Question about wait() and notify()

[Andres Gonzalez]

class Test {
   public static void main(String[] args) {
      final Foo f = new Foo();
      Thread t = new Thread(new Runnable() {
         public void run() {
            f.doStuff();
         }
      });
      Thread g = new Thread() {
         public void run() {
            f.doStuff();
         }
      };
      t.start();
      g.start();
   }
}
class Foo {
   int x = 5;
   public void doStuff()
   {
      if (x < 10) {
         // nothing to do
         try {
            wait();
         }
         catch (InterruptedException ex) {}
      }
      else {
         System.out.println("x is " + x++);
         if (x >= 10) {
            notify();
         }
      }
   }
}

My answer was that the code does not even compile because the call to notify()/wait() are not under a synchronized block. But it throws a runtime exception, which is fixed by adding the synchronized keyword to the doStuff() method.
So, when a wait()/notify()/notifyAll() are not placed under a synchronized block it'll throw a runtime exception, right? I just wanna make sure I'm concluding the right thing. There might be another reason and I haven't acknowledged ..yet..Any other observations will be highly appreciated.
thx

[Dave Johnson]

Hi Andres, the exception warning would be: java.lang.IllegalMonitorStateException: current thread not owner.
Which is the exception generated by code that uses wait() & notify() in non-synchronised blocks.

[Marlene Miller]

Here is an example to show that wait() and notify() do not have to be declared in synchronized methods. But the threads that execute wait() and notify() must hold the lock on the object.

class A {
  boolean condition = false;
  //
  synchronized void m1() { m2(); }
  //
  void m2() {
    System.out.println(Thread.currentThread().getName()
      + " holds lock= " + Thread.holdsLock(this));
    try {
      while (!condition) {
        wait();
      }
    } catch (InterruptedException e) {}
  }
  //
  synchronized void m3() { m4(); }
  //
  void m4() {
    System.out.println(Thread.currentThread().getName()
      + "  holds lock= " + Thread.holdsLock(this));
    condition = true;
    notifyAll();
  }
}

class Test1 {
  public static void main(String[] args) {
    final A a = new A();
    Thread t1 = new Thread(
      new Runnable() { public void run() { a.m1(); } });
    t1.start();
    Thread t2 = new Thread(
      new Runnable() { public void run() { a.m3(); } });
    t2.start();
  }
}
Thread-1 holds lock= true
Thread-2 holds lock= true
//
class Test2 {
  public static void main(String[] args) {
    final A a = new A();
    Thread t1 = new Thread(
      new Runnable() { public void run() { a.m2(); } });
    t1.start();
    Thread t2 = new Thread(
      new Runnable() { public void run() { a.m4(); } });
    t2.start();
  }
}
Thread-1 holds lock= false
java.lang.IllegalMonitorStateException: current thread not owner
Thread-2 holds lock= false
java.lang.IllegalMonitorStateException: current thread not owner

[ June 22, 2003: Message edited by: Marlene Miller ]

[Marlene Miller]

So, when a wait()/notify()/notifyAll() are not placed under a synchronized block
it'll throw a runtime exception, right?

Yes, right. Now, let�s describe what �placed under� means.
wait, notify and notifyAll may be invoked only by a thread that holds the
synchronization lock of the object on which the method is invoked.
The invocation can be made directly in a method or block of code declared as
synchronized, or can be made indirectly from a method invoked in such code.
Compliance generally cannot be verified at compile time. Failure to comply causes
these operations to throw an IllegalMonitorStateException at run time.
You can use the static method Thread.holdsLock(Object o) in your practice
programs to show you whether the current thread holds a lock.
[ June 22, 2003: Message edited by: Marlene Miller ]

[Andres Gonzalez]

thx!!