Why cache

In fact, we have been working for a long time, and we know that many bottlenecks are in the database.

The first level cache of mybatis

Of course, we still use the code to understand the first level cache of mybatis

Code demonstration

See github for detailed code. Only important code fragments are shown here

1. tempMapper.xml

    <select id="getById" resultType="entity.TempEntity">
       select * from  temp where id = #{id}
    </select>

1. TempTest

public class TempTest {

    Logger logger = Logger.getLogger(this.getClass());
    @Test
    public  void test() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis.xml");
        SqlSessionFactory build = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = build.openSession();
        TempEntity tempEntity1 = sqlSession.selectOne("dao.TempDao.getById", 1);
        logger.info(tempEntity1);
        TempEntity tempEntity2 = sqlSession.selectOne("dao.TempDao.getById", 1);
        logger.info(tempEntity2);
        logger.info(tempEntity1 == tempEntity2);
    }
}

3. Operation results

2020-06-26 08:57:37,453 DEBUG [dao.TempDao.getById] - ==>  Preparing: select * from temp where id = ? 
2020-06-26 08:57:37,513 DEBUG [dao.TempDao.getById] - ==> Parameters: 1(Integer)
2020-06-26 08:57:37,538 DEBUG [dao.TempDao.getById] - <==      Total: 1
2020-06-26 08:57:37,538 INFO [TempTest] - TempEntity{id=1, value1='11111', value2='aaaaa'}
2020-06-26 08:57:37,538 INFO [TempTest] - TempEntity{id=1, value1='11111', value2='aaaaa'}
2020-06-26 08:57:37,538 INFO [TempTest] - true

1. summary

4.1 from the above results, we can see that in the second query, there is no database to query directly, and the same object is returned. Prove that the second time you go is caching.
4.2 level 1 cache is enabled by default. We didn't configure any caching in our code
4.3 code review

mybatis first level cache hit principle

How does mybatis judge that two queries are identical?

1.statementId

1.1 mapper.xml

    <select id="getById1" resultType="entity.TempEntity">
       select * from  temp where id = #{id}
    </select>

    <select id="getById2" resultType="entity.TempEntity">
       select * from  temp where id = #{id}
    </select>

1.2 test

    @Test
    public  void test() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis.xml");
        SqlSessionFactory build = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = build.openSession();
        TempEntity tempEntity1 = sqlSession.selectOne("dao.Temp2Dao.getById1", 1);
        logger.info(tempEntity1);
        TempEntity tempEntity2 = sqlSession.selectOne("dao.Temp2Dao.getById2", 1);
        logger.info(tempEntity2);
        logger.info(tempEntity1 == tempEntity2);
    }

1.3 results

2020-06-26 09:19:09,926 DEBUG [dao.Temp2Dao.getById1] - ==>  Preparing: select * from temp where id = ? 
2020-06-26 09:19:09,957 DEBUG [dao.Temp2Dao.getById1] - ==> Parameters: 1(Integer)
2020-06-26 09:19:09,969 DEBUG [dao.Temp2Dao.getById1] - <==      Total: 1
2020-06-26 09:19:09,969 INFO [TempTest] - TempEntity{id=1, value1='11111', value2='aaaaa'}
2020-06-26 09:19:09,969 DEBUG [dao.Temp2Dao.getById2] - ==>  Preparing: select * from temp where id = ? 
2020-06-26 09:19:09,970 DEBUG [dao.Temp2Dao.getById2] - ==> Parameters: 1(Integer)
2020-06-26 09:19:09,970 DEBUG [dao.Temp2Dao.getById2] - <==      Total: 1
2020-06-26 09:19:09,971 INFO [TempTest] - TempEntity{id=1, value1='11111', value2='aaaaa'}

1.4 summary
It is required that the statementId of the query must be exactly the same, otherwise the cache cannot be hit, and the two query statements and parameters are exactly the same

2. Query parameters

We use different parameters to query, one to one and two to two
2.1 test

    @Test
    public  void testParam() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis.xml");
        SqlSessionFactory build = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = build.openSession();
        TempEntity tempEntity1 = sqlSession.selectOne("dao.Temp2Dao.getById1", 1);
        logger.info(tempEntity1);
        TempEntity tempEntity2 = sqlSession.selectOne("dao.Temp2Dao.getById1", 2);
        logger.info(tempEntity2);
        logger.info(tempEntity1 == tempEntity2);
    }

