org.metasyntactic.thread.concurrent
Class Rendezvous

java.lang.Object
  |
  +--org.metasyntactic.thread.concurrent.Rendezvous
All Implemented Interfaces:
Barrier

public class Rendezvous
extends java.lang.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();
   }
 }
 


Nested Class Summary
static interface Rendezvous.RendezvousFunction
          Interface for functions run at rendezvous points
static class Rendezvous.Rotator
          The default rendezvous function.
 
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
 java.lang.Object attemptRendezvous(java.lang.Object x, long msecs)
          Wait msecs to complete a rendezvous.
 boolean broken()
          Returns true if the barrier has been compromised by threads leaving the barrier before a synchronization point (normally due to interruption or timeout).
 int parties()
          Return the number of parties that must meet per barrier point.
 java.lang.Object rendezvous(java.lang.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.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

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.

Parameters:
parties -
Throws:
java.lang.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.

Parameters:
parties -
function -
Throws:
java.lang.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
Returns:
The number of barriers that must meet before all threads can continue

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
Returns:
True/False on whether or not the barrier has been comprimised by threads leaving

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 java.lang.Object rendezvous(java.lang.Object x)
                            throws java.lang.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.
java.lang.InterruptedException - if this thread was interrupted during the exchange. If so, broken status is also set.

attemptRendezvous

public java.lang.Object attemptRendezvous(java.lang.Object x,
                                          long msecs)
                                   throws java.lang.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.
java.lang.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.