MyBatis Parameter Passing: Trensferring parameters from Java code to mapper.xml files.

Single Simple Parameter

When a DAO interface method contains only one simple parameter (Java primitive type or String), the placeholder #{any_text} can be used, and it is independent of the method's parameter name.

Mappper XML Configuration (StudentDao.xml)

<!-- namespace: unique namespace, recommended to use the fully qualified name of the DAO interface -->
<mapper namespace="com.lifang.dao.StudentDao">
    <!--
    <select>: represents a query operation, must contain a select statement
    1. id: custom name for the SQL statement, must be unique, use the DAO interface method name
    2. resultType: recommended to use the fully qualified class name
    -->
    <select id="selectStudentById" resultType="com.lifang.domain.Student">
        SELECT id, name, email, age FROM student WHERE id = #{identifier}
        <!-- MyBatis creates Student object and assigns property values -->
    </select>
</mapper>

DAO Interface Definition

public interface StudentDao {
    // Retrieve all records from Student table
    List<Student> selectStudents();
    int insertStudent(Student student);
    Student selectStudentById(Integer id);
}

Test Implementation

@Test
public void testQueryById() {
    SqlSession session = MybatisUtils.getSqlSession();
    StudentDao dao = session.getMapper(StudentDao.class);
    Student result = dao.selectStudentById(1002);
    System.out.println(result);
}

ParameterType Attribute

The parameterType attribute specifies the data type of method parameters in the DAO interface. It accepts either fully qualified Java type names or MyBatis-defined aliases. This attribute is optional since MyBatis can infer parameter types through reflection.

<!--
    parameterType: data type of DAO interface method parameter (optional)
    Values can be Java fully qualified names or MyBatis aliases
    Examples: parameterType="java.lang.Integer"
              parameterType="int"
-->
<select id="selectStudentById" parameterType="int" resultType="com.lifang.domain.Student">
    SELECT id, name, email, age FROM student WHERE id = #{identifier}
</select>

<!-- Equivalent without parameterType -->
<select id="selectStudentById" resultType="com.lifang.domain.Student">
    SELECT id, name, email, age FROM student WHERE id = #{identifier}
</select>

Common MyBatis type aliases include:

• int or java.lang.Integer
• hashmap or java.util.HashMap
• list or java.util.ArrayList

The parameterType atttribute can be used with <select>, <insert>, <update>, and <delete> elements.