java实现信号量

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})

S示例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.

ReadWriteLock/*******************************************************************************

 * 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).

灵活的Semaphore/*******************************************************************************

 * 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.