关于读写锁的一些理解

同一线程的两种情况：

读读：

    public static void main(String[] args) throws InterruptedException {
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        readLock.lock();
        System.out.println("read1");
        
        readLock.lock();
        System.out.println("read2");
        readLock.unlock();
        
        readLock.unlock();
    }

输出：可重入

读写：

    public static void main(String[] args) throws InterruptedException {
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        readLock.lock();
        System.out.println("read1");

        writeLock.lock();
        System.out.println("write1");
        writeLock.unlock();

        readLock.unlock();
    }

输出：阻塞了。

写写：

    public static void main(String[] args) throws InterruptedException {
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        writeLock.lock();
        System.out.println("write1");

        writeLock.lock();
        System.out.println("write2");
        writeLock.unlock();

        writeLock.unlock();
    }

输出：可重入

写读：

    public static void main(String[] args) throws InterruptedException {
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        writeLock.lock();
        System.out.println("write1");

        readLock.lock();
        System.out.println("read1");
        readLock.unlock();

        writeLock.unlock();
    }

输出：文心一言说这种情况是锁降级，我不懂

不同线程的两种情况：

读读：

    public static void main(String[] args) throws InterruptedException {
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        readLock.lock();
        System.out.println("read1");

        Thread otherRead = new Thread(new Runnable() {
            @Override
            public void run() {
                readLock.lock();
                System.out.println("otherRead");
                readLock.unlock();
            }
        });
        otherRead.start();
        otherRead.join();
        readLock.unlock();
    }

输出：

read1
otherRead

读写：

    public static void main(String[] args) throws InterruptedException {
        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        readLock.lock();
        System.out.println("read1");

        Thread otherRead = new Thread(new Runnable() {
            @Override
            public void run() {
                writeLock.lock();
                System.out.println("otherWrite");
                writeLock.unlock();
            }
        });
        otherRead.start();
        otherRead.join();
        readLock.unlock();
    }

输出：

read1

写写：

    public static void main(String[] args) throws InterruptedException {

        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        writeLock.lock();
        System.out.println("write");

        Thread otherRead = new Thread(new Runnable() {
            @Override
            public void run() {
                writeLock.lock();
                System.out.println("otherWrite");
                writeLock.unlock();
            }
        });
        otherRead.start();
        otherRead.join();//等待其他线程完成。但是otherRead无法获取到锁，被阻塞。
        writeLock.unlock();
    }

输出：

write

写读：

    public static void main(String[] args) throws InterruptedException {

        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
        Lock readLock = lock.readLock();
        Lock writeLock = lock.writeLock();

        writeLock.lock();
        System.out.println("write");

        Thread otherRead = new Thread(new Runnable() {
            @Override
            public void run() {
                readLock.lock();
                System.out.println("otherRead");
                readLock.unlock();
            }
        });
        otherRead.start();
        otherRead.join();//依旧被阻塞，获取不到读锁
        writeLock.unlock();
    }

输出：

write