请稍等 ...

锁降级的问题

图片描述

锁降级的时候这个线程同时持有读锁和写锁，是否和要么多读要么一写这句话有冲突呢？

如果在降级的时候不释放写锁，我发现其他线程想要获取读锁还是获取不到呢老师，也就是说即使降级成功后，也得等释放了写锁其他读的线程才能得到读锁，大家还是不能跟它一起读取，除非先释放掉写锁。实验代码如下

public class Upgrading {
    private static ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock(false);
    private static ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    private static ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();
    private static void readUpgrading(){
        readLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"得到了读锁，正在读取");
            Thread.sleep(1000);
            System.out.println("升级会带来阻塞");
            writeLock.lock(); //升级为写锁
            System.out.println(Thread.currentThread().getName() + "获取到了写锁，升级成功");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println(Thread.currentThread().getName()+"释放读锁");
            readLock.unlock();
        }
    }

    private static void writeDowngrading(){
        writeLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"得到了写锁，正在写入");
            Thread.sleep(1000);
            readLock.lock();
            System.out.println("在不释放写锁的情况下，直接获取读锁，成功降级");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println(Thread.currentThread().getName()+"释放读锁");
            readLock.unlock();
            System.out.println(Thread.currentThread().getName()+"释放写锁");
            //writeLock.unlock();  //这里选择不释放写锁
        }
    }

    public static void main(String[] args) throws InterruptedException {
        System.out.println("先演示降级是可以的");
        Thread thread1 = new Thread(()->writeDowngrading(),"Thread1");
        thread1.start();
//        thread1.join();
//        System.out.println("----------------");
//
//        System.out.println("演示升级是不行的");
//        Thread thread2 = new Thread(()->readUpgrading(),"Thread2");
//        thread2.start();


        Thread thread3 = new Thread(()->readUpgrading(),"Thread3");
        thread3.start();

    }
}

控制台输出
图片描述