高效緩存系統示例

package com.fenxiang.jiyajie;

import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.*;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheTest {

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

 private ReadWriteLock rtl = new ReentrantReadWriteLock();//讀寫鎖
 
 public Object getData(String key){
  rtl.readLock().lock();
  Object data = null;
  try{
           data = cache.get(key);
     if(data == null){
           rtl.readLock().unlock();
           rtl.writeLock().lock();
     try{
         if(data == null){//雙重檢查，避免等待的多個寫鎖重複寫入數據
              data ="DB"; 
          }
     }finally{
         rtl.readLock().unlock();//在finally 中釋放讀鎖
     }
         rtl.readLock().lock();
   }
  
  }finally{
   rtl.readLock().unlock();//在finally 中釋放讀鎖
  }
  return data;
 }
}