mybatis一级缓存和二级缓存的使用

在mybatis中，有一级缓存和二级缓存的概念：

一级缓存：一级缓存 Mybatis的一级缓存是指SQLSession，一级缓存的作用域是SQLSession, Mabits默认开启一级缓存。在同一个SqlSession中，执行相同的SQL查询时；第一次会去查询数据库，并写在缓存中，第二次会直接从缓存中取。当执行SQL时候两次查询中间发生了增删改的操作，则SQLSession的缓存会被清空。

这么设计的原因是避免读取脏数据：假设A查询了某商品库存为10件，并将10件库存的数据存入缓存中，之后被客户买走了10件，数据被delete了，但是下次查询这件商品时，并不从数据库中查询，而是从缓存中查询，就会出现错误。

二级缓存：二级缓存是mapper级别的，Mybatis默认是没有开启二级缓存的。 第一次调用mapper下的SQL去查询用户的信息，查询到的信息会存放代该mapper对应的二级缓存区域。

看概念什么的最烦了是不是？用代码看看呗：

 package top.bigking;

 import org.apache.ibatis.io.Resources;
 import org.apache.ibatis.session.SqlSession;
 import org.apache.ibatis.session.SqlSessionFactory;
 import org.apache.ibatis.session.SqlSessionFactoryBuilder;
 import top.bigking.dao.DeptMapper;
 import top.bigking.dao.UserMapper;
 import top.bigking.pojo.Dept;
 import top.bigking.pojo.User;

 import java.io.InputStream;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;

 public class mybatisStudy_start {
     public static void main(String[] args) throws Exception{
         InputStream in = Resources.getResourceAsStream("mybatisStudy-config.xml");
         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
         SqlSession sqlSession = sqlSessionFactory.openSession(true);//true 表示 自动提交
         DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);
         List<Dept> deptList = deptMapper.query();
         for(Dept dept : deptList){
             System.out.println(dept);
         }
         System.out.println("-----------------------------------");
         //insert(deptMapper);
         //update(deptMapper);
         findById(deptMapper);

         findById(deptMapper);
         //delete(deptMapper);
         //findByIdDname(deptMapper);

 //        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
 //        List<User> userList = userMapper.query();
 //        for(User user : userList)
 //            System.out.println(user);
         sqlSession.close();

     }
     private static void insert(DeptMapper deptMapper){
         Dept dept = new Dept();
         dept.setDeptNo(11);
         dept.setDname("ABKing");
         dept.setLoc("111");
         int count = deptMapper.insert(dept);
         System.out.println(count);
     }
     public static void update(DeptMapper deptMapper){
         Dept dept = new Dept();
         dept.setDeptNo(11);
         dept.setDname("ABKing");
         dept.setLoc("上海");
         int count = deptMapper.update(dept);
         System.out.println(count);
     }
     public static void findById(DeptMapper deptMapper){
         Dept dept = deptMapper.findById(11);
         System.out.println(dept);
     }
     public static void delete(DeptMapper deptMapper){
         int count = deptMapper.delete(11);
         System.out.println(count);
     }
     public static void findByIdDname(DeptMapper deptMapper){
         Map map = new HashMap();
         map.put("deptNo", "11");
         map.put("dname", "ABKing");
         Dept dept = deptMapper.findByIdDname(map);
         System.out.println(dept);
     }
 }

看30行和32行，能看到调用了两次findById(deptMapper)，运行，查看控制台的日志（需要log4j包）

DEBUG [main] - ==>  Preparing: select deptNo, dname, loc from Dept where deptNo=? 

只打印出了一条SQL语句

而如果我们在31行加入sqlSession.clearCache();

 package top.bigking;

 import org.apache.ibatis.io.Resources;
 import org.apache.ibatis.session.SqlSession;
 import org.apache.ibatis.session.SqlSessionFactory;
 import org.apache.ibatis.session.SqlSessionFactoryBuilder;
 import top.bigking.dao.DeptMapper;
 import top.bigking.dao.UserMapper;
 import top.bigking.pojo.Dept;
 import top.bigking.pojo.User;

 import java.io.InputStream;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;

 public class mybatisStudy_start {
     public static void main(String[] args) throws Exception{
         InputStream in = Resources.getResourceAsStream("mybatisStudy-config.xml");
         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
         SqlSession sqlSession = sqlSessionFactory.openSession(true);//true 表示 自动提交
         DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);
         List<Dept> deptList = deptMapper.query();
         for(Dept dept : deptList){
             System.out.println(dept);
         }
         System.out.println("-----------------------------------");
         //insert(deptMapper);
         //update(deptMapper);
         findById(deptMapper);
         sqlSession.clearCache();
         findById(deptMapper);
         //delete(deptMapper);
         //findByIdDname(deptMapper);

 //        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
 //        List<User> userList = userMapper.query();
 //        for(User user : userList)
 //            System.out.println(user);
         sqlSession.close();

     }
     private static void insert(DeptMapper deptMapper){
         Dept dept = new Dept();
         dept.setDeptNo(11);
         dept.setDname("ABKing");
         dept.setLoc("111");
         int count = deptMapper.insert(dept);
         System.out.println(count);
     }
     public static void update(DeptMapper deptMapper){
         Dept dept = new Dept();
         dept.setDeptNo(11);
         dept.setDname("ABKing");
         dept.setLoc("上海");
         int count = deptMapper.update(dept);
         System.out.println(count);
     }
     public static void findById(DeptMapper deptMapper){
         Dept dept = deptMapper.findById(11);
         System.out.println(dept);
     }
     public static void delete(DeptMapper deptMapper){
         int count = deptMapper.delete(11);
         System.out.println(count);
     }
     public static void findByIdDname(DeptMapper deptMapper){
         Map map = new HashMap();
         map.put("deptNo", "11");
         map.put("dname", "ABKing");
         Dept dept = deptMapper.findByIdDname(map);
         System.out.println(dept);
     }
 }

此时查看控制台的日志

DEBUG [main] - ==>  Preparing: select deptNo, dname, loc from Dept where deptNo=?
DEBUG [main] - ==> Parameters: 11(Integer)
DEBUG [main] - <==      Total: 1
Dept{deptNo=11, dname='ABKing', loc='111'}
DEBUG [main] - ==>  Preparing: select deptNo, dname, loc from Dept where deptNo=? 

