Java多线程（八）：ReentrantReadWriteLock

读写锁ReentrantReadWriteLock概述

读写锁ReentrantReadWriteLock，使用它比ReentrantLock效率更高。

读写锁表示两个锁，一个是读操作相关的锁，称为共享锁；另一个是写操作相关的锁，称为排他锁。

1、读和读之间不互斥，因为读操作不会有线程安全问题

2、写和写之间互斥，避免一个写操作影响另外一个写操作，引发线程安全问题

3、读和写之间互斥，避免读操作的时候写操作修改了内容，引发线程安全问题

多个Thread可以同时进行读取操作，但是同一时刻只允许一个Thread进行写入操作。

读操作和读操作共享

请看例子

ThreadDomain44类和main方法

public class ThreadDomain44 extends ReentrantReadWriteLock
{
    public void read()
    {
        try
        {
            readLock().lock();
            System.out.println(Thread.currentThread().getName() + "获得了读锁, 时间为" +
                    System.currentTimeMillis());
            Thread.sleep(10000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            readLock().unlock();
        }
    }

    public static void main(String[] args)
    {
        final ThreadDomain44 td = new ThreadDomain44();
        Runnable readRunnable = new Runnable()
        {
            public void run()
            {
                td.read();
            }
        };
        Thread t0 = new Thread(readRunnable);
        Thread t1 = new Thread(readRunnable);
        t0.start();
        t1.start();
    }
}

输出结果如下

Thread-0获得了读锁, 时间为1564018325618
Thread-1获得了读锁, 时间为1564018325618

尽管加了Thread.sleep(10000);两个线程几乎同时执行lock()后面的方法。说明readLock().lock()允许多个线程执行并且能提高执行效率。

写操作和写操作互斥

请看例子

public class ThreadDomain45 extends ReentrantReadWriteLock {
    public void write()
    {
        try
        {
            writeLock().lock();
            System.out.println(Thread.currentThread().getName() + "获得了写锁, 时间为" +
                    new Date());
            Thread.sleep(10000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            writeLock().unlock();
        }
    }

    public static void main(String[] args)
    {
        final ThreadDomain45 td = new ThreadDomain45();
        Runnable readRunnable = new Runnable()
        {
            public void run()
            {
                td.write();
            }
        };
        Thread t0 = new Thread(readRunnable);
        Thread t1 = new Thread(readRunnable);
        t0.start();
        t1.start();
    }
}

输出结果

Thread-1获得了写锁, 时间为Tue Jul 30 11:26:21 CST 2019
Thread-0获得了写锁, 时间为Tue Jul 30 11:26:31 CST 2019

可以看到，后获得锁的Thread-0等待了十秒，说明写和写互斥

读操作和写操作互斥/写操作和读操作互斥

例子如下

public class ThreadDomain46 extends ReentrantReadWriteLock {
    public void write()
    {
        try
        {
            writeLock().lock();
            System.out.println(Thread.currentThread().getName() + "获得了写锁, 时间为" +
                    new Date());
            Thread.sleep(10000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            writeLock().unlock();
        }
    }

    public void read()
    {
        try
        {
            readLock().lock();
            System.out.println(Thread.currentThread().getName() + "获得了读锁, 时间为" +
                    new Date());
            Thread.sleep(10000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            readLock().unlock();
        }
    }

    public static void main(String[] args)
    {
        final ThreadDomain46 td = new ThreadDomain46();
        Runnable readRunnable = new Runnable()
        {
            public void run()
            {
                td.read();
            }
        };
        Runnable writeRunnable = new Runnable()
        {
            public void run()
            {
                td.write();
            }
        };
        Thread t0 = new Thread(readRunnable);
        Thread t1 = new Thread(writeRunnable);
        t0.start();
        t1.start();
    }
}

输出结果如下

Thread-0获得了读锁, 时间为Tue Jul 30 11:49:15 CST 2019
Thread-1获得了写锁, 时间为Tue Jul 30 11:49:26 CST 2019

可以看到Thread-0获得读锁执行完10s后Thread-1才获得了写锁，说明读操作和写操作互斥。

另一种输出结果如下

Thread-1获得了写锁, 时间为Tue Jul 30 14:14:07 CST 2019
Thread-0获得了读锁, 时间为Tue Jul 30 14:14:17 CST 2019

可以看到Thread-1获得写锁执行完10s后Thread-0才获得了读锁，说明写操作和读操作互斥。