Exchanger

public class Exchanger extends 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. An Exchanger may be viewed as a bidirectional form of a SynchronousQueue. Exchangers may be useful in applications such as genetic algorithms and pipeline designs.

Sample Usage: Here are the highlights of a class that uses an Exchanger 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 {
   Exchanger<DataBuffer> exchanger = new Exchanger<>();
   DataBuffer initialEmptyBuffer = ... a made-up type
   DataBuffer initialFullBuffer = ...

   class FillingLoop implements Runnable {
     public void run() {
       DataBuffer currentBuffer = initialEmptyBuffer;
       try {
         while (currentBuffer != null) {
           addToBuffer(currentBuffer);
           if (currentBuffer.isFull())
             currentBuffer = exchanger.exchange(currentBuffer);
         }
       } 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 Exchanger, actions prior to the exchange() in each thread happen-before those subsequent to a return from the corresponding exchange() in the other thread.

Public Constructor Summary

Exchanger() Creates a new Exchanger.

Public Method Summary

V	exchange(V x, long timeout, TimeUnit unit) Waits for another thread to arrive at this exchange point (unless the current thread is {@linkplain Thread#interrupt interrupted} or the specified waiting time elapses), and then transfers the given object to it, receiving its object in return.
V	exchange(V x) Waits for another thread to arrive at this exchange point (unless the current thread is {@linkplain Thread#interrupt interrupted}), and then transfers the given object to it, receiving its object in return.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

public Exchanger ()

Creates a new Exchanger.

Public Methods

public V exchange (V x, long timeout, TimeUnit unit)

Waits for another thread to arrive at this exchange point (unless the current thread is {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt 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

InterruptedException	if the current thread was interrupted while waiting
TimeoutException	if the specified waiting time elapses before another thread enters the exchange

public V exchange (V x)

Waits for another thread to arrive at this exchange point (unless the current thread is {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

• has its interrupted status set on entry to this method; or
• is {@linkplain Thread#interrupt 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

InterruptedException	if the current thread was interrupted while waiting