1. 讀寫鎖機制——ReadWriteLock介面

讀寫鎖適用於對資料結構頻繁讀而較少修改的場景。舉個栗子，你可以建立一個線上詞典供多條讀執行緒併發讀取，然而單條寫執行緒可能會不時新增新的定義或更新已有的定義。一個資源可以被多個執行緒同時讀，或者被一個執行緒寫，但是不能同時存在讀和寫執行緒。 

基本規則： 讀讀不互斥 、讀寫互斥 、寫寫互斥。

問：既然讀讀不互斥，為什麼還要加讀鎖？

答：如果讀可以不是最新資料，也不需要加鎖。要保證讀取資料的嚴格實時性，就必須加讀寫鎖。 

2. ReadWriteLock介面宣告的方法 

（1）Lock readLock()：返回用於讀的鎖。

（2）Lock writeLock()：返回用於寫的鎖。

3. ReadWriteLock介面的實現類——重入讀寫鎖ReentrantReadWriteLock

 常用方法

4. 示例

寫執行緒產生單詞定義的條目，而讀執行緒持續隨機地訪問這些條目並打印出來。

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("寫入單詞："+words[i]);
		                		 wlock.unlock();
		                		 
			                	 try {
										Thread.sleep(100);  //讓其他執行緒有機會執行
									} 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);  //隨機讀取一個單詞
	                    		System.out.println("讀取單詞："+words[i]+",它的意思是：" + dictionary.get(words[i]));
	                    		rlock.unlock();
	                    	}
	                    };
	es = Executors.newFixedThreadPool(1);
	es.submit(reader);                  
	}
}
 

 執行結果：

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

........