可以看到打印出了两条SQL语句，这足以证明，使用了sqlSession.clearCache();之后，缓存被清空了

接下来证明二级缓存：

应当明确的是，二级缓存是mapper级别的缓存，使用同一个Mapper.class的的Mapper类和SqlSession类都共用同一个二级缓存

代码如下：

 package top.bigking;

 import org.apache.ibatis.io.Resources;
 import org.apache.ibatis.session.SqlSession;
 import org.apache.ibatis.session.SqlSessionFactory;
 import org.apache.ibatis.session.SqlSessionFactoryBuilder;
 import top.bigking.dao.DeptMapper;
 import top.bigking.dao.UserMapper;
 import top.bigking.pojo.Dept;
 import top.bigking.pojo.User;

 import java.io.InputStream;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;

 public class mybatisStudy_start {
     public static void main(String[] args) throws Exception{
         InputStream in = Resources.getResourceAsStream("mybatisStudy-config.xml");
         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
         SqlSession sqlSession = sqlSessionFactory.openSession(true);//true 表示 自动提交
         SqlSession sqlSession1 = sqlSessionFactory.openSession(true);
         DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);
         DeptMapper deptMapper1 = sqlSession1.getMapper(DeptMapper.class);
         List<Dept> deptList = deptMapper.query();
         for(Dept dept : deptList){
             System.out.println(dept);
         }
         System.out.println("-----------------------------------");
         //insert(deptMapper);
         //update(deptMapper);
         findById(deptMapper);
         sqlSession.close();
         //sqlSession.clearCache();
         findById(deptMapper1);
         sqlSession1.close();
         //delete(deptMapper);
         //findByIdDname(deptMapper);

 //        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
 //        List<User> userList = userMapper.query();
 //        for(User user : userList)
 //            System.out.println(user);
         //sqlSession.close();

     }
     private static void insert(DeptMapper deptMapper){
         Dept dept = new Dept();
         dept.setDeptNo(11);
         dept.setDname("ABKing");
         dept.setLoc("111");
         int count = deptMapper.insert(dept);
         System.out.println(count);
     }
     public static void update(DeptMapper deptMapper){
         Dept dept = new Dept();
         dept.setDeptNo(11);
         dept.setDname("ABKing");
         dept.setLoc("上海");
         int count = deptMapper.update(dept);
         System.out.println(count);
     }
     public static void findById(DeptMapper deptMapper){
         Dept dept = deptMapper.findById(11);
         System.out.println(dept);
     }
     public static void delete(DeptMapper deptMapper){
         int count = deptMapper.delete(11);
         System.out.println(count);
     }
     public static void findByIdDname(DeptMapper deptMapper){
         Map map = new HashMap();
         map.put("deptNo", "11");
         map.put("dname", "ABKing");
         Dept dept = deptMapper.findByIdDname(map);
         System.out.println(dept);
     }
 }

在DeptMapper.xml中

    <select id="findById" resultType="Dept" useCache="true">
        select deptNo, dname, loc from Dept where deptNo=#{deptNo}
    </select>

通过useCache可以使用二级缓存

在mybatisStudy-config.xml中，在properties标签下，添加

   <settings>
        <setting name="cacheEnabled" value="true"/>
    </settings>

这是全局设置，表示开启二级缓存

查看控制台输出的日志：

DEBUG [main] - ==>  Preparing: select deptNo, dname, loc from Dept where deptNo=?
DEBUG [main] - ==> Parameters: 11(Integer)
DEBUG [main] - <==      Total: 1
Dept{deptNo=11, dname='ABKing', loc='111'}
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7a9273a8]

可以看到，由于使用了二级缓存，我们的日志中，只出现了一条SQLQL语句

而如果我们把DeptMapper.xml中的代码改为：

　　<select id="findById" resultType="Dept" useCache="false">
        select deptNo, dname, loc from Dept where deptNo=#{deptNo}
    </select>

表示不使用二级缓存

运行，查看控制台：

DEBUG [main] - ==>  Preparing: select deptNo, dname, loc from Dept where deptNo=?
DEBUG [main] - ==> Parameters: 11(Integer)
DEBUG [main] - <==      Total: 1
Dept{deptNo=11, dname='ABKing', loc='111'}
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7a9273a8]
DEBUG [main] - Returned connection 2056418216 to pool.
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Checked out connection 2056418216 from pool.
DEBUG [main] - ==>  Preparing: select deptNo, dname, loc from Dept where deptNo=?
DEBUG [main] - ==> Parameters: 11(Integer)
DEBUG [main] - <==      Total: 1
Dept{deptNo=11, dname='ABKing', loc='111'}
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7a9273a8]

可以看到两条SQL语句

可是我们的代码中，并没有使用到清空一级缓存的函数sqlSession.clearCache();

由此可证明，这里是二级缓存的开关

参考：

https://m.w3cschool.cn/kzsow/kzsow-qmod2gri.html

https://www.cnblogs.com/happyflyingpig/p/7739749.html

https://www.cnblogs.com/yuluoxingkong/p/8205858.html

https://www.cnblogs.com/charlypage/p/9747145.html

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