public class

Exchanger

extends Object
java.lang.Object
   ↳ java.util.concurrent.Exchanger<V>

Class Overview

A synchronization point at which two threads can exchange objects. Each thread presents some object on entry to the exchange method, and receives the object presented by the other thread on return.

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.full())
             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.empty())
             currentBuffer = exchanger.exchange(currentBuffer);
         }
       } catch (InterruptedException ex) { ... handle ...}
     }
   }

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

Summary

Public Constructors
Exchanger()
Create a new Exchanger.
Public Methods
V exchange(V 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.
V exchange(V x, long timeout, TimeUnit unit)
Waits for another thread to arrive at this exchange point (unless it is interrupted, or the specified waiting time elapses), and then transfers the given object to it, receiving its object in return.
[Expand]
Inherited Methods
From class java.lang.Object

Public Constructors

public Exchanger ()

Since: API Level 1

Create a new Exchanger.

Public Methods

public V exchange (V x)

Since: API Level 1

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
InterruptedException if current thread was interrupted while waiting

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

Since: API Level 1

Waits for another thread to arrive at this exchange point (unless it 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 current thread was interrupted while waiting
TimeoutException if the specified waiting time elapses before another thread enters the exchange.