2.2 results

2020-06-26 09:24:33,107 DEBUG [dao.Temp2Dao.getById1] - ==>  Preparing: select * from temp where id = ? 
2020-06-26 09:24:33,148 DEBUG [dao.Temp2Dao.getById1] - ==> Parameters: 1(Integer)
2020-06-26 09:24:33,162 DEBUG [dao.Temp2Dao.getById1] - <==      Total: 1
2020-06-26 09:24:33,162 INFO [TempTest] - TempEntity{id=1, value1='11111', value2='aaaaa'}
2020-06-26 09:24:33,162 DEBUG [dao.Temp2Dao.getById1] - ==>  Preparing: select * from temp where id = ? 
2020-06-26 09:24:33,163 DEBUG [dao.Temp2Dao.getById1] - ==> Parameters: 2(Integer)
2020-06-26 09:24:33,164 DEBUG [dao.Temp2Dao.getById1] - <==      Total: 1
2020-06-26 09:24:33,164 INFO [TempTest] - TempEntity{id=2, value1='22222', value2='bbbb'}
2020-06-26 09:24:33,164 INFO [TempTest] - false

2.3 summary

It is required to pass the same parameters to sql, otherwise the cache will not be hit

3. Paging parameters

3.1 transfer different paging parameters

    @Test
    public  void testPage() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis.xml");
        SqlSessionFactory build = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = build.openSession();
        RowBounds rowBounds1 = new RowBounds(0,1);
        List<TempEntity> tempEntity1 = sqlSession.selectList("dao.Temp2Dao.getList", null,rowBounds1);
        logger.info(tempEntity1);
        RowBounds rowBounds2 = new RowBounds(0,2);
        List<TempEntity> tempEntity2 = sqlSession.selectList("dao.Temp2Dao.getList",null, rowBounds2);
        logger.info(tempEntity2);
        logger.info(tempEntity1 == tempEntity2);
    }

3.2 results

2020-06-26 10:10:33,060 DEBUG [dao.Temp2Dao.getList] - ==>  Preparing: select * from temp where 1=1 
2020-06-26 10:10:33,101 DEBUG [dao.Temp2Dao.getList] - ==> Parameters: 
2020-06-26 10:10:33,116 INFO [TempTest] - [TempEntity{id=1, value1='11111', value2='aaaaa'}]
2020-06-26 10:10:33,116 DEBUG [dao.Temp2Dao.getList] - ==>  Preparing: select * from temp where 1=1 
2020-06-26 10:10:33,116 DEBUG [dao.Temp2Dao.getList] - ==> Parameters: 
2020-06-26 10:10:33,118 INFO [TempTest] - [TempEntity{id=1, value1='11111', value2='aaaaa'}, TempEntity{id=2, value1='22222', value2='bbbb'}]
2020-06-26 10:10:33,118 INFO [TempTest] - false

3.3 summary

It is required that the paging parameters must be the same, otherwise the cache cannot be hit. The granularity of the cache is the entire paging query result, not every object in the result

4. sql statement

4.1 mapper file

    <select id="getById" resultType="entity.TempEntity">
       select * from  temp 
       <where>
           <if test="type ==1">
           id = #{id}
            </if>
           <if test="type ==2">
               1=1 and id = #{id}
           </if>
       </where> 
    </select>

This one won't be tested.
4.2 summary
The sql passed to jdbc must be exactly the same. Even if 1 = 1 doesn't work

5. Environment

The environment here refers to that < environment id = "dev" > and < environment id = "test" > can also affect

    <environments default="dev">
        <environment id="dev">
            <!--Specify the type of transaction management, which is easy to use here Java Of JDBC Commit and rollback settings for-->
            <transactionManager type="JDBC"></transactionManager>
            <!--dataSource Refers to the connection source configuration, POOLED yes JDBC The implementation of data source connection pool of connection object-->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"></property>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/test"></property>
                <property name="username" value="root"></property>
                <property name="password" value="root"></property>
            </dataSource>
        </environment>
        <environment id="test">
            <!--Specify the type of transaction management, which is easy to use here Java Of JDBC Commit and rollback settings for-->
            <transactionManager type="JDBC"></transactionManager>
            <!--dataSource Refers to the connection source configuration, POOLED yes JDBC The implementation of data source connection pool of connection object-->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"></property>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/test"></property>
                <property name="username" value="root"></property>
                <property name="password" value="root"></property>
            </dataSource>
        </environment>
    </environments>

summary