mybatis文件映射之利用association进行关联查询之分步查询（三）

上一节我们

有Employee.java和Department.java

有EmployeeMapper.java和EmployeeMapper.xml

现在新增：DepartmentMapper.java和DepartmentMapper.xml

EmployeeMapper接口中有这么一个方法：

public Employee getEmpByIdStep(Integer id);

DepartmentMapper接口中有这么一个方法：

public Department getDeptById(Integer id);

DepartmentMapper.xml中：

<mapper namespace="com.gong.mybatis.dao.DepartmentMapper">
    <select id="getDeptById" resultType="com.gong.mybatis.bean.Department">
        select * from tbl_department where id=#{id}
    </select>
</mapper>

EmployeeMapper.xml中

<mapper namespace="com.gong.mybatis.dao.EmployeeMapperPlus">
    
    <resultMap type="com.gong.mybatis.bean.Employee" id="MySimpleByStep">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <association property="dept" select="com.gong.mybatis.dao.DepartmentMapper.getDeptById"
        column="d_id">            
        </association>
    </resultMap>
    <select id="getEmpByIdStep" resultMap="MySimpleByStep">
        select * from tbl_employee where id=#{id}
    </select>
    
</mapper>

说明：我们直接对association中的属性进行操作。property属性还是指定所需要关联的Employee类中的成员属性的名称。select属性对应着DepartmentMapper.xml中的getDeptById方法。column属性对应着传入到该方法中的参数，由于tbl_employee中的d_id字段是和tbl_department中的Id字段相关联，我们可以传入d_id。

最后进行测试：

package com.gong.mybatis.test;

import java.io.IOException;
import java.io.InputStream;
import java.util.Map;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.gong.mybatis.bean.Department;
import com.gong.mybatis.bean.Employee;
import com.gong.mybatis.dao.DepartmentMapper;
import com.gong.mybatis.dao.EmployeeMapperPlus;

public class TestMybatis2 {
    
    public SqlSessionFactory getSqlSessionFactory() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(resource);
        return new SqlSessionFactoryBuilder().build(is);
    }

    @Test
    public void test() throws IOException {
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession();
        try {
            EmployeeMapperPlus mapper = openSession.getMapper(EmployeeMapperPlus.class);
            Employee employee = mapper.getEmpByIdStep(3);
            System.out.println(employee);
            System.out.println(employee.getDept());
            openSession.commit();
        } finally {
            openSession.close();
        }
        
    }
}

输出：

DEBUG 01-20 14:36:37,371 ==> Parameters: 3(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 14:36:37,413 ====> Preparing: select * from tbl_department where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 14:36:37,414 ====> Parameters: 1(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 14:36:37,418 <==== Total: 1 (BaseJdbcLogger.java:145) DEBUG 01-20 14:36:37,419 <== Total: 1 (BaseJdbcLogger.java:145) Employee [id=3, lastName=小红, gender=0, email=xiaohong@qq.com, dept=Department [id=1, deptName=开发部]] Department [id=1, deptName=开发部]

说明通过这种方式也是可以成功的。