JDK并发包(concurrent) - 读写锁(ReadWriteLock)

ReadWriteLock是JDK1.5中提供的读写分离锁。读写分离可以有效的减少锁竞争，以此来高系统的性能。 读写锁允许多个线程同时读，但是考虑到数据完整性，写写操作，和读写操作依然是需要相互等待和持有锁的。如下图：

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了。上代码：

package com.example.thread; import java.util.Random; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; import java.util.concurrent.locks.ReentrantReadWriteLock; /** * Created by mazhenhua on 2017/3/8. */ public class ReadWriteLockDemo { private static Lock lock = new ReentrantLock(); 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) { final ReadWriteLockDemo demo = new ReadWriteLockDemo(); Runnable readRunable = new Runnable() { @Override public void run() { try { demo.handleRead(readLock); //demo.handleRead(lock); } catch (InterruptedException e) { e.printStackTrace(); } } }; final Runnable writeRunable = new Runnable() { @Override public void run() { try { demo.handleWrite(writeLock, new Random().nextInt()); //demo.handleWrite(lock, new Random().nextInt()); } catch (InterruptedException e) { e.printStackTrace(); } } }; for (int i = 0; i < 18; i ++){ new Thread(readRunable).start(); } for (int i = 18; i < 20; i ++){ new Thread(writeRunable).start(); } } }

这段代码很快就执行完了，如果把注掉的地方替换一下，可能就要执行好长时间了。