Java并发编程: 使用Exchanger实现线程间的数据交换

本文介绍Exchanger工具类, 然后采用Exchanger给出一个两个线程交换数值的简单实例。

1. Exchanger介绍

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

从上面的注释中可以看出：Exchanger提供了一个同步点，在这个同步点，两个线程可以交换数据。每个线程通过exchange()方法的入口提供数据给另外的线程，并接收其它线程提供的数据，并返回。

Exchanger通过Lock和Condition来完成功能，Exchanger的一个重要的public方法是exchange方法，用于线程的数据交换, 相关的类图以及详细的Exchanger类内容如下：

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * A synchronization point at which two threads can exchange objects.
 * Each thread presents some object on entry to the {@link #exchange
 * exchange} method, and receives the object presented by the other
 * thread on return.
 *
 * <p><b>Sample Usage:</b>
 * Here are the highlights of a class that uses an <tt>Exchanger</tt> 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.
 * <pre>
 * 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();
 *   }
 * }
 * </pre>
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The type of objects that may be exchanged
 */
public class Exchanger<V> {
    private final ReentrantLock lock = new ReentrantLock();
    private final Condition taken = lock.newCondition();

    /** Holder for the item being exchanged */
    private V item;
    
    /**
     * Arrival count transitions from 0 to 1 to 2 then back to 0
     * during an exchange.
     */
    private int arrivalCount;

    /**
     * Main exchange function, handling the different policy variants.
     */
    private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
        lock.lock();
        try {
            V other;

            // If arrival count already at two, we must wait for
            // a previous pair to finish and reset the count;
            while (arrivalCount == 2) {
                if (!timed)
                    taken.await();
                else if (nanos > 0) 
                    nanos = taken.awaitNanos(nanos);
                else 
                    throw new TimeoutException();
            }

            int count = ++arrivalCount;

            // If item is already waiting, replace it and signal other thread
            if (count == 2) { 
                other = item;
                item = x;
                taken.signal();
                return other;
            }

            // Otherwise, set item and wait for another thread to
            // replace it and signal us.

            item = x;
            InterruptedException interrupted = null;
            try { 
                while (arrivalCount != 2) {
                    if (!timed)
                        taken.await();
                    else if (nanos > 0) 
                        nanos = taken.awaitNanos(nanos);
                    else 
                        break; // timed out
                }
            } catch (InterruptedException ie) {
                interrupted = ie;
            }

            // Get and reset item and count after the wait.
            // (We need to do this even if wait was aborted.)
            other = item;
            item = null;
            count = arrivalCount;
            arrivalCount = 0; 
            taken.signal();
            
            // If the other thread replaced item, then we must
            // continue even if cancelled.
            if (count == 2) {
                if (interrupted != null)
                    Thread.currentThread().interrupt();
                return other;
            }

            // If no one is waiting for us, we can back out
            if (interrupted != null) 
                throw interrupted;
            else  // must be timeout
                throw new TimeoutException();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Create a new Exchanger.
     **/
    public Exchanger() {
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * it is {@link Thread#interrupt interrupted}),
     * and then transfers the given object to it, receiving its object
     * in return.
     * <p>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.
     * <p>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:
     * [list]
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread.
     * [/list]
     * <p>If the current thread:
     * [list]
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * for the exchange, 
     * [/list]
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * @param x the object to exchange
     * @return the object provided by the other thread.
     * @throws InterruptedException if current thread was interrupted 
     * while waiting
     **/
    public V exchange(V x) throws InterruptedException {
        try {
            return doExchange(x, false, 0);
        } catch (TimeoutException cannotHappen) { 
            throw new Error(cannotHappen);
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * it is {@link Thread#interrupt interrupted}, or the specified waiting
     * time elapses),
     * and then transfers the given object to it, receiving its object
     * in return.
     *
     * <p>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.
     *
     * <p>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:
     * [list]
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * [/list]
     * <p>If the current thread:
     * [list]
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * for the exchange, 
     * [/list]
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown.
     * If the time is 
     * less than or equal to zero, the method will not wait at all.
     *
     * @param x the object to exchange
     * @param timeout the maximum time to wait
     * @param unit the time unit of the <tt>timeout</tt> argument.
     * @return the object provided by the other thread.
     * @throws InterruptedException if current thread was interrupted
     * while waiting
     * @throws TimeoutException if the specified waiting time elapses before
     * another thread enters the exchange.
     **/
    public V exchange(V x, long timeout, TimeUnit unit) 
        throws InterruptedException, TimeoutException {
        return doExchange(x, true, unit.toNanos(timeout));
    }

}

2. Exchanger工具类的使用案例
本文给出一个简单的例子，实现两个线程之间交换数据，用Exchanger来做非常简单。

package my.concurrent.exchanger;

import java.util.concurrent.Exchanger;
import java.util.concurrent.atomic.AtomicReference;

public class ThreadA implements Runnable {

	private final Exchanger<Integer> exchanger;

	private final AtomicReference<Integer> last = new AtomicReference<Integer>(
			5);

	public ThreadA(Exchanger<Integer> exchanger) {
		this.exchanger = exchanger;
	}

	public void run() {
		try {
			while (true) {
				last.set(exchanger.exchange(last.get()));
				System.out.println(" After calling exchange. Thread A has value: " + last.get());
				Thread.sleep(2000);
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

}

package my.concurrent.exchanger;

import java.util.concurrent.Exchanger;
import java.util.concurrent.atomic.AtomicReference;

public class ThreadB implements Runnable {

	private Exchanger<Integer> exchanger;

	private final AtomicReference<Integer> last = new AtomicReference<Integer>(
			10);

	public ThreadB(Exchanger<Integer> exchanger) {
		this.exchanger = exchanger;
	}

	public void run() {
		try {
			while (true) {
				last.set(exchanger.exchange(last.get()));
				System.out.println(" After calling exchange. Thread B has value: " + last.get());
				Thread.sleep(2000);
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

}

package my.concurrent.exchanger;

import java.util.concurrent.Exchanger;

public class ExchangerTest {

	public static void main(String[] args) {
		Exchanger<Integer> exchanger = new Exchanger<Integer>();
		new Thread(new ThreadA(exchanger)).start();
		new Thread(new ThreadB(exchanger)).start();
	}

}

运行一段时间之后的输出结果如下：
After calling exchange. Thread B has value: 5
After calling exchange. Thread A has value: 10
After calling exchange. Thread B has value: 10
After calling exchange. Thread A has value: 5
After calling exchange. Thread A has value: 10
After calling exchange. Thread B has value: 5
After calling exchange. Thread B has value: 10
After calling exchange. Thread A has value: 5
After calling exchange. Thread A has value: 10
After calling exchange. Thread B has value: 5
After calling exchange. Thread B has value: 10
After calling exchange. Thread A has value: 5
After calling exchange. Thread A has value: 10
After calling exchange. Thread B has value: 5
After calling exchange. Thread A has value: 5
After calling exchange. Thread B has value: 10
After calling exchange. Thread A has value: 10
After calling exchange. Thread B has value: 5
After calling exchange. Thread B has value: 10
After calling exchange. Thread A has value: 5
After calling exchange. Thread B has value: 5
After calling exchange. Thread A has value: 10
After calling exchange. Thread A has value: 5
After calling exchange. Thread B has value: 10
After calling exchange. Thread B has value: 5
After calling exchange. Thread A has value: 10
After calling exchange. Thread B has value: 10
After calling exchange. Thread A has value: 5
After calling exchange. Thread B has value: 5
After calling exchange. Thread A has value: 10



可以看出：两个线程的数据一直都在相互交换。