java实现信号量

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();
	}
}  

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();
	}
}  

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 + ")"; //$NON-NLS-1$ //$NON-NLS-2$
	}