深入浅出Mybatis系列（八）---mapper映射文件配置之select、resultMap

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上篇《深入浅出Mybatis系列（七）---mapper映射文件配置之insert、update、delete》介绍了insert、update、delete的用法，本篇将介绍select、resultMap的用法。select无疑是我们最常用，也是最复杂的，mybatis通过resultMap能帮助我们很好地进行高级映射。下面就开始看看select 以及 resultMap的用法：

先看select的配置吧：

<select
 id="selectPerson" 
parameterType="int" 
resultType="hashmap" 
resultMap="personResultMap" 
flushCache="false" 
useCache="true" 
timeout="10000" 
fetchSize="256" 
statementType="PREPARED" 
resultSetType="FORWARD_ONLY">

配置看起来总是这么多，不过实际常用的配置也就那么几个，根据自己的需要吧，上面都已注明是否必须配置。

下面还是上个demo及时练练手吧：

------------------------------------------------------------------------下面是针对select 的练手demo---------------------------------------------------------------------------------------

数据库：新增两张表（t_course, t_student）

t_course:

t_student:

其中，1个student可选择多个course进行学习。

我们还是拿上篇文章的demo, 继续写：

增加后，项目目录如下所示：

Course.java:

package com.dy.entity;public class Course { private int id;
private String name;
private int deleteFlag; public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getDeleteFlag() {
return deleteFlag;
}
public void setDeleteFlag(int deleteFlag) {
this.deleteFlag = deleteFlag;
}}

Student.java:

package com.dy.entity;import java.util.List;public class Student { private int id;
private String idCard;
private String name;
private List<Course> courseList;
private int deleteFlag; public Student(int id, String idCard, String name, List<Course> courseList, int deleteFlag) {
this.id = id;
this.idCard = idCard;
this.name = name;
this.courseList = courseList;
this.deleteFlag = deleteFlag;
} public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getIdCard() {
return idCard;
}
public void setIdCard(String idCard) {
this.idCard = idCard;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Course> getCourseList() {
return courseList;
}
public void setCourseList(List<Course> courseList) {
this.courseList = courseList;
}
public int getDeleteFlag() {
return deleteFlag;
}
public void setDeleteFlag(int deleteFlag) {
this.deleteFlag = deleteFlag;
}
}

CourseDao.java:

package com.dy.dao;import com.dy.entity.Course;public interface CourseDao { public Course findCourseById(int courseId);}

StudentDao.java:

package com.dy.dao;import com.dy.entity.Student;public interface StudentDao { public Student findStudentById(String idCard);
}

courseDao.xml:

<mapper namespace="com.dy.dao.CourseDao"> 
<select id="findCourseById" resultType="course" >
select course_id as id, course_name as name, course_delete_flg as deleteFlag from t_course where course_id=#{courseId}
</select></mapper>

CourseDaoTest.java:

package com.dy.dao;import java.io.IOException;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.dy.entity.Course;public class CourseDaoTest { @Test
public void findCourseById() {
SqlSessionFactory sqlSessionFactory = getSessionFactory();
SqlSession sqlSession = sqlSessionFactory.openSession();
CourseDao courseDao = sqlSession.getMapper(CourseDao.class);
Course course = courseDao.findCourseById(1);
} //Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互
private static SqlSessionFactory getSessionFactory() {
SqlSessionFactory sessionFactory = null;
String resource = "mybatis-conf.xml";
try {
sessionFactory = new SqlSessionFactoryBuilder().build(Resources
.getResourceAsReader(resource));
} catch (IOException e) {
e.printStackTrace();
}
return sessionFactory;
}
}

上面的示例，我们针对course, 简单演示了 select的用法，不过有个问题值得思考：一个student可以对应多个course, 那么，在mybatis中如何处理这种一对多，甚至于多对多，一对一的关系呢？

这儿，就不得不提到 resultMap 这个东西， mybatis的resultMap功能可谓十分强大，能够处理复杂的关系映射，那么resultMap 该怎么配置呢？别急，这就来了：

resultMap的配置：

<resultMap type="" id=""> 
<id property="" column=""/> 
<result property="" column=""/> 
<constructor>

<idArg column=""/>

<arg column=""/>
</constructor> 
<collection property="" column="" ofType=""></collection> 
<association property="" column="" javaType=""></association>
</resultMap>

好啦，知道resutMap怎么配置后，咱们立即接着上面的demo来练习一下吧：

------------------------------------------------------------------下面是用resultMap处理一对多关系的映射的示例-------------------------------------------------------------

一个student对应多个course，典型的一对多，咱们就来看看mybatis怎么配置这种映射吧：

studentDao.xml:

<mapper namespace="com.dy.dao.StudentDao"> 
<resultMap type="student" id="studentMap"> 
<id property="idCard" column="stu_id_card"/>
<result property="id" column="stu_id"/>
<result property="name" column="stu_name"/>
<result property="deleteFlag" column="stu_delete_flg"/> 
<collection property="courseList" column="stu_course_id" ofType="Course">
<id property="id" column="course_id"/>
<result property="name" column="course_name"/>
<result property="deleteFlag" column="course_delete_flg"/>
</collection>
</resultMap> 
<select id="findStudentById" resultMap="studentMap">
SELECT s.*, c.* FROM t_student s LEFT JOIN t_course c ON s.stu_course_id=c.course_id WHERE s.stu_id_card=#{idCard}
</select></mapper>

StudentDaoTest.java:

package com.dy.dao;import java.io.IOException;
import java.util.List;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.dy.entity.Course;
import com.dy.entity.Student;public class StudentDaoTest { @Test
public void findCourseById() {
SqlSessionFactory sqlSessionFactory = getSessionFactory();
SqlSession sqlSession = sqlSessionFactory.openSession();
StudentDao studentDao = sqlSession.getMapper(StudentDao.class);
Student student = studentDao.findStudentById("20140101");
List<Course> courseList = student.getCourseList();
for (Course course: courseList) {
System.out.println(course.getId() + " " + course.getName());
}
} //Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互
private static SqlSessionFactory getSessionFactory() {
SqlSessionFactory sessionFactory = null;
String resource = "mybatis-conf.xml";
try {
sessionFactory = new SqlSessionFactoryBuilder().build(Resources
.getResourceAsReader(resource));
} catch (IOException e) {
e.printStackTrace();
}
return sessionFactory;
}
}