下面這段程式碼 如果1W個人同時訪問的話,  從redis 拿 allUser 時， userList為空時，那麼1W個人都要進入if判斷語句，查詢資料庫，資料庫壓力承受不住

package com.tb.service;
 
import com.tb.dao.TUserMapper;
import com.tb.pojo.TUser;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
 
import javax.annotation.Resource;
import java.util.List;
 
@Service
@Transactional
public class TuserServicelmpl implements  TuserService {
 
    @Resource
    private TUserMapper  tUserMapper;
 
    @Resource
    private RedisTemplate  redisTemplate;
 
    @Override
    public List<TUser> userlist() {
 
        //高併發下有點問題，此處問題：快取穿透
        //查詢快取
        List<TUser>  userList =  ( List<TUser>) redisTemplate.opsForValue().get("allUser");
        if (userList==null){
            //查詢資料庫
            userList =  tUserMapper.userlist();
            redisTemplate.opsForValue().set("allUser",userList);
        }
        return userList;
    }
}
 

解決方案

     Synchronized   關鍵字

雙重檢測鎖 解決

package com.tb.service;
 
import com.tb.dao.TUserMapper;
import com.tb.pojo.TUser;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
 
import javax.annotation.Resource;
import java.util.List;
 
@Service
@Transactional
public class TuserServicelmpl implements  TuserService {
 
    @Resource
    private TUserMapper  tUserMapper;
 
    @Resource
    private RedisTemplate  redisTemplate;
 
    @Override
    public List<TUser> userlist() {
 
        //高併發下有點問題，此處問題：快取穿透
        //查詢快取
        List<TUser>  userList =  ( List<TUser>) redisTemplate.opsForValue().get("allUser");
 
        //雙重檢測鎖
        if (userList==null){
            synchronized(this){
                //從資料庫獲取資料
                userList =  ( List<TUser>) redisTemplate.opsForValue().get("allUser");
                if (userList==null){
                    //快取為空，查詢資料庫
                    userList =  tUserMapper.userlist();
                    redisTemplate.opsForValue().set("allUser",userList);
                }
            }
 
 
        }
        return userList;
    }
}