Interface Proxy Implementation

Proxy Development Approach

MyBatis enables DAO layer creation through interface proxies. The framework dynamical generates proxy objects implementing mapper interfaces, eliminating manual DAO implementations.

Development Requirements:

1. XML mapper namespace must match the interface's fully qualified name
2. Method names must correspond to statement IDs in XML
3. Parameter types must align with SQL parameter definitions
4. Return types must match result type declarations

Mechanism:

• Developer defines interface methods
• MyBatis generates proxy implementation at runtime
• Proxy handles database operations via SqlSession

Testing Proxy Implementation

public Student retrieveEntity(Integer identifier) {
    try (InputStream cfgStream = Resources.getResourceAsStream("Configuration.xml");
         SqlSession dbSession = new SqlSessionFactoryBuilder().build(cfgStream).openSession(true)) {
        StudentRepository repo = dbSession.getMapper(StudentRepository.class);
        return repo.findById(identifier);
    } catch (IOException ex) {
        handleException(ex);
        return null;
    }
}

Source Analysis

• Proxies are instances of org.apache.ibatis.binding.MapperProxy
• Method execution routes through MapperMethod.execute()
• Command patterns determine CRUD operation routing

Dynamic SQL Constructs

Conditional Logic with <if>

Conditionally include SQL segments based on runtime parameters:

<select id="queryEntities" resultType="Student">
    SELECT * FROM students
    <where>
        <if test="id != 0">AND id = #{id}</if>
        <if test="name != null">AND name = #{name}</if>
    </where>
</select>

Collection Processing with <foreach>

Iterate over collections for batch operations:

<select id="batchFetch" resultType="Student">
    SELECT * FROM students
    <where>
        <foreach collection="idCollection" 
                 open="id IN (" close=")" 
                 item="itemId" separator=",">
            #{itemId}
        </foreach>
    </where>
</select>

SQL Fragmetn Reuse

Extract reusable SQL components:

<sql id="baseSelect">SELECT * FROM students</sql>
<select id="findById" resultType="Student">
    <include refid="baseSelect"/> WHERE id = #{id}
</select>

Key Tags:

• <where>: Dynamic WHERE clause handling
• <if>: Conditional inclusion
• <foreach>: Collection iteration
• <sql>/<include>: SQL snippet reuse

Pagination Integration

PageHelper Configuration

1. Add dependencies: pagehelper and jsqlparser
2. Configure plugin in MyBatis settings:

<plugins>
    <plugin interceptor="com.github.pagehelper.PageInterceptor">
        <property name="helperDialect" value="mysql"/>
    </plugin>
</plugins>

Usage Pattern

PageHelper.startPage(2, 10); // Page 2, 10 items/page
List<User> results = userRepository.findAll();
PageInfo<User> paginationData = new PageInfo<>(results);

Pagination Metrics:

• Total records: paginationData.getTotal()
• Total pages: paginationData.getPages()
• Current page: paginationData.getPageNum()
• Page size: paginationData.getPageSize()

Multi-Table Relationships

Association Types

• One-to-One: Primary key ↔ Foreign key reference
• One-to-Many: Foreign key in child table
• Many-to-Many: Join table with dual foreign keys

One-to-One Mapping

<resultMap id="passportMap" type="Passport">
    <id column="pass_id" property="id"/>
    <result column="number" property="serial"/>
    <association property="holder" javaType="Person">
        <id column="person_id" property="id"/>
        <result column="full_name" property="name"/>
    </association>
</resultMap>

One-to-Many Mapping

<resultMap id="departmentMap" type="Department">
    <id column="dept_id" property="id"/>
    <result column="dept_name" property="name"/>
    <collection property="staff" ofType="Employee">
        <id column="emp_id" property="id"/>
        <result column="emp_name" property="name"/>
    </collection>
</resultMap>

Many-to-Many Mapping

<resultMap id="studentMap" type="Student">
    <id column="stu_id" property="id"/>
    <collection property="enrollments" ofType="Course">
        <id column="course_id" property="id"/>
        <result column="course_title" property="title"/>
    </collection>
</resultMap>

Mapping Elements:

• <association>: Single-object relationships
• <collection>: Multi-object relationships
• javaType/ofType: Target entity types