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ResultMap
简介
resultMap– 描述如何从数据库结果集中加载对象，是最复杂也是最强大的元素。resultMap元素是 MyBatis 中最重要最强大的元素。它可以让你从 90% 的 JDBCResultSets数据提取代码中解放出来，并在一些情形下允许你进行一些 JDBC 不支持的操作。实际上，在为一些比如连接的复杂语句编写映射代码的时候，一份resultMap能够代替实现同等功能的数千行代码。- ResultMap 的设计思想是，对简单的语句做到零配置，对于复杂一点的语句，只需要描述语句之间的关系就行了。
多对一
a. 建表
CREATE TABLE `teacher` (
`id` INT(10) NOT NULL PRIMARY KEY,
`name` VARCHAR(30) DEFAULT NULL
)ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO teacher (`id`, `name`) VALUES (1, 'li');
CREATE TABLE `student` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
`tid` INT(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fktid` (`tid`),
CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
)ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO student (`id`, `name`, `tid`) VALUES (1, 'Tom', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (2, 'Jerry', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (3, 'Alice', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (4, 'Sam', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (5, 'Merry', 1);
b. 创建实体类
package com.pojo;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
//学生需要关联一个老师
private Teacher teacher;
}
package com.pojo;
import lombok.Data;
@Data
public class Teacher {
private int id;
private String name;
}
c. 在mybatis-config.xml中绑定接口
<!-- 接口绑定 -->
<!-- 接口和它的Mapper必须同名-->
<!-- 接口和它的Mapper必须在同一包下-->
<mappers>
<mapper class="com.dao.StudentMapper"></mapper>
<mapper class="com.dao.TeacherMapper"></mapper>
</mappers>
d. 创建Mapper.xml文件和Mapper接口
package com.dao;
import com.pojo.Student;
import java.util.List;
public interface StudentMapper {
// 查询所有的学生信息以及对应的老师的信息
List<Student> getStudent();
List<Student> getStudent2();
}
package com.dao;
import com.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
import java.util.List;
public interface TeacherMapper {
@Select("select * from teacher where id = #{tid}")
Teacher getTeacher(@Param("tid") int id);
}
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.StudentMapper">
<!-- select s.id sid,s.name sname,t.name tname-->
<!-- from student s,teacher t where s.tid=t.id;-->
<!-- 按照结果嵌套处理-->
<select id="getStudent2" resultMap="StudentTeacher2">
select s.id sid,s.name sname,t.name tname
from student s,teacher t where s.tid=t.id;
</select>
<resultMap id="StudentTeacher2" type="com.pojo.Student">
<result property="id" column="sid"></result>
<result property="name" column="sname"></result>
<association property="teacher" javaType="com.pojo.Teacher">
<result property="name" column="tname"></result>
</association>
</resultMap>
<!-- 按照查询嵌套处理 -->
<select id="getStudent" resultMap="StudentTeacher">
select * from student;
</select>
<resultMap id="StudentTeacher" type="com.pojo.Student">
<result property="id" column="id"></result>
<result property="name" column="name"></result>
<!--对象使用assiociation-->
<!--集合用collection-->
<association property="teacher" column="tid"
javaType="com.pojo.Teacher"
select="getTeacher"></association>
</resultMap>
<select id="getTeacher" resultType="com.pojo.Teacher">
select * from teacher where id = #{id};
</select>
</mapper>
c. 编写测试类
package com.dao;
import com.pojo.Student;
import com.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class StudentMapperTest {
@Test
public void getStudentTest() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> student = mapper.getStudent();
for (Student stu : student) {
System.out.println(stu);
}
sqlSession.close();
}
@Test
public void getStudent2Test() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> student = mapper.getStudent2();
for (Student stu : student) {
System.out.println(stu);
}
sqlSession.close();
}
}
输出：
Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Opening JDBC Connection
Created connection 1780132728.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6a1aab78]
==> Preparing: select * from student;
==> Parameters:
<== Columns: id, name, tid
<== Row: 1, Tom, 1
====> Preparing: select * from teacher where id = ?;
====> Parameters: 1(Integer)
<==== Columns: id, name
<==== Row: 1, li
<==== Total: 1
<== Row: 2, Jerry, 1
<== Row: 3, Alice, 1
<== Row: 4, Sam, 1
<== Row: 5, Merry, 1
<== Total: 5
Student(id=1, name=Tom, teacher=Teacher(id=1, name=li))
Student(id=2, name=Jerry, teacher=Teacher(id=1, name=li))
Student(id=3, name=Alice, teacher=Teacher(id=1, name=li))
Student(id=4, name=Sam, teacher=Teacher(id=1, name=li))
Student(id=5, name=Merry, teacher=Teacher(id=1, name=li))
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6a1aab78]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6a1aab78]
Returned connection 1780132728 to pool.
