EDU.oswego.cs.dl.util.concurrent

Class Rendezvous

Implemented Interfaces:
Barrier

public class Rendezvous
extends Object
implements Barrier

A rendezvous is a barrier that:

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. ]

Nested Class Summary

static interface
Rendezvous.RendezvousFunction
Interface for functions run at rendezvous points
static class
Rendezvous.Rotator
The default rendezvous function.

Field Summary

protected boolean
broken_
protected long
departures_
Number of threads that are permitted to depart rendezvous
protected int
entries_
Number of threads that have entered rendezvous
protected Semaphore
entryGate_
Incoming threads pile up on entry until last set done.
protected int
parties_
protected Rendezvous.RendezvousFunction
rendezvousFunction_
The function to run at rendezvous point
protected Object[]
slots_
Temporary holder for items in exchange

Constructor Summary

Rendezvous(int parties)
Create a Barrier for the indicated number of parties, and the default Rotator function to run at each barrier point.
Rendezvous(int parties, Rendezvous.RendezvousFunction function)
Create a Barrier for the indicated number of parties.

Method Summary

Object
attemptRendezvous(Object x, long msecs)
Wait msecs to complete a rendezvous.
boolean
broken()
protected Object
doRendezvous(Object x, boolean timed, long msecs)
int
parties()
Object
rendezvous(Object x)
Enter a rendezvous; returning after all other parties arrive.
void
restart()
Reset to initial state.
Rendezvous.RendezvousFunction
setRendezvousFunction(Rendezvous.RendezvousFunction function)
Set the function to call at the point at which all threads reach the rendezvous.

Field Details

broken_

protected boolean broken_


departures_

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


entries_

protected int entries_
Number of threads that have entered rendezvous


entryGate_

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


parties_

protected final int parties_


rendezvousFunction_

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


slots_

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

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.


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.

Method Details

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.
TimeoutException - if this thread timed out waiting for the exchange. If the timeout occured while already in the exchange, broken status is also set.


broken

public boolean broken()
Specified by:
broken in interface Barrier


doRendezvous

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


parties

public int parties()
Specified by:
parties in interface 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.


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.


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