Java多线程（八）：ReentrantReadWriteLock

青旅半醒 2023-08-17 15:16 134阅读 0赞

## 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。

转载于:https://www.cnblogs.com/Java-Starter/p/11242643.html