Question

In Groovy code something simple:
#!/usr/bin/env groovy

public class test {
  boolean val
  def obj=new Object()

  def dos() {
    val=false
    Thread.start() {
      synchronized(obj) {
    val=true
    obj.notifyAll()
      }
    }
    Thread.sleep(5000)
    synchronized(obj) {
      while (!val) {
    obj.wait()
      }
    }
  }

  static void main(String[] args) {
    def t=new test()
    t.dos()
  }
}

---

Ok, here is my problem in more detail.

Thread (A)
start an action in a separate thread, then wait for its completion
— OK THIS IS NOT EXACTLY TRUE OTHERWISE COULD USE thread.join(). This
Thread actually starts a task that then signals methodOne eventually

Thread (B)
we get a signal when action is completed

class A {
   private boolean finished

   public synchronized void methodOne() {
       finished=true;
       notifyAll();
   } 

   public void methodTwo() {
       new ThreadThatCallsMethodOneWhenDone().start();
       synchronized(this) {
           while (!finished) {
                 wait();
           }
       }
   }
}

Is this code okay or am I still running into potential problems? What’s a better way to solve?

Misha

---

I was wondering, which is correct:

Option One

class A {
   public void methodOne() {
       synchronized(this) {
           modifyvalue
           notifyAll()
       }
   }

   public void methodTwo() {
       while (valuenotmodified) {
           synchronized(this) {
              wait()
           }
       }
   }

Option Two

class A {
   public void methodOne() {
       modifyvalue
       synchronized(this) {
           notifyAll()
       }
   }

   public void methodTwo() {
       while (valuenotmodified) {
           synchronized(this) {
              wait()
           }
       }
   }

and why?

Answer 1

I think that both are dangerous because your valuenotmodified check is performed without synchronisation. So, there is no telling what happens if methodOne modifies the value while methodTwo is in process of verifying whether it has changed.

And I see no difference between your two “options”. Both have this potential problem.