7.4 读写锁原理

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()，写锁保护数据的write()方法。

@Slf4j(topic = "c.DataContainer")
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");
            Thread.sleep(1000);
            return data;
        } catch (InterruptedException e) {
   
            throw new RuntimeException(e);
        } 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");
            Thread.sleep(1000);
        } catch (InterruptedException e) {
   
            throw new RuntimeException(e);
        } finally {
   
            log.debug("释放写锁...");
            w.unlock();
        }
    }
}

测试读锁-读锁可以并发

public class TestReentrantReadWriteLock {
   
    public static void main(String[] args) {
   
        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

17:41:10.773 [t2] - 获取读锁...
17:41:10.773 [t1] - 获取读锁...
17:41:10.774 [t2] - 读取
17:41:10.774 [t1] - 读取
17:41:11.776 [t1] - 释放读锁...
17:41:11.779 [t2] - 释放读锁...

测试读锁-写锁相互阻塞

public class TestReentrantReadWriteLock {
   
    public static void main(String[] args) throws InterruptedException {
   
        DataContainer dataContainer = new DataContainer();
        new Thread(() -> {
   
            dataContainer.read();
        }, "t1").start();

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

输出结果

17:50:57.280 [t1] - 获取读锁...
17:50:57.281 [t1] - 读取
17:50:57.384 [t2] - 获取写锁...
17:50:58.287 [t1] - 释放读锁...
17:50:58.287 [t2] - 写入
17:50:59.291 [t2] - 释放写