Java并发——锁框架（三）读写锁

1. 读写锁机制——ReadWriteLock接口

读写锁适用于对数据结构频繁读而较少修改的场景。举个栗子，你可以创建一个在线词典供多条读线程并发读取，然而单条写线程可能会不时添加新的定义或更新已有的定义。一个资源可以被多个线程同时读，或者被一个线程写，但是不能同时存在读和写线程。 

基本规则： 读读不互斥 、读写互斥 、写写互斥。

问：既然读读不互斥，为什么还要加读锁？

答：如果读可以不是最新数据，也不需要加锁。要保证读取数据的严格实时性，就必须加读写锁。 

2. ReadWriteLock接口声明的方法 

（1）Lock readLock()：返回用于读的锁。

（2）Lock writeLock()：返回用于写的锁。

3. ReadWriteLock接口的实现类——重入读写锁ReentrantReadWriteLock

 常用方法

方法名称	描述
ReentrantReadWriteLock()	创建一个重入读写锁的实例。
ReentrantReadWriteLock(boolean fair)	创建一个具有公平策略的重入读写锁实例。
ReadLock readLock()	返回用于读的锁。
WriteLock writeLock()	返回用于写的锁。
int getReadHoldCount()	返回被调用线程在这个锁上持有读锁的数量，当调用线程没有持有这个读锁，返回0.
int getWriteHoldCount()	返回被调用线程在这个锁上持有写锁的数量，当调用线程没有持有这个写锁，返回0.

4. 示例

写线程产生单词定义的条目，而读线程持续随机地访问这些条目并打印出来。

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;



public class ReentrantReadWriteLockDemo {

	public static void main(String[] args)
	{
		final String[] words = {"dog","cat","bird","fish","human"};
		final String[] definitions = {"a member of the genus Canis",
				"feline mammal usually having thick soft fur ",
				"A bird is a feathered, winged, bipedal, warm-blooded, egg-laying, vertebrate animal.",
				"Fish are vertebrates with gills that live in water",
				"characterized by superior intelligence, articulate speech, and erect carriage"};
		final Map<String,String> dictionary = new HashMap<String,String>();
		
		ReadWriteLock rwlock = new ReentrantReadWriteLock();
		final Lock rlock = rwlock.readLock();
		final Lock wlock = rwlock.writeLock();
		
		Runnable writer = ()->
		                 {
		                	 for(int i = 0; i < words.length; i++)
		                	 {
		                		 wlock.lock();
		                		 dictionary.put(words[i], definitions[i]);
		                		 System.out.println("写入单词："+words[i]);
		                		 wlock.unlock();
		                		 
			                	 try {
										Thread.sleep(100);  //让其他线程有机会运行
									} catch (InterruptedException e) {
										// TODO Auto-generated catch block
										e.printStackTrace();
									}
		                	 }		                	 			                			
		                 };
	   ExecutorService es = Executors.newFixedThreadPool(1);
	   es.submit(writer);
	   
	   Runnable reader = ()->
	                    {
	                    	while(true)
	                    	{
	                    		rlock.lock();
	                    		int i = (int) (Math.random() * words.length);  //随机读取一个单词
	                    		System.out.println("读取单词："+words[i]+",它的意思是：" + dictionary.get(words[i]));
	                    		rlock.unlock();
	                    	}
	                    };
	es = Executors.newFixedThreadPool(1);
	es.submit(reader);                  
	}
}

 运行结果：

读取单词：human,它的意思是：characterized by superior intelligence, articulate speech, and erect carriage
读取单词：cat,它的意思是：feline mammal usually having thick soft fur 
读取单词：fish,它的意思是：Fish are vertebrates with gills that live in water
读取单词：dog,它的意思是：a member of the genus Canis
读取单词：fish,它的意思是：Fish are vertebrates with gills that live in water
读取单词：cat,它的意思是：feline mammal usually having thick soft fur 
读取单词：fish,它的意思是：Fish are vertebrates with gills that live in water
读取单词：cat,它的意思是：feline mammal usually having thick soft fur 

........