File: CondVar.java

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/*
  File: ConditionVariable.java

  Originally written by Doug Lea and released into the public domain.
  This may be used for any purposes whatsoever without acknowledgment.
  Thanks for the assistance and support of Sun Microsystems Labs,
  and everyone contributing, testing, and using this code.

  History:
  Date       Who                What
  11Jun1998  dl               Create public version
*/

package EDU.oswego.cs.dl.util.concurrent;

/**
 * This class is designed for fans of POSIX pthreads programming.
 * If you restrict yourself to Mutexes and CondVars, you can
 * use most of your favorite constructions. Don't randomly mix them
 * with synchronized methods or blocks though.
 * <p>
 * Method names and behavior are as close as is reasonable to
 * those in POSIX.
 * <p>
 * <b>Sample Usage.</b> Here is a full version of a bounded buffer
 * that implements the BoundedChannel interface, written in
 * a style reminscent of that in POSIX programming books.
 * <pre>
 * class CVBuffer implements BoundedChannel {
 *   private final Mutex mutex;
 *   private final CondVar notFull;
 *   private final CondVar notEmpty;
 *   private int count = 0;
 *   private int takePtr = 0;     
 *   private int putPtr = 0;
 *   private final Object[] array;
 * 
 *   public CVBuffer(int capacity) { 
 *     array = new Object[capacity];
 *     mutex = new Mutex();
 *     notFull = new CondVar(mutex);
 *     notEmpty = new CondVar(mutex);
 *   }
 * 
 *   public int capacity() { return array.length; }
 * 
 *   public void put(Object x) throws InterruptedException {
 *     mutex.acquire();
 *     try {
 *       while (count == array.length) {
 *         notFull.await();
 *       }
 *       array[putPtr] = x;
 *       putPtr = (putPtr + 1) % array.length;
 *       ++count;
 *       notEmpty.signal();
 *     }
 *     finally {
 *       mutex.release();
 *     }
 *   }
 * 
 *   public Object take() throws InterruptedException {
 *     Object x = null;
 *     mutex.acquire();
 *     try {
 *       while (count == 0) {
 *         notEmpty.await();
 *       }
 *       x = array[takePtr];
 *       array[takePtr] = null;
 *       takePtr = (takePtr + 1) % array.length;
 *       --count;
 *       notFull.signal();
 *     }
 *     finally {
 *       mutex.release();
 *     }
 *     return x;
 *   }
 * 
 *   public boolean offer(Object x, long msecs) throws InterruptedException {
 *     mutex.acquire();
 *     try {
 *       if (count == array.length) {
 *         notFull.timedwait(msecs);
 *         if (count == array.length)
 *           return false;
 *       }
 *       array[putPtr] = x;
 *       putPtr = (putPtr + 1) % array.length;
 *       ++count;
 *       notEmpty.signal();
 *       return true;
 *     }
 *     finally {
 *       mutex.release();
 *     }
 *   }
 *   
 *   public Object poll(long msecs) throws InterruptedException {
 *     Object x = null;
 *     mutex.acquire();
 *     try {
 *       if (count == 0) {
 *         notEmpty.timedwait(msecs);
 *         if (count == 0)
 *           return null;
 *       }
 *       x = array[takePtr];
 *       array[takePtr] = null;
 *       takePtr = (takePtr + 1) % array.length;
 *       --count;
 *       notFull.signal();
 *     }
 *     finally {
 *       mutex.release();
 *     }
 *     return x;
 *   }
 * }
 *
 * </pre>
 * @see Mutex
 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]

 **/

public class CondVar {

  /** The mutex **/
  protected final Sync mutex_;

  /** 
   * Create a new CondVar that relies on the given mutual
   * exclusion lock. 
   * @param mutex A non-reentrant mutual exclusion lock.
   * Standard usage is to supply an instance of <code>Mutex</code>,
   * but, for example, a Semaphore initialized to 1 also works.
   * On the other hand, many other Sync implementations would not
   * work here, so some care is required to supply a sensible 
   * synchronization object.
   * In normal use, the mutex should be one that is used for <em>all</em>
   * synchronization of the object using the CondVar. Generally, 
   * to prevent nested monitor lockouts, this
   * object should not use any native Java synchronized blocks.
   **/
 
  public CondVar(Sync mutex) {
    mutex_ = mutex;
  }

  /** 
   * Wait for notification. This operation at least momentarily
   * releases the mutex. The mutex is always held upon return, 
   * even if interrupted.
   * @exception InterruptedException if the thread was interrupted
   * before or during the wait. However, if the thread is interrupted
   * after the wait but during mutex re-acquisition, the interruption 
   * is ignored, while still ensuring
   * that the currentThread's interruption state stays true, so can
   * be probed by callers.
   **/
  public void await() throws InterruptedException {
    if (Thread.interrupted()) throw new InterruptedException();
    try {
      synchronized(this) {
        mutex_.release();
        try {
          wait(); 
        }
        catch (InterruptedException ex) {
          notify();
          throw ex;
        }
      }
    }
    finally { 
      // Must ignore interrupt on re-acquire
      boolean interrupted = false;
      for (;;) {
        try {
          mutex_.acquire();
          break;
        }
        catch (InterruptedException ex) {
          interrupted = true;
        }
      }
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
  }

  /**
   * Wait for at most msecs for notification. 
   * This operation at least momentarily
   * releases the mutex. The mutex is always held upon return, 
   * even if interrupted.
   * @param msecs The time to wait. A value less than or equal to zero 
   * causes a momentarily release
   * and re-acquire of the mutex, and always returns false.
   * @return false if at least msecs have elapsed
   * upon resumption; else true. A 
   * false return does NOT necessarily imply that the thread was
   * not notified. For example, it might have been notified 
   * after the time elapsed but just before resuming.
   * @exception InterruptedException if the thread was interrupted
   * before or during the wait.
   **/

  public boolean timedwait(long msecs) throws  InterruptedException {
    if (Thread.interrupted()) throw new InterruptedException();
    boolean success = false;
    try {
      synchronized(this) {
        mutex_.release();
        try {
          if (msecs > 0) {
            long start = System.currentTimeMillis();
            wait(msecs); 
            success = System.currentTimeMillis() - start <= msecs;
          }
        }
        catch (InterruptedException ex) {
          notify();
          throw ex;
        }
      }
    }
    finally {
      // Must ignore interrupt on re-acquire
      boolean interrupted = false;
      for (;;) {
        try {
          mutex_.acquire();
          break;
        }
        catch (InterruptedException ex) {
          interrupted = true;
        }
      }
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
    return success;
  }
  
  /** 
   * Notify a waiting thread.
   * If one exists, a non-interrupted thread will return
   * normally (i.e., not via InterruptedException) from await or timedwait.
   **/
  public synchronized void signal() {
    notify();
  }

  /** Notify all waiting threads **/
  public synchronized void broadcast() {
    notifyAll();
  }
}