JAVA ReentrantReadWriteLock AND 缓存系统

package cn.itcast.hemai2;

import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
//notify和wait必须 在同步代码块里面写，而不能存在某个普通的代码或者代码块中。
public class ReadWriteLockTest {

public static void main(String[] args) {
final Queue3 q3 = new Queue3();
for(int i=0;i<3;i++)
{
new Thread(){
public void run(){
while(true){
q3.get();
}
}

}.start();

new Thread(){
public void run(){
while(true){
q3.put(new Random().nextInt(10000));
}
}

}.start();
}
}

}
/**
* 在写的情况 下就不能再读。
* @author admin
*
*/
class Queue3{
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。
ReadWriteLock rwl = new ReentrantReadWriteLock();
public void get(){
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();
}
}

public void put(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 cn.itcast.hemai2;

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

public class CacheDemo {
private Map<String, Object> cache = new HashMap<String, Object>();
public static void main(String[] args) {
// TODO Auto-generated method stub

}
/**
* 读写互斥。多线程 并发读取，写读取互斥 。
*/
private ReadWriteLock rwl = new ReentrantReadWriteLock();
public  Object getData(String key){
rwl.readLock().lock();
Object value = null;
try{
value = cache.get(key);
if(value == null){
rwl.readLock().unlock();
rwl.writeLock().lock();
try{
if(value==null){
value = "aaaa";//实际失去queryDB();
}
}finally{
rwl.writeLock().unlock();
}
rwl.readLock().lock();
}
}finally{
rwl.readLock().unlock();
}
return value;
}
}