Process finished with exit code 0
一对多
a. 创建实体类
package com.pojo;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
private int tid;
}
package com.pojo;
import lombok.Data;
import java.util.List;
@Data
public class Teacher {
private int id;
private String name;
private List<Student> studentList;
}
b. 创建Mapper接口和对应的Mapper.xml
package com.dao;
import com.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface TeacherMapper {
List<Teacher> getTeacher();
// 获取指定老师下的所有学生以及老师的信息
// 按照结果查询
Teacher getTeacher(@Param("tid") int id);
// 按照查询嵌套处理
Teacher getTeacher2(@Param("tid") int id);
}
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.TeacherMapper">
<!-- 按照结果查询-->
<select id="getTeacher" resultMap="TeacherStudent">
select s.id sid, s.name sname, t.name tname, t.id tid
from student s, teacher t
where s.tid = t.id and t.id = #{tid};
</select>
<resultMap id="TeacherStudent" type="com.pojo.Teacher">
<result property="id" column="tid"></result>
<result property="name" column="tname"></result>
<!--集合中的泛型信息，我们用oftype获取-->
<collection property="studentList" ofType="com.pojo.Student">
<result property="id" column="sid"></result>
<result property="name" column="sname"></result>
</collection>
</resultMap>
<!-- 按照查询嵌套处理-->
<select id="getTeacher2" resultMap="TeacherStudent2">
select * from mybatis.teacher where id = #{tid}
</select>
<resultMap id="TeacherStudent2" type="com.pojo.Teacher">
<collection property="studentList" column="id" javaType="ArrayList"
ofType="com.pojo.Student"
select="getStudentByTeacherId"></collection>
</resultMap>
<select id="getStudentByTeacherId" resultType="com.pojo.Student">
select * from mybatis.student where tid = #{id}
</select>
</mapper>
package com.dao;
import com.pojo.Student;
public interface StudentMapper {
Student getStudentByTeacherId(int id);
}
c. 编写测试类
package com.dao;
import com.pojo.Teacher;
import com.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class TeacherMapperTest {
@Test
public void getTeacherTest() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);
System.out.println(teacher);
sqlSession.close();
}
@Test
public void getTeachar2Test() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher2(1);
System.out.println(teacher);
sqlSession.close();
}
}
输出：
com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 com.dao.TeacherMapperTest,getTeachar2Test
Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Opening JDBC Connection
Created connection 1032986144.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d921e20]
==> Preparing: select * from mybatis.teacher where id = ?
==> Parameters: 1(Integer)
<== Columns: id, name
<== Row: 1, li
====> Preparing: select * from mybatis.student where tid = ?
====> Parameters: 1(Integer)
<==== Columns: id, name, tid
<==== Row: 1, Tom, 1
<==== Row: 2, Jerry, 1
<==== Row: 3, Alice, 1
<==== Row: 4, Sam, 1
<==== Row: 5, Merry, 1
<==== Total: 5
<== Total: 1
com.pojo.Teacher@1a3869f4
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d921e20]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d921e20]
Returned connection 1032986144 to pool.
Process finished with exit code 0
总结
-
关联 - association 多对一
-
集合 - collection 一对多
-
javaType & ofType
- JavaType用来指定实体中属性类型
- ofType映射到list中的类型，泛型中的约束类型
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