# ThreadSafeQueue.java

    /*
    ** Module   : ThreadSafeQueue.java
    ** Abstract : a multi-thread safe object queue
    **
    ** Copyright (c) 2005-2017, Golden Code Development Corporation.
    **
    ** -#- -I- --Date-- --JPRM-- ------------------Description-------------------
    ** 001 SIY 20050113   @19510 Created initial implementation of the queue.
    **                           LinkedList is used as an internal storage.
    ** 002 SIY 20050339   @20557 Added capability to terminate queue.
    ** 003 SIY 20051001   @22919 Added counter for waiting threads. Fixed
    **                           formatting.
    ** 004 GES 20060501   @25883 Added synchronization for accessing the
    **                           count, made the dequeuing loop until an
    **                           element is available or termination occurs
    **                           and cleaned up formatting and javadoc
    **                           problems.  The dequeuing change was needed
    **                           because it was possible to be interrupted
    **                           and attempt to get an element from the list
    **                           while the list was empty.
    ** 005 ECF 20071106   @35895 Implemented as a generic class.
    ** 006 GES 20090816   @43641 Added isTerminated() method.
    ** 007 IAS 20160805   @43641 Replaced List with BlockingQueue
    */
    /*
    ** This program is free software: you can redistribute it and/or modify
    ** it under the terms of the GNU Affero General Public License as
    ** published by the Free Software Foundation, either version 3 of the
    ** License, or (at your option) any later version.
    **
    ** This program is distributed in the hope that it will be useful,
    ** but WITHOUT ANY WARRANTY; without even the implied warranty of
    ** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    ** GNU Affero General Public License for more details.
    **
    ** You may find a copy of the GNU Affero GPL version 3 at the following
    ** location: https://www.gnu.org/licenses/agpl-3.0.en.html
    **
    ** Additional terms under GNU Affero GPL version 3 section 7:
    **
    **   Under Section 7 of the GNU Affero GPL version 3, the following additional
    **   terms apply to the works covered under the License.  These additional terms
    **   are non-permissive additional terms allowed under Section 7 of the GNU
    **   Affero GPL version 3 and may not be removed by you.
    **
    **   0. Attribution Requirement.
    **
    **     You must preserve all legal notices or author attributions in the covered
    **     work or Appropriate Legal Notices displayed by works containing the covered
    **     work.  You may not remove from the covered work any author or developer
    **     credit already included within the covered work.
    **
    **   1. No License To Use Trademarks.
    **
    **     This license does not grant any license or rights to use the trademarks
    **     Golden Code, FWD, any Golden Code or FWD logo, or any other trademarks
    **     of Golden Code Development Corporation. You are not authorized to use the
    **     name Golden Code, FWD, or the names of any author or contributor, for
    **     publicity purposes without written authorization.
    **
    **   2. No Misrepresentation of Affiliation.
    **
    **     You may not represent yourself as Golden Code Development Corporation or FWD.
    **
    **     You may not represent yourself for publicity purposes as associated with
    **     Golden Code Development Corporation, FWD, or any author or contributor to
    **     the covered work, without written authorization.
    **
    **   3. No Misrepresentation of Source or Origin.
    **
    **     You may not represent the covered work as solely your work.  All modified
    **     versions of the covered work must be marked in a reasonable way to make it
    **     clear that the modified work is not originating from Golden Code Development
    **     Corporation or FWD.  All modified versions must contain the notices of
    **     attribution required in this license.
    */

    package com.goldencode.p2j.util;

    import java.util.*;
    import java.util.concurrent.*;

    /**
     * This class implements a thread safe queue of the objects. Any number of
     * threads can access the queue to store/retrieve objects to/from the queue.
     * If the queue is empty then the thread which tries to retrieve an object
     * from the queue is suspended until another thread stores an object into
     * the queue.
     *
     * @param   <E>
     *          Type of element object to be queued.
     */
    public class ThreadSafeQueue<E>
    {
       /** Termination token. */
       private static final Poison POISON = new Poison();

       /** Internal storage for the objects. */
       private final BlockingQueue<Object> queue = new LinkedBlockingQueue<>();

       /** Flag indicating if all access to the queue should be ended. */
       private volatile boolean terminated = false;

       /**
        * Construct an empty queue.
        */
       public ThreadSafeQueue()
       {
       }

       /**
        * Put an object into the queue and notify one waiting thread.
        *
        * @param   obj
        *          An object to place into queue.
        *
        * @throws  InterruptedException
        *          When the queue is terminated.
        */
       public void enqueue(E obj)
       throws InterruptedException
       {
          if (terminated)
          {
             throw new InterruptedException("The queue has been terminated.");
          }
          queue.add(obj);
       }

       /**
        * Get the next object from the queue.  Wait indefinitely if the queue is
        * empty.
        *
        * @return  The next object from the queue.
        *
        * @throws  InterruptedException
        *          When the queue is terminated or if interrupted while waiting.
        */
       public E dequeue()
       throws InterruptedException
       {
          // Retrieve the next object when it will be available
          // At this point the InterruptedException can be thrown
          Object e = queue.take();

          // re-insert the poison object so other waiters are unblocked
          // this may add a small overhead with terminated queue but
          // checking the 'terminated' flag before 'take'
          // adds (smaller) overhead to each dequeue operation.
          if (e == POISON)
          {
             queue.add(POISON);
             // As the POISON was retrieved the 'terminated' flag is already set
             // and the InterruptedException will be thrown below
          }

          if (terminated)
          {
             throw new InterruptedException("The queue has been terminated.");
          }

          // this cast is in fact safe as the only way for put the non-E object is via terminate
          return (E)e;
       }

       /**
        * Terminate queue processing and release all waiting threads. All
        * subsequent calls to this instance will generate an exception.
        */
       public void terminate()
       {
          terminated = true;
          queue.add(POISON);
       }

       /**
        * Report if the queue processing has been terminated.
        *
        * @return    <code>true</code> if the queue can no longer be used.
        */
       public boolean isTerminated()
       {
          return terminated;
       }

       /**
        * Class for the termination token
        */
       private static class Poison
       {
       }
    }