java并发包：读写锁

青旅半醒 2024-02-17 21:17 24阅读 0赞

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

![QQ20160714-0_402x.png][]

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();
            }
    
        }
    }

[http_blog.csdn.net_a910626_article_details_51900954]: http://blog.csdn.net/a910626/article/details/51900954
[QQ20160714-0_402x.png]: http://7xvzr8.com1.z0.glb.clouddn.com/QQ20160714-0%402x.png