Class Rendezvous

java.lang.Object
EDU.oswego.cs.dl.util.concurrent.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 Details

    • 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

    • rendezvousFunction_

      protected Rendezvous.RendezvousFunction rendezvousFunction_
      The function to run at rendezvous point

  • Constructor Details

    • 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 Details

    • 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.

    • doRendezvous

      protected Object doRendezvous(Object x, boolean timed, long msecs) throws InterruptedException, TimeoutException, BrokenBarrierException
      Throws:
      InterruptedException
      TimeoutException
      BrokenBarrierException