锁的升降级

本文基于ReentrantReadWriteLock来说明它自己的锁升级的策略。

读写锁总结

这里我先说明下读写锁进行下总结：

要么多读，要么一写；读写锁适用去读多写锁的情况。

公平情况下，读写锁都不能插队；非公平情况下，写锁可以插队，读锁只有在头结点不是写等待的情况下插队。

读锁是否可以升级到写锁

　　这里我先给出结论：读锁不可以升级为写锁；若两个线程的读锁都想升级写锁，则需要对方都释放自己锁，而双方都不释放，就会产生死锁。

　　实现代码如下所示

　　

package com.yang.lock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * 本实例演示读锁的升级
 */
public class ReentrantReadWriteUpgardeDemo {
    private static ReentrantReadWriteLock reentrantReadWriteLock=new ReentrantReadWriteLock(false);
    private static ReentrantReadWriteLock.ReadLock readLock=reentrantReadWriteLock.readLock();
    private static ReentrantReadWriteLock.WriteLock writeLock=reentrantReadWriteLock.writeLock();

    public void upgarde(){
        System.out.println(Thread.currentThread().getName()+"尝试获取读锁");
        readLock.lock();
        try{
            System.out.println(Thread.currentThread().getName()+"获取到了读锁");
            System.out.println(Thread.currentThread().getName()+"阻塞获取写锁");
            writeLock.lock();

        }finally {
            readLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReentrantReadWriteUpgardeDemo reentrantReadWriteUpgardeDemo=new ReentrantReadWriteUpgardeDemo();
        new Thread(()->reentrantReadWriteUpgardeDemo.upgarde(),"线程1").start();
        new Thread(()->reentrantReadWriteUpgardeDemo.upgarde(),"线程2").start();

    }
}

　　

写锁是否可以降级为读锁

　　这里我先给出结论：写锁是可以降级为读锁的；写锁只有一个，当写锁降级为读锁时，所有的都是读。

　　实现代码如下所示

　　

package com.yang.lock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * 本实例演示读锁的升级
 */
public class ReentrantReadWriteDowngradeDemo {
    private static ReentrantReadWriteLock reentrantReadWriteLock=new ReentrantReadWriteLock(false);
    private static ReentrantReadWriteLock.ReadLock readLock=reentrantReadWriteLock.readLock();
    private static ReentrantReadWriteLock.WriteLock writeLock=reentrantReadWriteLock.writeLock();

    public void downgrade(){
        System.out.println(Thread.currentThread().getName()+"尝试获取写锁");
        writeLock.lock();
        try{
            System.out.println(Thread.currentThread().getName()+"获取到了写锁");
            System.out.println(Thread.currentThread().getName()+"降级获取读锁");
            readLock.lock();
            try{

            }finally {
                readLock.unlock();
            }
        }finally {
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReentrantReadWriteDowngradeDemo reentrantReadWriteUpgardeDemo=new ReentrantReadWriteDowngradeDemo();
        new Thread(()->reentrantReadWriteUpgardeDemo.downgrade(),"线程1").start();
        new Thread(()->reentrantReadWriteUpgardeDemo.downgrade(),"线程2").start();

    }
}

　　运行结果如下图所示：