edu.emory.mathcs.backport.java.util.concurrent
Class Exchanger

java.lang.Object
  edu.emory.mathcs.backport.java.util.concurrent.Exchanger

public class Exchanger

extends java.lang.Object

A synchronization point at which threads can pair and swap elements within pairs. Each thread presents some object on entry to the exchange method, matches with a partner thread, and receives its partner's object on return.

Sample Usage: Here are the highlights of a class that uses an 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.

       } catch (InterruptedException ex) { ... handle ... }
     }
   }

   class EmptyingLoop implements Runnable {
     public void run() {
       DataBuffer currentBuffer = initialFullBuffer;
       try {
         while (currentBuffer != null) {
           takeFromBuffer(currentBuffer);
           if (currentBuffer.isEmpty())
             currentBuffer = exchanger.exchange(currentBuffer);
         }
       } catch (InterruptedException ex) { ... handle ...}
     }
   }

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

Memory consistency effects: For each pair of threads that successfully exchange objects via an , actions prior to the in each thread happen-before those subsequent to a return from the corresponding in the other thread.

Since:

1.5

Author:

Doug Lea and Bill Scherer and Michael Scott

Constructor Summary

Exchanger()
Creates a new Exchanger.

Method Summary

java.lang.Object	exchange(java.lang.Object x) Waits for another thread to arrive at this exchange point (unless it is interrupted), and then transfers the given object to it, receiving its object in return.
java.lang.Object	exchange(java.lang.Object x, long timeout, TimeUnit unit) Waits for another thread to arrive at this exchange point (unless the current thread is interrupted or the specified waiting time elapses), and then transfers the given object to it, receiving its object in return.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Exchanger

public Exchanger()

Creates a new Exchanger.

Method Detail

exchange

public java.lang.Object exchange(java.lang.Object x)
                          throws java.lang.InterruptedException

Waits for another thread to arrive at this exchange point (unless it is interrupted), and then transfers the given object to it, receiving its object in return.

If another thread is already waiting at the exchange point then it is resumed for thread scheduling purposes and receives the object passed in by the current thread. The current thread returns immediately, receiving the object passed to the exchange by that other thread.

If no other thread is already waiting at the exchange then the current thread is disabled for thread scheduling purposes and lies dormant until one of two things happens:

• Some other thread enters the exchange; or
• Some other thread interrupts the current thread.

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted while waiting for the exchange,

then InterruptedException is thrown and the current thread's interrupted status is cleared.

Parameters:

x - the object to exchange

Returns:

the object provided by the other thread

Throws:

java.lang.InterruptedException - if the current thread was interrupted while waiting

exchange

public java.lang.Object exchange(java.lang.Object x,
                                 long timeout,
                                 TimeUnit unit)
                          throws java.lang.InterruptedException,
                                 TimeoutException

Waits for another thread to arrive at this exchange point (unless the current thread is interrupted or the specified waiting time elapses), and then transfers the given object to it, receiving its object in return.

If another thread is already waiting at the exchange point then it is resumed for thread scheduling purposes and receives the object passed in by the current thread. The current thread returns immediately, receiving the object passed to the exchange by that other thread.

If no other thread is already waiting at the exchange then the current thread is disabled for thread scheduling purposes and lies dormant until one of three things happens:

• Some other thread enters the exchange; or
• Some other thread interrupts the current thread; or
• The specified waiting time elapses.

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted while waiting for the exchange,

then InterruptedException is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then TimeoutException is thrown. If the time is less than or equal to zero, the method will not wait at all.

Parameters:

x - the object to exchange

timeout - the maximum time to wait

unit - the time unit of the timeout argument

Returns:

the object provided by the other thread

Throws:

java.lang.InterruptedException - if the current thread was interrupted while waiting

TimeoutException - if the specified waiting time elapses before another thread enters the exchange