jdk1.5读写锁技术的妙用

/**
 * ReadWriteLockTest.java
 * cn.com.songjy.test.socket.thread
 * Function： TODO 
 *
 *   version    date      author
 * ──────────────────────────────────
 *   	1.0	 2013-8-19    songjy
 *
 * Copyright (c) 2013, TNT All Rights Reserved.
 */

package cn.com.songjy.test.socket.thread;

import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * ClassName:ReadWriteLockTest 
 * jdk1.5读写锁技术的妙用
 * 
 * @author songjy
 * @version 1.0
 * @since v1.0
 * @Date 2013-8-19 下午9:11:37
 */

public class ReadWriteLockTest {

	private Log log = LogFactory.getLog(ReadWriteLockTest.class);

	public static void main(String[] args) {

		final Queue queue = new Queue();
		for (int i = 0; i < 3; i++) {
			new Thread() {
				public void run() {
					while (true) {
						queue.get();
					}
				}

			}.start();

			new Thread() {
				public void run() {
					while (true) {
						queue.put(new Random().nextInt(10000));
					}
				}

			}.start();
		}

	}

	/* 缓存模拟 */
	final ReadWriteLock rwl = new ReentrantReadWriteLock();
	Map<String, Object> cache = new HashMap<String, Object>();

	public Object getData(String key) {

		rwl.readLock().lock();// 上读锁
		Object obj = null;

		try {
			obj = cache.get(key);
			if (null == obj) {
				rwl.readLock().unlock();// 开读锁
				rwl.writeLock().lock();// 上写锁
				try {
					if (null == obj) {// 为啥还要判断一次呢，去掉本行判断难道会影响业务逻辑吗，答案是否定，去掉不会影响业务逻辑， 但该判断是必须的，是为了提高程序性能！原因是当2（多）个线程同时执行到了75行，则第一个线程上写锁并写入数据，第二个线程就阻塞，第一个线程接着开写锁上读锁，当最后开读锁完毕时，第二个线程马上进行上写锁，并再次写入数据，这时发生了不必要的第二次数据写入，因为第一次已经写入了,若有if (null == obj)的判断就不会发生第二次写数据的重复写入！
						obj = "obj";// 通常是去查询数据库等操作
					}
				} catch (Exception e) {
					log.error(e.getMessage(), e);
				} finally {
					rwl.writeLock().unlock();// 开写锁
				}
				rwl.readLock().lock();// 上读锁
			}
		} catch (Exception e) {
			log.error(e.getMessage(), e);
		} finally {
			rwl.readLock().unlock();// 开读锁
		}
		return obj;
	}

}

class Queue {
	private Log log = LogFactory.getLog(Queue.class);
	private Object data = null;// 共享数据，只能有一个线程能写该数据，但可以有多个线程同时读该数据。
	private ReadWriteLock rwl = new ReentrantReadWriteLock();

	public void get() {
		rwl.readLock().lock();
		try {
			log.info(Thread.currentThread().getName()
					+ " be ready to read data!");
			Thread.sleep((long) (Math.random() * 1000));
			log.info(Thread.currentThread().getName() + "have read data :"
					+ data);
		} catch (InterruptedException e) {
			log.error(e.getMessage(), e);
		} finally {
			rwl.readLock().unlock();
		}
	}

	public void put(Object data) {

		rwl.writeLock().lock();
		try {
			log.info(Thread.currentThread().getName()
					+ " be ready to write data!");
			Thread.sleep((long) (Math.random() * 1000));
			this.data = data;
			log.info(Thread.currentThread().getName() + " have write data: "
					+ data);
		} catch (InterruptedException e) {
			log.error(e.getMessage(), e);
		} finally {
			rwl.writeLock().unlock();
		}

	}
}

来自:http://down.51cto.com/data/443434