[MyBatis] mybatis cache mechanism

1. Cache basics:

• Cache: cache

• The role of the cache: to improve the execution efficiency of the program by reducing IO

• mybatis cache includes:

• Level 1 cache: The data of speech query is stored in SqlSession

• Second-level cache: store the queried data in SqlSessionFactory

• Or integrate a third-party cache: such as EhCache...

• Mybatis cache is only for DQL statements, that is to say, the cache mechanism only corresponds to select statements.

2. Level 1 cache:

• The first-level cache is enabled by default, and no configuration is required

• Principle: As long as the same SQL statement is executed using the same SqlSession object, the cache will be taken

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    // mapper1和mapper2的sqlsession都是相同的, 因此会走缓存
    CarMapper mapper1 = sqlSession.getMapper(CarMapper.class);
    Car car1 = mapper1.selectById(16L);
    // 会缓存
    CarMapper mapper2 = sqlSession.getMapper(CarMapper.class);
    Car car2 = mapper2.selectById(16L);
    
    sqlSession.close();
}

• When is the cache not taken?

• The SqlSession object is not the same, and the cache must not be taken

• The query conditions are different, and the cache must not be taken

• When is the L1 cache invalidated?

• Doing any of the following between the first DQL and the second DQL will clear the first-level cache

• The clearCache() method of sqlSession is executed, which is to manually clear the cache

• The INSERT/UPDATE/DELETE statement is executed, no matter which table is operated, the first-level cache will be cleared

3. Second level cache:

• The scope of the second level cache is SqlSessionFactory

• The following conditions are required to use the second-level cache

• Globally enable or disable any cache configured in all mapping configuration files, the default is true, no need to set

• Add configuration to the SqlMapper.xml file that needs to use the second-level cache:

• The entity class object using the second-level cache must be serializable, that is, it must implement the java.io.Serializable interface

• After the SqlSession object is closed or submitted, the data in the first-level cache will be written to the second-level cache, and the second-level cache will be available at this time.

// @test
public static void main(String[] args) {

    SqlSessionFactory sqlsessionfactory = new SqlSessionFactoryBuilder().
        	build(Resources.getResourceAsStream("mybatis-config.xml"));
    
    SqlSession session1 = sqlsessionfactory.openSession();
    SqlSession session2 = sqlsessionfactory.openSession();

    CarMapper mapper1 = sqlSession1.getMapper(CarMapper.class);
    CarMapper mapper2 = sqlSession2.getMapper(CarMapper.class);
    
    // 这行代码执行结束之后, 实际上数据是缓存到一级缓存当中. (sqlSession1是一级缓存)
    Car car1 = mapper1.selectById(16L);
    System.out.println(car1);

    // 这里如果不关闭sqlSession1对象的话, 二级缓存中还是没有数据
    // 如果执行了这行代码, sqlSession1的一级缓存中的数据会放到二级缓存当中
    sqlSession1.close();
    
    // 这行代码执行结束之后, 实际上数据是缓存到一级缓存当中. (sqlSession2是一级缓存)
    Car car2 = mapper2.selectById(16L);
    System.out.println(car1);
    
    // 程序执行到这里的时候, 会将sqlSession1这个一级缓存中的数据写入到二级缓存当中
    sqlSession1.close();
    // 程序执行到这里的时候, 会将sqlSession2这个一级缓存中的数据写入到二级缓存当中
    sqlSession2.close();
}

• Invalidation of the second-level cache: It only needs to add, delete, and modify operations between two queries, and the second-level cache will be invalid (the first-level cache will also be invalid)