package com.ljq.test.thread;import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;/*** 读写锁* * @author Administrator* */
public class ReadWriteLockTest {public static void main(String[] args) {final ReadWrite rw = new ReadWrite();for (int i = 0; i < 3; i++) {new Thread() {public void run() {while (true) {rw.read();}}}.start();new Thread() {public void run() {while (true) {rw.write(new Random().nextInt(10000));}}}.start();}}
}/*** 读和写要互斥，因此要把它们放在同一个类中* * @author Administrator**/
class ReadWrite {private Object data = null;//共享数据，只能有一个线程写该数据，但可以有多个线程同时读该数据。ReadWriteLock rwl = new ReentrantReadWriteLock();/*** 读数据*/public void read() {rwl.readLock().lock();try {System.out.println(Thread.currentThread().getName() + " be ready to read data!");Thread.sleep((long) (Math.random() * 1000));System.out.println(Thread.currentThread().getName() + "have read data :" + data);} catch (InterruptedException e) {e.printStackTrace();} finally {rwl.readLock().unlock();}}/*** 写数据* * @param data*/public void write(Object data) {rwl.writeLock().lock();try {System.out.println(Thread.currentThread().getName() + " be ready to write data!");Thread.sleep((long) (Math.random() * 1000));this.data = data;System.out.println(Thread.currentThread().getName() + " have write data: " + data);} catch (InterruptedException e) {e.printStackTrace();} finally {rwl.writeLock().unlock();}}
}

package com.ljq.test.thread;import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;/*** 设计一个缓存系统* * * @author Administrator**/
public class CacheDemo {private Map<String, Object> cache = new HashMap<String, Object>();public static void main(String[] args) {String key = "name";CacheDemo cacheDemo = new CacheDemo();System.out.println(cacheDemo.getData(key)); //从数据库获取数据System.out.println(cacheDemo.getData(key)); //从缓存获取数据System.out.println(cacheDemo.getData(key)); //从缓存获取数据}private ReadWriteLock rwl = new ReentrantReadWriteLock();public Object getData(String key) {rwl.readLock().lock(); //上读锁Object value = null;try {value = cache.get(key); //先查询内部存储器中有没有要的值if (value == null) { //如果没有，就去数据库中查询，并将查到的结果存入内部存储器中//释放读锁、上写锁rwl.readLock().unlock();rwl.writeLock().lock();try {if (value == null) { //再次进行判断，防止多个写线程堵在这个地方重复写System.out.println("read data from database");value = "张三";cache.put(key, value);}} finally {//设置完成 释放写锁rwl.writeLock().unlock();}//恢复读写状态rwl.readLock().lock();}else{System.out.println("read data from cache");}} finally {rwl.readLock().unlock(); //释放读锁}return value;}
}