Class Exchanger<V>

java.lang.Object

java.util.concurrent.Exchanger<V>

Type Parameters:

V - The type of objects that may be exchanged

public class Exchanger<V>
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.

Since:

1.5

Constructor Summary

Constructors

Constructor	Description
Exchanger()	Creates a new Exchanger.

Method Summary

Modifier and Type	Method	Description
V	exchange(V x)	Waits for another thread to arrive at this exchange point (unless the current thread is interrupted), and then transfers the given object to it, receiving its object in return.
V	exchange(V 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 declared in class Object

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

Modifier and Type	Method	Description
protected Object	clone()	Answers a new instance of the same class as the receiver, whose slots have been filled in with the values in the slots of the receiver.
boolean	equals(Object o)	Compares the argument to the receiver, and answers true if they represent the same object using a class specific comparison.
protected void	finalize()	Deprecated, for removal: This API element is subject to removal in a future version. May cause performance issues, deadlocks and hangs.
final Class<? extends Object>	getClass()	Answers the unique instance of java.lang.Class which represents the class of the receiver.
int	hashCode()	Answers an integer hash code for the receiver.
final void	notify()	Causes one thread which is waiting on the receiver to be made ready to run.
final void	notifyAll()	Causes all threads which are waiting on the receiver to be made ready to run.
String	toString()	Answers a string containing a concise, human-readable description of the receiver.
final void	wait()	Causes the thread which sent this message to be made not ready to run pending some change in the receiver (as indicated by notify or notifyAll).
final void	wait(long time)	Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.
final void	wait(long time, int frac)	Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.

Constructor Details

Exchanger

public Exchanger()

Creates a new Exchanger.

Method Details

exchange

public V exchange(V x)
           throws InterruptedException

Waits for another thread to arrive at this exchange point (unless the current thread 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:

InterruptedException - if the current thread was interrupted while waiting

exchange

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

InterruptedException - if the current thread was interrupted while waiting

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