Java Wait and Notify: IllegalMonitorStateException

Question

I don't completely understand how wait and notify (of Object) work, and as a result I'm forced to slim down my attempts into the following section of code.

Main.java:

import java.util.ArrayList;  class Main {   public static Main main = null;    public static int numRunners = 4;   public static ArrayList<Runner> runners = null;    public static void main(String[] args)   {     main = new Main();   }    Main()   {     runners = new ArrayList<Runner>(numRunners);      for (int i = 0; i < numRunners; i++)     {       Runner r = new Runner();       runners.add(r);       new Thread(r).start();     }      System.out.println("Runners ready.");     notifyAll();   } } 

Runner.java:

class Runner implements Runnable {   public void run()   {     try     {       Main.main.wait();     } catch (InterruptedException e) {}     System.out.println("Runner away!");   } } 

Currently I get an IllegalMonitorStateException when calling Main.main.wait();, but I don't understand why. From what I can see, I need to synchronize Runner.run, but in doing so I assume it would only notify one thread, when the idea is to notify them all.

I've looked at java.util.concurrent, but I can't find a suitable replacement (maybe I'm just missing something).

Answer 1

You can't wait() on an object unless the current thread owns that object's monitor. To do that, you must synchronize on it:

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

The same rule applies to notify()/notifyAll() as well.

The Javadocs for wait() mention this:

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

Throws:

IllegalMonitorStateException – if the current thread is not the owner of this object's monitor.

And from notify():

A thread becomes the owner of the object's monitor in one of three ways:

• By executing a synchronized instance method of that object.
• By executing the body of a synchronized statement that synchronizes on the object.
• For objects of type Class, by executing a synchronized static method of that class.