IllegalMonitorStateException

Why This Happens

The wait/notify mechanism requires the calling thread to own the object's monitor (lock). You must call wait() or notify() inside a synchronized block that synchronizes on the same object. Calling them outside synchronized context or on a different object causes this exception.

The Problem

Object lock = new Object();
lock.wait(); // IllegalMonitorStateException - not in synchronized block

The Fix

Object lock = new Object();
synchronized (lock) {
    lock.wait(); // Must be inside synchronized(lock)
}

Step-by-Step Fix

1. 1

   Identify the unsynchronized call

   Find the wait(), notify(), or notifyAll() call that is not inside a synchronized block.

2. 2

   Check the synchronized object

   If inside a synchronized block, verify that you are synchronizing on the same object you are calling wait/notify on.

3. 3

   Add proper synchronization

   Wrap the wait/notify call in a synchronized block on the same object, or consider using java.util.concurrent utilities instead.