Why Am I Getting IllegalMonitorStateException When Using Java\'s Wait and Notify?

Java's Wait and Notify: Understanding IllegalMonitorStateException

In Java, wait and notify methods allow threads to synchronize their execution. However, using these methods incorrectly can lead to IllegalMonitorStateException.

To understand why, let's analyze the provided code:

Main.java

• In the constructor, the main thread creates multiple threads and adds them to an ArrayList.
• After printing "Runners ready," it calls notifyAll() to notify all threads.

Runner.java

• In the run() method, each runner enters a wait() condition, waiting for a notification from the main thread.

The issue lies in the wait() call in Runner.run(). When a thread calls wait() on an object, it must own that object's monitor. Ownership is established by synchronizing on the object.

To fix the issue, synchronize on Main.main within the wait() call:

<code class="java">class Runner implements Runnable {
  public void run() {
    try {
      synchronized (Main.main) {
        Main.main.wait();
      }
    } catch (InterruptedException e) {}
    System.out.println("Runner away!");
  }
}</code>

This ensures that the current thread owns Main.main's monitor before entering the wait() condition.

The same principle applies to notify() and notifyAll(). A thread must own the object's monitor before issuing these methods.

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