package com.henry;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheDataTest {
 static Map<Integer,Object> dataMap=new HashMap<Integer,Object>();
 static
 
 ReadWriteLock lock=new ReentrantReadWriteLock();//建立讀寫鎖的例項
 static Object getData(Integer key){
  lock.readLock().lock();//讀取前先上鎖
  Object val=null;
  try{
   val=dataMap.get(key);
   if(val == null){
    // Must release read lock before acquiring write lock
    lock.readLock().unlock();
    lock.writeLock().lock
 
();
    try{
     //可能已經由其他執行緒寫入資料
     if(val==null){
      //dataMap.put(key, "");//query from db
      val=queryDataFromDB(key);
     }
    }finally{
     //Downgrade by acquiring read lock before releasing write lock
     lock.readLock().lock();
     // Unlock write, still hold read
     lock.writeLock().unlock();
    }
   }
  }finally
 
{
   lock.readLock().unlock();//最後一定不要忘記釋放鎖
  }
  System.out.println("get data key="+key+">val="+val);
  return val;
 }

 static Object queryDataFromDB(Integer key){
  Object val=new Random().nextInt(1000);
  dataMap.put(key, val);
  System.out.println("write into data key="+key+">val="+val);
  return val;
 }

 public static void main(String[] args) {
  for(int i=0;i<10;i++){
   new Thread(new Runnable(){public void run() {
    getData(new Random().nextInt(5));
   }}).start();
  }
 }
}