Re: understanding: synchronized reference

From:

Robert Klemme <shortcutter@googlemail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 09 Aug 2006 13:23:45 +0200

Message-ID:

<4jtuu1F9neitU1@individual.net>

On 09.08.2006 13:00, John_Woo wrote:

Hi,

I want some classification on understanding the context of synchronized
reference:

say

LinkedList queue = new LinkedList();

public Object pop() throws InterruptedException {
    synchronized(queue) {
      while (queue.isEmpty()) {
        queue.wait();
      }
      return queue.removeFirst();
    }
  }

public void put(Object o){queue.add(o);}

public void do(){
   pup();
   put(o);
   put(o);
   pup();
  ..
}

case 1:pop never called
case 2: pop called only once
cass 3: pop/put called many times

I'm wondering, in above 3 cases, what happened to queue in terms of
synchronization?
like,
in case 1, is it queue not synchronized?
in case 2, if queue size always > 0 ; queue not synchronized?

The question doesn't really make sense to me. It reads as if
"synchronizing" does something to the queue content which it does not.
Synchronizing ensures exclusive access to a lock for a single thread.

If queue should be accessed from multiple threads access needs to be
synchronized. Note, that in Java 1.5 there is are queue classes that
has proper synchronization built in. See here for example

http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/BlockingQueue.html

when queue is synchronized in pop, did that imply it synchronized
anywhere in same class?

No. But it must be synchronized in put. Also, put must invoke
queue.notifyAll() to make pop() wake up.

Kind regards

robert