Threads synchronizing in Java, IllegalMonitorStateException

Question

I am trying to synchronize two threads - the "Main" thread, and a runnable. I get the IllegalMonitorStateException, but I do not completelty understand what "you do not have the lock of the object" means.

Here is my code:

public class ThreadsTest {
    private static ThreadsTest instance;
    public volatile boolean flag = false;

    public void doStuff() {
        System.out.println("first");

        this.flag = true;

    }

    public Runnable mDrawer = new Runnable() {

        public void run() {
            synchronized (ThreadsTest.getInstance()) {
                while (flag == false)
                    try {
                        wait();
                    } catch (InterruptedException ie) {
                        System.out.println("second");
                    }
            }



        }
    };

    public static ThreadsTest getInstance() {
        if (ThreadsTest.instance == null) {
            ThreadsTest.instance = new ThreadsTest();
        }
        return ThreadsTest.instance;
    }

    private ThreadsTest() {
    }

    public static void main(String[] args) {
        ThreadsTest t = ThreadsTest.getInstance();
        t.mDrawer.run();
        t.doStuff();

    }
}

Answer 1

You may call wait() method only on objects you synchronizing on.
So, since you have synchronized (ThreadsTest.getInstance()), you must write ThreadsTest.getInstance().wait().

Not sure what you trying to test here exactly, if it's just basic thread sync sample, then you should change your code to

public class ThreadsTest {
    private static ThreadsTest instance;
    public volatile boolean flag = false;

    public void doStuff() {
        System.out.println("first");
        this.flag = true;
        synchronized (getInstance()) {
            getInstance().notifyAll();
        }
    }

    public Runnable mDrawer = new Runnable() {

        public void run() {
            synchronized (ThreadsTest.getInstance()) {
                while (flag == false)
                    try {
                        ThreadsTest.getInstance().wait();
                    } catch (InterruptedException ie) {
                        System.out.println("second");
                    }
            }



        }
    };

    public static ThreadsTest getInstance() {
        if (ThreadsTest.instance == null) {
            ThreadsTest.instance = new ThreadsTest();
        }
        return ThreadsTest.instance;
    }

    private ThreadsTest() {
    }

    public static void main(String[] args) throws InterruptedException {
        ThreadsTest t = ThreadsTest.getInstance();
        new Thread(t.mDrawer).start();
        Thread.sleep(1000L); // let other thread start, so it won't skip our notify()
        t.doStuff();
    }
}