Package EDU.oswego.cs.dl.util.concurrent

Class Rendezvous

  • All Implemented Interfaces:
    Barrier
    public class Rendezvous
extends Object
implements Barrier
    A rendezvous is a barrier that:
    • Unlike a CyclicBarrier, is not restricted to use with fixed-sized groups of threads. Any number of threads can attempt to enter a rendezvous, but only the predetermined number of parties enter and later become released from the rendezvous at any give time.
    • Enables each participating thread to exchange information with others at the rendezvous point. Each entering thread presents some object on entry to the rendezvous, and returns some object on release. The object returned is the result of a RendezvousFunction that is run once per rendezvous, (it is run by the last-entering thread). By default, the function applied is a rotation, so each thread returns the object given by the next (modulo parties) entering thread. This default function faciliates simple application of a common use of rendezvous, as exchangers.

    Rendezvous use an all-or-none breakage model for failed synchronization attempts: If threads leave a rendezvous point prematurely because of timeout or interruption, others will also leave abnormally (via BrokenBarrierException), until the rendezvous is restarted. This is usually the simplest and best strategy for sharing knowledge about failures among cooperating threads in the most common usages contexts of Rendezvous.

    While any positive number (including 1) of parties can be handled, the most common case is to have two parties.

    Sample Usage

    Here are the highlights of a class that uses a Rendezvous to swap buffers between threads so that the thread filling the buffer gets a freshly emptied one when it needs it, handing off the filled one to the thread emptying the buffer. 

     class FillAndEmpty {
   Rendezvous exchanger = new Rendezvous(2);
   Buffer initialEmptyBuffer = ... a made-up type
   Buffer initialFullBuffer = ...

   class FillingLoop implements Runnable {
     public void run() {
       Buffer currentBuffer = initialEmptyBuffer;
       try {
         while (currentBuffer != null) {
           addToBuffer(currentBuffer);
           if (currentBuffer.full()) 
             currentBuffer = (Buffer)(exchanger.rendezvous(currentBuffer));
         }
       }
       catch (BrokenBarrierException ex) {
         return;
       }
       catch (InterruptedException ex) {
         Thread.currentThread().interrupt();
       }
     }
   }

   class EmptyingLoop implements Runnable {
     public void run() {
       Buffer currentBuffer = initialFullBuffer;
       try {
         while (currentBuffer != null) {
           takeFromBuffer(currentBuffer);
           if (currentBuffer.empty()) 
             currentBuffer = (Buffer)(exchanger.rendezvous(currentBuffer));
         }
       }
       catch (BrokenBarrierException ex) {
         return;
       }
       catch (InterruptedException ex) {
         Thread.currentThread().interrupt();
       }
     }
   }

   void start() {
     new Thread(new FillingLoop()).start();
     new Thread(new EmptyingLoop()).start();
   }
 }

    [ Introduction to this package. ]

    • Field Detail

      • parties_

        protected final int parties_

      • broken_

        protected boolean broken_

      • entries_

        protected int entries_
        Number of threads that have entered rendezvous

      • departures_

        protected long departures_
        Number of threads that are permitted to depart rendezvous

      • entryGate_

        protected final Semaphore entryGate_
        Incoming threads pile up on entry until last set done.

      • slots_

        protected final Object[] slots_
        Temporary holder for items in exchange

    • Constructor Detail

      • Rendezvous

        public Rendezvous​(int parties)
        Create a Barrier for the indicated number of parties, and the default Rotator function to run at each barrier point.
        Throws:
        IllegalArgumentException - if parties less than or equal to zero.

      • Rendezvous

        public Rendezvous​(int parties,
                  Rendezvous.RendezvousFunction function)
        Create a Barrier for the indicated number of parties. and the given function to run at each barrier point.
        Throws:
        IllegalArgumentException - if parties less than or equal to zero.

    • Method Detail

      • setRendezvousFunction

        public Rendezvous.RendezvousFunction setRendezvousFunction​(Rendezvous.RendezvousFunction function)
        Set the function to call at the point at which all threads reach the rendezvous. This function is run exactly once, by the thread that trips the barrier. The function is not run if the barrier is broken.
        Parameters:
        function - the function to run. If null, no function is run.
        Returns:
        the previous function

      • parties

        public int parties()
        Description copied from interface: Barrier
        Return the number of parties that must meet per barrier point. The number of parties is always at least 1.
        Specified by:
        parties in interface Barrier

      • broken

        public boolean broken()
        Description copied from interface: Barrier
        Returns true if the barrier has been compromised by threads leaving the barrier before a synchronization point (normally due to interruption or timeout). Barrier methods in implementation classes throw throw BrokenBarrierException upon detection of breakage. Implementations may also support some means to clear this status.
        Specified by:
        broken in interface Barrier

      • restart

        public void restart()
        Reset to initial state. Clears both the broken status and any record of waiting threads, and releases all currently waiting threads with indeterminate return status. This method is intended only for use in recovery actions in which it is somehow known that no thread could possibly be relying on the the synchronization properties of this barrier.

      • rendezvous

        public Object rendezvous​(Object x)
                  throws InterruptedException,
                         BrokenBarrierException
        Enter a rendezvous; returning after all other parties arrive.
        Parameters:
        x - the item to present at rendezvous point. By default, this item is exchanged with another.
        Returns:
        an item x given by some thread, and/or processed by the rendezvousFunction.
        Throws:
        BrokenBarrierException - if any other thread in any previous or current barrier since either creation or the last restart operation left the barrier prematurely due to interruption or time-out. (If so, the broken status is also set.) Also returns as broken if the RendezvousFunction encountered a run-time exception. Threads that are noticed to have been interrupted after being released are not considered to have broken the barrier. In all cases, the interruption status of the current thread is preserved, so can be tested by checking Thread.interrupted.
        InterruptedException - if this thread was interrupted during the exchange. If so, broken status is also set.

      • attemptRendezvous

        public Object attemptRendezvous​(Object x,
                                long msecs)
                         throws InterruptedException,
                                TimeoutException,
                                BrokenBarrierException
        Wait msecs to complete a rendezvous.
        Parameters:
        x - the item to present at rendezvous point. By default, this item is exchanged with another.
        msecs - The maximum time to wait.
        Returns:
        an item x given by some thread, and/or processed by the rendezvousFunction.
        Throws:
        BrokenBarrierException - if any other thread in any previous or current barrier since either creation or the last restart operation left the barrier prematurely due to interruption or time-out. (If so, the broken status is also set.) Also returns as broken if the RendezvousFunction encountered a run-time exception. Threads that are noticed to have been interrupted after being released are not considered to have broken the barrier. In all cases, the interruption status of the current thread is preserved, so can be tested by checking Thread.interrupted.
        InterruptedException - if this thread was interrupted during the exchange. If so, broken status is also set.
        TimeoutException - if this thread timed out waiting for the exchange. If the timeout occured while already in the exchange, broken status is also set.