LinkedBlockingDeque offerFirst

[Gary Blomquist]

Is it possible for offerFirst(E e) of LinkedBlockingDeque to block? The javadoc says:

"Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available."

The verbiage "if it is possible to do so immediately" seems to indicate that it can NOT block but I have a Thread that is calling this method that either dies or blocks indefinitely. The run loop is inside a try/catch block so the thread should not die. I am wondering if the thread might be seizing up on the call to offerFirst.

[Ireneusz Kordal]

Look into source code:
http://www.docjar.com/html/api/java/util/concurrent/LinkedBlockingDeque.java.html

This is an implementation of offerFirst:

  271       public boolean offerFirst(E e) {
  272           if (e == null) throw new NullPointerException();
  273           lock.lock();
  274           try {
  275               return linkFirst(e);
  276           } finally {
  277               lock.unlock();
  278           }
  279       }
 

This is a declaration of 'lock' variable:

  107       /** Main lock guarding all access */
  108       private final ReentrantLock lock = new ReentrantLock();
  

It seems that offerFirst blocks.

[Adam Smolnik]

Hey.

Thread that is calling this method that either dies or blocks indefinitely.

It is rather a strange situation. It could have been locked only during some longer traversal, for example:

    public int drainTo(Collection<? super E> c) {
        ...
        lock.lock();
        try {
            for (Node<E> p = first; p != null; p = p.next)
                c.add(p.item);
            ...
            notFull.signalAll();
            return n;
        } finally {
            lock.unlock();
        }
    }

Could you show more your LinkedBlockingDeque's client code?

Adam

[Henry Wong]

Ireneusz Kordal wrote:
It seems that offerFirst blocks.

When the JavaDoc implies that it doesn't block, it is saying that there is no situation that can cause it to block indefinitely -- which is generally caused by a call to the condition variable await() method, because the queue is full.

The locks are just held long enough to do the task, and maintain thread safety.

Henry

[Henry Wong]

Adam Smolnik wrote:
It is rather a strange situation.

Could show more your LinkedBlockingDeque's client code?

Agreed. We need to see the client code -- as it is likely something else, and not being blocked while calling the offerFirst() method.

Henry