Mapper agent development

Mapper Agent development overview

There is a hard coding problem in writing code before , As shown in the figure below .

This call selectList() The parameters passed by the method are in the mapping configuration file namespace.id value . This is not convenient for later maintenance .
If you use Mapper Agency mode （ Here's the picture ） There is no hard coding problem .

Use Mapper The purpose of agency ：

• Solve the hard coding in the native way
• Simplify late execution SQL

Use Mapper agent

step1: Definition and SQL Mapping files with the same name Mapper Interface , And will Mapper Interface and SQL The mapping files are placed in the same directory .

step2: Set up SQL Mapping files namespace The attribute is Mapper The interface is fully qualified .

step3: stay Mapper Define methods in interfaces , The method name is SQL In the mapping file sql Of the statement id, And keep the parameter type consistent with the return value type


step4: change UserMapper.xml Configuration in progress mapper route

step5: code

MybatiDemo2.java

public class MybatiDemo2 {
    
    public static void main(String[] args) throws IOException {
    
        //1.  load mybatis Core profile for , obtain  SqlSessionFactory
        String resource = "mybatis-config.xml";

        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        //2.  obtain SqlSession object , Use it to execute sql
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //3.  perform sql
        //List<User> users = sqlSession.selectList("test.selectAll"); // The argument is a string , The string must be the of the mapping profile namespace.id
        //System.out.println(users);
// 3.1 obtain UserMapper The proxy object of the interface 
        UserMapper userMapper=sqlSession.getMapper(UserMapper.class);
        List<User> users=userMapper.selectAll();
        System.out.println(users);
        //4.  Release resources 
        sqlSession.close();
    }
}

Be careful ：

If Mapper Interface name and SQL The mapping file name is the same , And in the same directory , You can use packet scanning to simplify SQL Loading of mapping file . That is, modify the configuration of the loading mapping configuration file of the core configuration file to

<mappers>
    <!-- load sql The mapping file -->
    <!-- <mapper resource="com/itheima/mapper/UserMapper.xml"/>-->
    <!--Mapper Agency mode -->
    <package name="com.itheima.mapper"/>
</mappers>

Return value type alias

In the mapping configuration file resultType The type of data that needs to be encapsulated （ The fully qualified name of the class ）. And it's particularly troublesome to write like this every time ,Mybatis Provides Type the alias (typeAliases) It can simplify the writing of this part .

First, you need to configure the type alias in the core configuration file , Which means giving pojo All classes under the package are aliased （ An alias is a class name ）, Case insensitive . The contents are as follows ：

With the above configuration , We can simplify the mapping configuration file resultType Attribute value writing

<mapper namespace="com.itheima.mapper.UserMapper">
    <select id="selectAll" resultType="user">
        select * from tb_user;
    </select>
</mapper>

Reference resources
JavaWebNotes note