Issue with synchronization

[David Spades]

public class ObjectDemo extends Object {

   private List synchedList;

   public ObjectDemo() {
      // create a new synchronized list to be used
      synchedList = Collections.synchronizedList(new LinkedList());
   }

   // method used to remove an element from the list
   public String removeElement() throws InterruptedException {
      synchronized (synchedList) {

         // while the list is empty, wait
         while (synchedList.isEmpty()) {
            System.out.println("List is empty...");
            synchedList.wait();
            System.out.println("Waiting...");
         }
         String element = (String) synchedList.remove(0);

         return element;
      }
   }

   // method to add an element in the list
   public void addElement(String element) {
      System.out.println("Opening...");
      synchronized (synchedList) {

         // add an element and notify all that an element exists
         synchedList.add(element);
         System.out.println("New Element:'" + element + "'");

         synchedList.notifyAll();
         System.out.println("notifyAll called!");
      }
      System.out.println("Closing...");
   }

   public static void main(String[] args) {
      final ObjectDemo demo = new ObjectDemo();

      Runnable runA = new Runnable() {

         public void run() {
            try {
               String item = demo.removeElement();
               System.out.println(Thread.currentThread().getName() + " " + item);
            } catch (InterruptedException ix) {
               System.out.println("Interrupted Exception!");
            } catch (Exception x) {
               System.out.println("Exception thrown.");
            }
         }
      };

      Runnable runB = new Runnable() {

         // run adds an element in the list and starts the loop
         public void run() {
            demo.addElement("Hello!");
         }
      };

      try {
         Thread threadA1 = new Thread(runA, "A");
         threadA1.start();

         Thread threadA2 = new Thread(runA, "B");
         threadA2.start();

         Thread threadB = new Thread(runB, "C");
         threadB.start();

         threadA1.interrupt();
         threadA2.interrupt();

         Thread.sleep(1000);
      } catch (InterruptedException x) {
      }
   }
}

first of all, is there any chance that this program will ouput the line "A Hello!"? I always got "B Hello!".
if it'll never print "A Hello!", could anybody explain why?
thanks

[Henry Wong]

David Spades wrote:
first of all, is there any chance that this program will ouput the line "A Hello!"? I always got "B Hello!".
if it'll never print "A Hello!", could anybody explain why?

Scheduling of threads is not defined by the JVM specification. This is mainly to allow the native threading system to schedule threads as they like. This is also true with the use of condition variables (ie. wait/notify).

So, to answer your question... The behavior is not defined.

Henry

[Rahul Mahindrakar]

Hi

I have just added a while loop and see that both B Hello and A Hello do appear but not in any deterministic order.

while(true){
Thread threadA1 = new Thread(runA, "A");  
...
}

This should be normal.

[Rahul Mahindrakar]

It will be good to know what are you trying to prove/test here.

[S Majumder]

David Spades wrote:
first of all, is there any chance that this program will ouput the line "A Hello!"? I always got "B Hello!".
if it'll never print "A Hello!", could anybody explain why?
thanks

David ,
When I'm running your code am getting below output :

List is empty...
List is empty...
Opening...
Interrupted Exception!
New Element:'Hello!'
notifyAll called!
Closing...
Interrupted Exception!


Your code it is quite self explanatory ..

If I modify the code like this

try {
			Thread threadA1 = new Thread(runA, "A");
			threadA1.setPriority(1);
			threadA1.start();

			Thread threadA2 = new Thread(runA, "B");
			threadA2.setPriority(5);
			threadA2.start();

			Thread threadB = new Thread(runB, "C");
			threadB.setPriority(10);
			threadB.start();

			threadA1.interrupt();
			threadA2.interrupt();

			Thread.sleep(1000);
		} catch (InterruptedException x) {
		}

then I get :

List is empty...
Interrupted Exception!
Opening...
New Element:'Hello!'
notifyAll called!
Closing...
A Hello!

Thanks ,
Satya