4 互斥锁的弱化版本：读写锁

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package Test;import java.util.Random;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;/* * 互斥锁稍微弱化的版本：读写锁，可以一起读，读写互斥写写互斥 * */public class RWLock {static int k = 1;static ReadWriteLock rwl = new ReentrantReadWriteLock();public static void main(String[] args){for(int i=0;i<2;i++){new Thread(new Runnable(){public void run(){while(true){rwl.readLock().lock();try{System.out.println("读："+Thread.currentThread().getName()+":"+k);try{Thread.sleep(500);}catch(InterruptedException e){}}finally{rwl.readLock().unlock();}}}}).start();}for(int i=0;i<2;i++){new Thread(new Runnable(){public void run(){while(true){rwl.writeLock().lock();try{k = new Random().nextInt(10);try {Thread.sleep(500);System.out.println("写："+Thread.currentThread().getName()+":"+k);} catch (InterruptedException e) {}}finally{rwl.writeLock().unlock();}}}}).start();}}}

读写锁的一个应用：

package Test;import java.util.HashMap;import java.util.Map;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;/* * 读写锁的例子：cache * */public class SynCache {private Map<String,Object> map = new HashMap<String,Object>();private ReadWriteLock rwl = new ReentrantReadWriteLock();public Object get(String s){Object o = null;rwl.readLock().lock();try{o = map.get(s);if(o==null){rwl.readLock().unlock();rwl.writeLock().lock();try{if(map.get(s)==null) o = "get data from base";else   //之前已经有个线程去DB得到数据o = map.get(s);}finally{rwl.writeLock().unlock();}}}finally{rwl.readLock().unlock();}return o;}}