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  • 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.

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  • 原文地址:https://www.cnblogs.com/balaamwe/p/2612210.html
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