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关于读写锁的一些理解

news 来源：原创 2025/5/13 9:43:04

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() {@Overridepublic 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() {@Overridepublic void run() {writeLock.lock();System.out.println("otherWrite");writeLock.unlock();}});otherRead.start();otherRead.join();readLock.unlock();}

输出：

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() {@Overridepublic 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() {@Overridepublic 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