How to use the second-level cache of Mybatis in Java

Overview and classification of cache

Overview

Cache is a piece of memory space that saves temporary data

Why use Cache

Read the data from the data source (database or file) and store it in the cache. When retrieving it, obtain it directly from the cache, which can reduce the number of interactions with the database, which can improve the performance of the program. Performance!

Applicability of cache

Suitable for cache: frequently queried but not frequently modified (eg: provinces, cities, category data), the accuracy of the data has no impact on the final result Big

Not suitable for caching: frequently changing data, sensitive data (for example: stock market prices, bank exchange rates, money in bank cards), etc.

MyBatis Cache Category

Level 1 cache: It is the cache of the sqlSession object. It comes with it (no configuration required) and cannot be uninstalled (if you don’t want to use it). The life cycle of the first level cache is consistent with sqlSession.

Second level cache: It is the cache of SqlSessionFactory. As long as the SqlSession created by the same SqlSessionFactory shares the contents of the second-level cache, it can operate the second-level cache. If we want to use the second-level cache, we need to manually enable it ourselves (configuration is required).

Use of second-level cache

1. Enable the second-level cache in the core configuration file of mybatis

    <!--**因为 cacheEnabled 的取值默认就为 true**，所以这一步可以省略不配置。为 true 代表开启二级缓存；为 false 代表不开启二级缓存。  -->
<settings>
    <setting name="cacheEnabled" value="true"/>
</settings>

2. Configure the use of the second-level cache in the Dao mapping file

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.dao.UserDao">
    <!--配置二级缓存-->
    <cache/>
   
    <select id="findAll" resultType="user">
        select * from t_user
    </select>
 
    <delete id="deleteById" parameterType="int">
        delete from t_user where uid=#{id}
    </delete>
</mapper>

3. The Pojo class for second-level caching must implement the Serializable interface

public class User implements Serializable {
    private int uid;
    private String username;
    private String sex;
    private Date birthday;
    private String address;
    // 省略setter,getter,构造...等方法
}

4. Test the use of second-level cache

Test code

@Test
      public void testFindAll() throws Exception{
          // 1.加载mybatis核心配置文件
          InputStream is = Resources.getResourceAsStream("SqlMapConfig.xml");
 
          // 2.创建SqlSessionFactoryBuilder对象
          SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
 
          // 3.构建SqlSessionFactory对象
          SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
 
          // 4.获取SqlSession对象
          SqlSession sqlSession = sqlSessionFactory.openSession();
 
          // 5.获得dao接口的代理对象
          UserDao userDao = sqlSession.getMapper(UserDao.class);
 
          // 6.执行sql语句,得到结果
          List<User> list = userDao.findAll();
          for (User user : list) {
              System.out.println("user = " + user);
          }
          sqlSession.close();//清除一级缓存
 
          System.out.println("分割线----------------------------------");
 
          SqlSession sqlSession2 = sqlSessionFactory.openSession();
          UserDao userDao2 = sqlSession2.getMapper(UserDao.class);
          List<User> userList2 = userDao2.findAll();
          for (User user : userList2) {
              System.out.println(user);
          }
          // 7.释放资源
          sqlSession2.close();
      }

- Test results:

#- After the above test, we found that two queries were executed, and after executing the first query, we turned off the first-level cache and then went to When executing the second query, we found that no SQL statement was issued to the database, so the data at this time can only come from what we call the second-level cache.

5. Test to turn off the second-level cache

-Test code

   @Test
      public void testFindAll() throws Exception{
          // 1.加载mybatis核心配置文件
          InputStream is = Resources.getResourceAsStream("SqlMapConfig.xml");
 
          // 2.创建SqlSessionFactoryBuilder对象
          SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
 
          // 3.构建SqlSessionFactory对象
          SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
 
          // 4.获取SqlSession对象
          SqlSession sqlSession = sqlSessionFactory.openSession();
 
          // 5.获得dao接口的代理对象
          UserDao userDao = sqlSession.getMapper(UserDao.class);
 
          // 6.执行sql语句,得到结果
          List<User> list = userDao.findAll();
          for (User user : list) {
              System.out.println("user = " + user);
          }
          sqlSession.close();//清除一级缓存
 
          System.out.println("分割线----------------------------------");
 
          SqlSession sqlSession2 = sqlSessionFactory.openSession();
          UserDao userDao2 = sqlSession2.getMapper(UserDao.class);
          userDao2.deleteById(5);// 关闭二级缓存
 
          List<User> userList2 = userDao2.findAll();
          for (User user : userList2) {
              System.out.println(user);
          }
          // 7.释放资源
          sqlSession2.close();
      }

-Test result

After the above Testing, we found that two queries were executed, and after executing the first query, we closed the first-level cache and the second-level cache. When we executed the second query, we found that a sql statement was issued to the database, so The data at this time comes from the database, not the cache.

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