Java并发编程-Condition使用实例

本文将简单介绍用于线程协作的Condition, 并给出一个例子，实现一个多线程题目--有三个线程分别打印A、B、C,请用多线程编程实现，在屏幕上循环打印10次ABCABC…
。

Condition是从JDK 1.5开始有的。API是这么描述的：





以上是Condition接口定义的方法，await()对应于Object#wait()，signal()对应于Object#notify()，signalAll()对应于Object#notifyAll()。

Condition是与Lock结合使用的，通过Lock.newCondition()方法能够创建与Lock绑定的Condition实例。

Lock和Condition的关系就如同 Object.wait(),Object.notify()方法和synchronized一样，它们都可以配合使用完成对多线程协作的控制。

JDK提供的ArrayBlockingQueue很好地诠释了Condition的使用。

public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    /** Condition for waiting takes */
    private final Condition notEmpty;
    /** Condition for waiting puts */
    private final Condition notFull;


    /**
     * Inserts element at current put position, advances, and signals.
     * Call only when holding lock.
     */
    private void insert(E x) {
        items[putIndex] = x;
        putIndex = inc(putIndex);
        ++count;
        notEmpty.signal();
    }

    /**
     * Extracts element at current take position, advances, and signals.
     * Call only when holding lock.
     */
    private E extract() {
        final Object[] items = this.items;
        E x = this.<E>cast(items[takeIndex]);
        items[takeIndex] = null;
        takeIndex = inc(takeIndex);
        --count;
        notFull.signal();
        return x;
    }

    /**
     * Deletes item at position i.
     * Utility for remove and iterator.remove.
     * Call only when holding lock.
     */
    void removeAt(int i) {
        final Object[] items = this.items;
        // if removing front item, just advance
        if (i == takeIndex) {
            items[takeIndex] = null;
            takeIndex = inc(takeIndex);
        } else {
            // slide over all others up through putIndex.
            for (;;) {
                int nexti = inc(i);
                if (nexti != putIndex) {
                    items[i] = items[nexti];
                    i = nexti;
                } else {
                    items[i] = null;
                    putIndex = i;
                    break;
                }
            }
        }
        --count;
        notFull.signal();
    }

/**
     * Inserts the specified element at the tail of this queue, waiting
     * for space to become available if the queue is full.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
                notFull.await();
            insert(e);
        } finally {
            lock.unlock();
        }
    }

    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();
            return extract();
        } finally {
            lock.unlock();
        }
    }
... ... 
}

下面就使用Condition来完成一个线程面试题：有三个线程分别打印A、B、C,请用多线程编程实现，在屏幕上循环打印10次ABCABC…

程序如下：

package my.thread.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Logger;

/**
 * 题目：有三个线程分别打印A、B、C,请用多线程编程实现，在屏幕上循环打印10次ABCABC…
 * 
 * 本程序采用Lock和Condition来实现。
 * 
 * @author Eric
 * 
 */
public class ConditionExample {

	private Lock lock = new ReentrantLock();

	private Condition conditionA = lock.newCondition();
	private Condition conditionB = lock.newCondition();
	private Condition conditionC = lock.newCondition();

	/** 当前线程的名字 */
	private char currentThreadName = 'A';

	private static final Logger logger = Logger
			.getLogger("my.thread.test.OrderPrintTest");

	public static void main(String[] args) {

		ConditionExample ce = new ConditionExample();

		ExecutorService service = Executors.newFixedThreadPool(3);
		service.execute(ce.new ThreadA());
		service.execute(ce.new ThreadB());
		service.execute(ce.new ThreadC());

		service.shutdown();
	}

	private class ThreadA implements Runnable {
		public void run() {

			for (int i = 0; i < 10; i++) {
				lock.lock();
				try {
					while (currentThreadName != 'A') {
						try {
							/*
							 * 如果当前线程名字不是A，那么ThreadA就处理等待状态
							 */
							conditionA.await();
						} catch (InterruptedException e) {
							logger.severe(e.getLocalizedMessage());
						}
					}

					/*
					 * 打印出第几遍以及A信息
					 */
					System.out.println(String.format("第%d遍", i + 1));
					System.out.println("A");

					/*
					 * 将当前线程名置为B， 然后通知ThreadB执行
					 */
					currentThreadName = 'B';
					conditionB.signal();

				} finally {
					lock.unlock();
				}
			}
		}

	}

	private class ThreadB implements Runnable {
		public void run() {
			for (int i = 0; i < 10; i++) {
				lock.lock();
				try {
					while (currentThreadName != 'B') {
						try {
							/*
							 * 如果当前线程名字不是B，那么ThreadB就处理等待状态
							 */
							conditionB.await();
						} catch (InterruptedException e) {
							logger.severe(e.getLocalizedMessage());
						}
					}

					/*
					 * 打印信息B
					 */
					System.out.println("B");

					/*
					 * 将当前线程值置为C 并通过ThreadC来执行
					 */
					currentThreadName = 'C';
					conditionC.signal();

				} finally {
					lock.unlock();
				}
			}

		}

	}

	private class ThreadC implements Runnable {

		public void run() {
			for (int i = 0; i < 10; i++) {
				lock.lock();
				try {
					while (currentThreadName != 'C') {
						try {
							/*
							 * 如果当前线程名字不是C，那么ThreadC就处理等待状态
							 */
							conditionC.await();
						} catch (InterruptedException e) {
							logger.severe(e.getLocalizedMessage());
						}
					}

					/*
					 * 打印信息C
					 */
					System.out.println("C");
					System.out.println();

					/*
					 * 将当前线程值置为A 并通过ThreadA来执行
					 */
					currentThreadName = 'A';
					conditionA.signal();

				} finally {
					lock.unlock();
				}

			}
		}
	}
}

程序运行结果如下：
第1遍
A
B
C

第2遍
A
B
C

第3遍
A
B
C

第4遍
A
B
C

第5遍
A
B
C

第6遍
A
B
C

第7遍
A
B
C

第8遍
A
B
C

第9遍
A
B
C

第10遍
A
B
C