在Integer上同步时notify（）上发生IllegalMonitorStateException

Question

I'm new to using wait() and notify() in Java and I'm getting an IllegalMonitorStateException.

Main Code

public class ThreadTest {

    private static Integer state = 0;
    public static void main(String[] args) {

        synchronized(state) {
            System.out.println("Starting thread");

            Thread t = new Thread(new AnotherTest());
            t.start();

            synchronized(state) {
                state = 0;
                while(state == 0) {
                    try {
                        state.wait(1000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println("State is: " + state);
            }
        }
    }   

    public static class AnotherTest implements Runnable {

        @Override
        public void run() {
            synchronized(state) {
                state = 1;
                state.notify();
            }

        }

    }
}

I'm getting an IllegalMonitorStateException what state.notify() is called. Any ideas?

Edit : Based on answer below here is code that works. As a side note, I was first trying this with an enum which has the same problem of using Integer.

public class ThreadTest {

    private static int state = 0;
    private static Object monitor = new Object();
    public static void main(String[] args) {

        synchronized(monitor) {
            System.out.println("Starting thread");

            Thread t = new Thread(new AnotherTest());
            t.start();

            state = 0;
            while(state == 0) {
                try {
                    for(int i = 0; i < 5; i++) {
                        System.out.println("Waiting " + (5 - i) + " Seconds");
                        Thread.sleep(1000);
                    }
                    monitor.wait(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            System.out.println("State is: " + state);
        }
    }   

    public static class AnotherTest implements Runnable {

        @Override
        public void run() {
            synchronized(monitor) {
                state = 1;
                monitor.notify();
            }

        }

    }
}

Answer 1

This

private static Integer state = 0;

is equivalent to

private static Integer state = Integer.valueOf(0);

The invocation of valueOf(0) returns a reference to an Integer object, call it A.

You then do

synchronized(state) {

your thread acquires the lock on the object referenced by state , currently that is A.

You then do

state = 1;

which is equivalent to

state = Integer.valueOf(1);

which gives you a different reference to an Integer object, call it B, and assigns it to state . When you then call

state.notify();

you're invoking notify() on an object, B, for which your thread doesn't own the monitor. You can't call notify or wait on objects for which your thread doesn't own the monitor.