package cn.itcast.heima2;

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

public class ReadWriteLockTest {

    static Random rd = new Random();

    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(rd.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。
    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();
        }

    }
}

一个线程在写的时候其他线程都不能打断写入过程，就是写入的时候不能读取

package cn.itcast.heima2;

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

    }

    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();//A
                try{
                    if(value==null){//如果有多个线程同时到达A点了，那这个数据要写入多次，这是不对的
                        value = "aaaa";//实际失去queryDB();
                    }
                }finally{
                    rwl.writeLock().unlock();
                }
                rwl.readLock().lock();
            }
        }finally{
            rwl.readLock().unlock();
        }
        return value;
    }
}