommended against in its documentation;
• you extend Thread instead of using that class as-is and only passing it an instance of your implementation of Runnable.

For almost a whole decade now, the general advice has been to avoid the wait-notify mechanism altogether and instead use one of synchronization aids from java.util.concurrent, such as the CountDownLatch.