12.Java5读写锁技术的妙用

import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
 * 
 * @author LiTaiQing
 *
 */
public class ReadWriteLockTest {
    public static void main(String[] args){
        final Queue3 q3 = new Queue3();
        
        for (int i = 0; i < 3; i++) {
            new Thread(){
                @Override
                public void run(){
                    while(true){
                        q3.get();
                    }
                }
            }.start();
            
            new Thread(){
                @Override
                public void run() {
                    q3.put(new Random().nextInt(10000));
                };
            }.start();
            
        }
        
    }
    
}
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。
    //读写锁-实现类ReentrantReadWriteLock
    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();
        }
    }
}

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
 * 面试题，写一个缓存系统
 * @author LiTaiQing
 *
 */
public class CacheDemo {
    
    private Map<String,Object> cache = new HashMap<String,Object>();
    
    public static void main(String[] args) {
        
    }
    /**
     *    读写锁的应用
     */
    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 = "abc";//实际是去queryDB();
                    }
                }finally{
                    rwl.writeLock().unlock();
                }
                rwl.readLock().lock();
            }
        }finally{
            rwl.readLock().unlock();
        }
        return value;
    }
    
}