Java多线程 ReentrantReadWriteLock原理及实例详解

Z时代
2024-01-10
分类：综合

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();
  }
}