java中的读/写锁

ReadWriteLock rtLock = new ReentrantReadWriteLock();
 rtLock.readLock().lock();
 System.out.println("get readLock.");
 rtLock.writeLock().lock();
 System.out.println("blocking");

ReadWriteLock rtLock = new ReentrantReadWriteLock();
rtLock.writeLock().lock();
System.out.println("writeLock");

rtLock.readLock().lock();
System.out.println("get read lock");

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

public class CacheDemo {
    /**
     * 缓存器,这里假设需要存储1000左右个缓存对象，按照默认的负载因子0.75，则容量=750，大概估计每一个节点链表长度为5个
     * 那么数组长度大概为：150,又有雨设置map大小一般为2的指数，则最近的数字为：128
     */
    private Map<String, Object> map = new HashMap<>(128);
    private ReadWriteLock rwl = new ReentrantReadWriteLock();
    public static void main(String[] args) {

    }
    public Object get(String id){
        Object value = null;
        rwl.readLock().lock();//首先开启读锁，从缓存中去取
        try{
            value = map.get(id);
            if(value == null){  //如果缓存中没有释放读锁，上写锁
                rwl.readLock().unlock();
                rwl.writeLock().lock();
                try{
                    if(value == null){ //防止多写线程重复查询赋值
                        value = "redis-value";  //此时可以去数据库中查找，这里简单的模拟一下
                    }
                    rwl.readLock().lock(); //加读锁降级写锁,不明白的可以查看上面锁降级的原理与保持读取数据原子性的讲解
                }finally{
                    rwl.writeLock().unlock(); //释放写锁
                }
            }
        }finally{
            rwl.readLock().unlock(); //最后释放读锁
        }
        return value;
    }
}