**锁降级：将写入锁降级为读锁，读锁不能升级为写锁，ReentrantReadWriter 可重入读写锁。

ReentrantReadWriteLock rwlock=new ReentrantReadWriteLock();
        ReentrantReadWriteLock.ReadLock readLock=rwlock.readLock();
        ReentrantReadWriteLock.WriteLock writeLock= rwlock.writeLock();
//        readLock.lock();
//        System.out.println("--read");
        //锁降级
        //获取写锁
        writeLock.lock();
        System.out.println("atguigu");
        //获取读锁
        readLock.lock();
        System.out.println("--read");
        //释放写锁
        writeLock.unlock();
        //释放读锁
        readLock.unlock();