读写锁实现缓存系统

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package com.icss.thread;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheDemo {

   private Map<String, Object> cache = new HashMap<String, Object>();
   public static void main(String[] args) {

   }

   private ReadWriteLock rwl = new ReentrantReadWriteLock();
   public /*synchronized*/ Object getData(String key){
       rwl.readLock().lock();
       Object value = null;
       try {
           value = cache.get(key);
           if(value == null){
               rwl.readLock().unlock();
               rwl.writeLock().lock();
               try {
                   if(value == null){
                       value = "aaaa"; //实际失去queryDB();
                   }
               } finally {
                   rwl.writeLock().unlock();
               }
               rwl.readLock().lock();
           }
       } finally {
           rwl.readLock().unlock();
       }
       return value;
   }

}

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