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();//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(); }
                              }