java实现信号量

http://www.cnblogs.com/bronte/articles/2321188.html

       本文介绍的Semaphore实现基于synchronized,wait()和notify/notifyAll(),这是java并发包之前的典型实现方式.在eclipse的源码中可以找到不少这样的案例,下文中也会把eclipse中的几个实现类作为案例以分析之.

       注,这里介绍的信号量实现是基于java语言机制,用于实现多线程间的同步操作,所以对S,P(S),V(S)等概念的介绍将结合本文内容,做合适的调整,读者可阅读操作系统相关书籍的信号量章节获取标准定义.

*本文内容

---信号量简介

---典型案例

*Semaphore概述

---通常把一个非负整数称为Semaphore,表示为S.

S可以理解为可用的资源数量.这里不涉及进程问题,所以就假定S>=0.

---S实现的同步机制表示为PV原语操作

P(S):若S=0,线程进入等待队列;否则,—S;

V(S):++S,唤醒处于等待中的线程.

(注,P是荷兰语的Passeren，相当于英文的pass, V是荷兰语的Verhoog,相当于英文中的incremnet).

*案例

1)典型实现

这段程序源自ibm的一本并发书籍,实现了计数信号量{S|S∈{0,N}}和二元信号量(S={0,1})

public abstract class Semaphore {
	private int value = 0;

	public Semaphore() {
	}

	public Semaphore(int initial) {
		if (initial >= 0)
			value = initial;
		else
			throw new IllegalArgumentException("initial < 0");
	}

	public final synchronized void P() throws InterruptedException {
		while (value == 0)
			wait();
		value--;
	}

	protected final synchronized void Vc() {
		value++;
		notifyAll();
	}

	protected final synchronized void Vb() {
		value++;
		notifyAll();
		if (value > 1)
			value = 1;
	}

	public abstract void V();

	public String toString() {
		return ".value=" + value;
	}

}


public final class BinarySemaphore extends Semaphore {
	public BinarySemaphore() {
		super();
	}

	public BinarySemaphore(int initial) {
		super(initial);
		if (initial > 1)
			throw new IllegalArgumentException("initial > 1");
	}

	public final synchronized void V() {
		super.Vb();
	}
}

public final class CountingSemaphore extends Semaphore {
	public CountingSemaphore() {
		super();
	}

	public CountingSemaphore(int initial) {
		super(initial);
	}

	public final synchronized void V() {
		super.Vc();
	}
}

2)实现读写锁

eclipse使用它,解决日志操作相关类在map,数组中的同步问题.

/*******************************************************************************
 * Copyright (c) 2008, 2011 IBM Corporation and others
 * All rights reserved. This program and the accompanying materials are made
 * available under the terms of the Eclipse Public License v1.0 which
 * accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 ******************************************************************************/
package org.eclipse.equinox.log.internal;

public class BasicReadWriteLock {
	private int currentReaders = 0;
	private int writersWaiting = 0;
	private boolean writing = false;

	public synchronized void readLock() {
		while (writing || writersWaiting != 0) {
			try {
				wait();
			} catch (InterruptedException e) {
				// reset interrupted state but keep waiting
				Thread.currentThread().interrupt();
			}
		}
		currentReaders++;
	}

	public synchronized void readUnlock() {
		currentReaders--;
		notifyAll();
	}

	public synchronized void writeLock() {
		writersWaiting++;
		while (writing || currentReaders != 0) {
			try {
				wait();
			} catch (InterruptedException e) {
				// reset interrupted state but keep waiting
				Thread.currentThread().interrupt();
			}
		}
		writersWaiting--;
		writing = true;
	}

	public synchronized void writeUnlock() {
		writing = false;
		notifyAll();
	}
}

3)延迟信号量

这个信号量的亮点在acquire(long delay).

/*******************************************************************************
 * Copyright (c) 2003, 2006 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.core.runtime.internal.adaptor;

/**
 * Internal class.
 */
public class Semaphore {
	protected long notifications;

	public Semaphore(int count) {
		notifications = count;
	}

	/**
	 * Attempts to acquire this semaphore.  Returns only when the semaphore has been acquired.
	 */
	public synchronized void acquire() {
		while (true) {
			if (notifications > 0) {
				notifications--;
				return;
			}
			try {
				wait();
			} catch (InterruptedException e) {
				//Ignore
			}
		}
	}

	/**
	 * Attempts to acquire this semaphore.  Returns true if it was successfully acquired,
	 * and false otherwise.
	 */
	public synchronized boolean acquire(long delay) { //若传入负数,用于判断是否资源已被占
		long start = System.currentTimeMillis();
		long timeLeft = delay;
		while (true) {
			if (notifications > 0) {
				notifications--;
				return true;
			}
			if (timeLeft <= 0)        //在延迟后不再继续尝试获取锁
				return false;
			try {
				wait(timeLeft);
			} catch (InterruptedException e) {
				//Ignore
			}
			timeLeft = start + delay - System.currentTimeMillis();
		}
	}

	public synchronized void release() {
		notifications++;
		notifyAll();
	}

	// for debug only
	public String toString() {
		return "Semaphore(" + notifications + ")"; //$NON-NLS-1$ //$NON-NLS-2$
	}

*总结

---通过java的对象锁,wait/notify机制模拟的信号量,可以呈现多种形态以应对各种的互斥需求.

---本文给出的例子,具有普遍的适用性.在实践中,咱们可以根据需求定制各种信号量实现.

---jdk1.5提供了Semaphore的另一种实现机制.