java.lang.IllegalMonitorStateException whilst calling wait from synchronized block

Question

Before I move onto using Condition variables I'm trying to understand object wait principles. I wrote a little code to understand more but its not working as expected.

What is supposed to happen is a Waiter class waits upon thread start. Meanwhile the Notifier class adds some elements to a list using a loop. Once the Notifier has done this it notifies the Waiter which should simply print that it has been notified but i get illegalmonitorstate exception

This is my output

Exception in thread "Waiter Thread" java.lang.IllegalMonitorStateException
at java.lang.Object.wait(Native Method)
at tutorials.waitnotify.Waiter.run(Waiter.java:26)
at java.lang.Thread.run(Thread.java:745)
0 [Waiter Thread] DEBUG tutorials.waitnotify.Waiter  - Starting waiter....
2002 [Notifier Thread] DEBUG tutorials.waitnotify.Waiter  - Starting     
notifier...
3005 [Notifier Thread] DEBUG tutorials.waitnotify.Waiter  - Element added [1]
4007 [Notifier Thread] DEBUG tutorials.waitnotify.Waiter  - Element added [2]
5012 [Notifier Thread] DEBUG tutorials.waitnotify.Waiter  - Element added [3]
Exception in thread "Notifier Thread" java.lang.IllegalMonitorStateException
7022 [Notifier Thread] DEBUG tutorials.waitnotify.Waiter  - Object about to    
notify
at java.lang.Object.notify(Native Method)
at tutorials.waitnotify.Notifier.run(Notifier.java:42)
at java.lang.Thread.run(Thread.java:745)

This is the code As you can see I'm trying to synchronise on a common lock object.

public class Notifier implements Runnable {

private static final Logger LOGGER = LoggerFactory.getLogger(Waiter.class);
private List<Integer> list;
private Object commonLock;

public Notifier(Object lock) {
    this.list = new ArrayList<>();
    this.commonLock = lock;
}

public void run() {
    LOGGER.debug("Starting notifier....");
    synchronized (commonLock) {
        for (int i = 1; i <= 3; i++) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ie) {
                LOGGER.debug("Interrupted");
            }
            list.add(i);
            LOGGER.debug("Element added [{}]", i);
        }
        LOGGER.debug("About to notify");
        notify();
    }
}
}

public class Waiter implements Runnable {

private static final Logger LOGGER = LoggerFactory.getLogger(Waiter.class);
private final Object commonLock;

public Waiter(Object lock) {
    this.commonLock = lock;
}

public void run() {
    LOGGER.debug("Starting waiter....");
    synchronized (commonLock) {
        try {
            wait(10000);
        } catch (InterruptedException ie) {
            LOGGER.debug("Interrupted");
        }
    }
    LOGGER.debug("Object been notified");
}
}

public class WaiterNotifierMain {

public static void main(String[] args) {
    BasicConfigurator.configure();

    Object lock = new Object();
    Waiter waiter = new Waiter(lock);
    Notifier notifier = new Notifier(lock);

    Thread waiterThread = new Thread(waiter);
    Thread notifierThread = new Thread(notifier);

    notifierThread.setName("Notifier Thread");
    waiterThread.setName("Waiter Thread");
    waiterThread.start();
    try {
        Thread.sleep(2000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    notifierThread.start();
}
}

Any pointers would be appreciated. Thanks in advance

Answer 1

You should do

• commonLock .wait(10000)
• commonLock .notify()

You get an exception because you're waiting and notifying on "this". And you didn't synchronize on "this".

You might consider using notifyAll() instead of notify():

If you plan to have multiple Waiter threads, keep in mind that notify() will only wake up one Waiter thread at a time. If you want to wake up ALL Waiter threads at once, you should use the notifyAll() method instead.

Even if know that you will never have more than one Waiter thread, I consider it best practice to use notifyAll() instead of notify(), since the Notifier object doesn't know how many threads are listening on the object...