java并发编程十五 ReentrantReadWriteLock和StampedLock介绍

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读写锁 ReentrantReadWriteLock

当读操作远远高于写操作时，这时候使用 读写锁 让 读-读 可以并发，提高性能。 类似于数据库中的 select …from … lock in share mode 提供一个 数据容器类 内部分别使用读锁保护数据的 read() 方法，写锁保护数据的 write() 方法

class DataContainer {private Object data;private ReentrantReadWriteLock rw = new ReentrantReadWriteLock();private ReentrantReadWriteLock.ReadLock r = rw.readLock();private ReentrantReadWriteLock.WriteLock w = rw.writeLock();public Object read() {log.debug("获取读锁...");r.lock();try {log.debug("读取");sleep(1);return data;} finally {log.debug("释放读锁...");r.unlock();}}public void write() {log.debug("获取写锁...");w.lock();try {log.debug("写入");sleep(1);} finally {log.debug("释放写锁...");w.unlock();}}
}

测试 读锁-读锁 可以并发

DataContainer dataContainer = new DataContainer();
new Thread(() -> {dataContainer.read();
}, "t1").start();
new Thread(() -> {dataContainer.read();
}, "t2").start();

输出结果，从这里可以看到 Thread-0 锁定期间，Thread-1 的读操作不受影响

14:05:14.341 c.DataContainer [t2] - 获取读锁... 
14:05:14.341 c.DataContainer [t1] - 获取读锁... 
14:05:14.345 c.DataContainer [t1] - 读取
14:05:14.345 c.DataContainer [t2] - 读取
14:05:15.365 c.DataContainer [t2] - 释放读锁... 
14:05:15.386 c.DataContainer [t1] - 释放读锁...

测试 读锁-写锁 相互阻塞

DataContainer dataContainer = new DataContainer();
new Thread(() -> {dataContainer.read();
}, "t1").start();
Thread.sleep(100);
new Thread(() -> {dataContainer.write();
}, "t2").start();

输出结果

14:04:21.838 c.DataContainer [t1] - 获取读锁... 
14:04:21.838 c.DataContainer [t2] - 获取写锁... 
14:04:21.841 c.DataContainer [t2] - 写入
14:04:22.843 c.DataContainer [t2] - 释放写锁... 
14:04:22.843 c.DataContainer [t1] - 读取
14:04:23.843 c.DataContainer [t1] - 释放读锁... 

注意事项

• 读锁不支持条件变量
• 重入时升级不支持：即持有读锁的情况下去获取写锁，会导致获取写锁永久等待

r.lock();
try {// ...w.lock();try {// ...} finally{w.unlock();}
} finally{r.unlock();
}

• 重入时降级支持：即持有写锁的情况下去获取读锁

class CachedData {Object data;// 是否有效，如果失效，需要重新计算 datavolatile boolean cacheValid;final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();void processCachedData() {rwl.readLock().lock();if (!cacheValid) {// 获取写锁前必须释放读锁rwl.readLock().unlock();rwl.writeLock().lock();try {// 判断是否有其它线程已经获取了写锁、更新了缓存, 避免重复更新if (!cacheValid) {data = ...cacheValid = true;}// 降级为读锁, 释放写锁, 这样能够让其它线程读取缓存rwl.readLock().lock();} finally {rwl.writeLock().unlock();}}// 自己用完数据, 释放读锁 try {use(data);} finally {rwl.readLock().unlock();}}
}

StampedLock

该类自 JDK 8 加入，是为了进一步优化读性能，它的特点是在使用读锁、写锁时都必须配合【戳】使用
加解读锁

long stamp = lock.readLock();
lock.unlockRead(stamp);

加解写锁

long stamp = lock.writeLock();
lock.unlockWrite(stamp);

乐观读，StampedLock 支持 tryOptimisticRead() 方法（乐观读），读取完毕后需要做一次 戳校验 如果校验通过，表示这期间确实没有写操作，数据可以安全使用，如果校验没通过，需要重新获取读锁，保证数据安全。

long stamp = lock.tryOptimisticRead();
// 验戳
if(!lock.validate(stamp)){// 锁升级
}

提供一个 数据容器类 内部分别使用读锁保护数据的 read() 方法，写锁保护数据的 write() 方法

class DataContainerStamped {private int data;private final StampedLock lock = new StampedLock();public DataContainerStamped(int data) {this.data = data;}public int read(int readTime) {long stamp = lock.tryOptimisticRead();log.debug("optimistic read locking...{}", stamp);sleep(readTime);if (lock.validate(stamp)) {log.debug("read finish...{}, data:{}", stamp, data);return data;}// 锁升级 - 读锁log.debug("updating to read lock... {}", stamp);try {stamp = lock.readLock();log.debug("read lock {}", stamp);sleep(readTime);log.debug("read finish...{}, data:{}", stamp, data);return data;} finally {log.debug("read unlock {}", stamp);lock.unlockRead(stamp);}}public void write(int newData) {long stamp = lock.writeLock();log.debug("write lock {}", stamp);try {sleep(2);this.data = newData;} finally {log.debug("write unlock {}", stamp);lock.unlockWrite(stamp);}}
}

测试 读-读 可以优化

public static void main(String[] args) {DataContainerStamped dataContainer = new DataContainerStamped(1);new Thread(() -> {dataContainer.read(1);}, "t1").start();sleep(0.5);new Thread(() -> {dataContainer.read(0);}, "t2").start();
}

输出结果，可以看到实际没有加读锁

15:58:50.217 c.DataContainerStamped [t1] - optimistic read locking...256 
15:58:50.717 c.DataContainerStamped [t2] - optimistic read locking...256 
15:58:50.717 c.DataContainerStamped [t2] - read finish...256, data:1 
15:58:51.220 c.DataContainerStamped [t1] - read finish...256, data:1 

测试 读-写 时优化读补加读锁

public static void main(String[] args) {DataContainerStamped dataContainer = new DataContainerStamped(1);new Thread(() -> {dataContainer.read(1);}, "t1").start();sleep(0.5);new Thread(() -> {dataContainer.write(100);}, "t2").start();
}

输出结果

输出结果
15:57:00.219 c.DataContainerStamped [t1] - optimistic read locking...256 
15:57:00.717 c.DataContainerStamped [t2] - write lock 384 
15:57:01.225 c.DataContainerStamped [t1] - updating to read lock... 256 
15:57:02.719 c.DataContainerStamped [t2] - write unlock 384 
15:57:02.719 c.DataContainerStamped [t1] - read lock 513 
15:57:03.719 c.DataContainerStamped [t1] - read finish...513, data:1000 
15:57:03.719 c.DataContainerStamped [t1] - read unlock 513

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