IllegalMonitorStateException on wait() call

Question

I am using multi-threading in java for my program. I have run thread successfully but when I am using Thread.wait(), it is throwing java.lang.IllegalMonit

Answer 1

wait is defined in Object, and not it Thread. The monitor on Thread is a little unpredictable.

Although all Java objects have monitors, it is generally better to have a dedicated lock:

private final Object lock = new Object();

You can get slightly easier to read diagnostics, at a small memory cost (about 2K per process) by using a named class:

private static final class Lock { }
private final Object lock = new Lock();

In order to wait or notify/notifyAll an object, you need to be holding the lock with the synchronized statement. Also, you will need a while loop to check for the wakeup condition (find a good text on threading to explain why).

synchronized (lock) {
    while (!isWakeupNeeded()) {
        lock.wait();
    }
}

To notify:

synchronized (lock) {
    makeWakeupNeeded();
    lock.notifyAll();
}

It is well worth getting to understand both Java language and java.util.concurrent.locks locks (and java.util.concurrent.atomic) when getting into multithreading. But use java.util.concurrent data structures whenever you can.