java并发包：读写锁

本文转载至：http://blog.csdn.net/a910626/article/details/51900954

ReadWriteLock是jdk5中提供的读写分离锁。读写分离锁可以有效的帮助减少锁竞争，以提升性能。用锁分离的机制来提升性能非常容易理解，比如线程A1,A2,A3进行写操作，B1,B2,B3进行读操作，如果使用重入锁或者内部锁，则理论上说所有读之间、读和写之间、写和写之间都是串行操作。当B1进行读取时，B2,B3则需要等待锁的释放。由于读操作并不对数据的完整性造成破坏，这种等待显然是不合理。因此，读写锁就有了发挥功能的余地。

  在这种情况下，读写锁允许多个线程同时读，使得B1,B2,B3之间真正并行。但是，考虑到数据完整性，写写操作和读写操作间仍然是需要相互等待和持有锁的。总的来说，读写锁的访问约束如下：

  如果在系统中，读操作次数远远大于写操作，则读写锁就可以发挥最大的功效，提升系统的性能。

一个例子

/**
 * Created by niehongtao on 16/7/12.
 * 读写锁
 */
public class ReadWriteLockDemo {
    private static ReentrantReadWriteLock reentrantReadWriteLock =
            new ReentrantReadWriteLock();
    private static Lock readLock = reentrantReadWriteLock.readLock();
    private static Lock writeLock = reentrantReadWriteLock.writeLock();
    private int value;


    public Object handleRead(Lock lock) throws InterruptedException {
        try {
            lock.lock();
            Thread.sleep(1000);
            return value;
        } finally {
            lock.unlock();
        }
    }


    public void handleWrite(Lock lock, int index) throws InterruptedException {
        try {
            lock.lock();
            Thread.sleep(1000);
            value = index;
        } finally {
            lock.unlock();
        }
    }


    public static void main(String[] args) {
        ReadWriteLockDemo demo = new ReadWriteLockDemo();
        Runnable readRunnable = new Runnable() {
            @Override
            public void run() {
                try {
                    demo.handleRead(readLock);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };

        Runnable writeRunnalbe = new Runnable() {
            @Override
            public void run() {
                try {
                    demo.handleWrite(writeLock, new Random().nextInt());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };


        for (int i = 0; i < 18; i++) {
            new Thread(readRunnable).start();
        }

        for (int i = 18; i < 20; i++) {
            new Thread(writeRunnalbe).start();
        }

    }
}