java中数据缓存的核心机制

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();//创建读写锁的实例
 static Object getData(Integer key){
  lock.readLock().lock();//读取前先上锁
  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{      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();//最后一定不要忘记释放锁   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();  }
}