MyBatis学习笔记(三) 关联关系
首先给大家推荐几个网页：
http://blog.csdn.net/isea533/article/category/2092001 没事看看 - MyBatis工具：www.mybatis.tk
http://www.mybatis.org/mybatis-3/zh/getting-started.html 入门
http://edu.51cto.com/course/course_id-1354.html mybatis视频教程
一、多对多关系。
一般我们在设置多对多关系的时候，都是建立第三张关系表。
例子：学生t_student与课程t_courses，一个学生可以对应学习多门课程，一门课程对应可以有多名学生学习。第三张关系表t_stu_cou。
1）关系整理好，接着建立数据库表，添加信息。
1 create table t_student( 2 id int primary key auto_increment, 3 student_name varchar(20) 4 ); 5 6 create table t_courses( 7 id int primary key auto_increment, 8 courses_name varchar(20) 9 ); 10 11 create table t_stu_cou( 12 id int primary key auto_increment, 13 fk_stu_id int, 14 fk_cou_id int 15 );
添加数据。
1 insert into t_student values (null,'米兰'); 2 insert into t_student values (null,'凌雪'); 3 insert into t_student values (null,'成成'); 4 insert into t_student values (null,'睿懿'); 5 insert into t_student values (null,'瑞瑞'); 6 7 insert into t_courses values (null,'语文'); 8 insert into t_courses values (null,'数学'); 9 insert into t_courses values (null,'计算机'); 10 insert into t_courses values (null,'java编程'); 11 insert into t_courses values (null,'html'); 12 13 insert into t_stu_cou values (null,1,1); 14 insert into t_stu_cou values (null,1,2); 15 insert into t_stu_cou values (null,2,3); 16 insert into t_stu_cou values (null,2,4); 17 insert into t_stu_cou values (null,3,1); 18 insert into t_stu_cou values (null,3,5); 19 insert into t_stu_cou values (null,4,4); 20 insert into t_stu_cou values (null,4,2);
2）建立JavaBean。
CoursesBean.java
1 package com.cy.mybatis.beans;
2
3 import java.io.Serializable;
4 import java.util.List;
5 /**
6 * manyTOmany
7 * @author acer
8 *
9 */
10 public class CoursesBean implements Serializable{
11
12
13 private static final long serialVersionUID = 1L;
14 private Integer id;
15 private String name;
16 // 使用 List<StudentBean>集合，是说明学习这门课程的所有学生
17 private List<StudentBean> student;
18 public CoursesBean() {
19 super();
20 }
21 public CoursesBean(Integer id, String name, List<StudentBean> student) {
22 super();
23 this.id = id;
24 this.name = name;
25 this.student = student;
26 }
27 public Integer getId() {
28 return id;
29 }
30 public void setId(Integer id) {
31 this.id = id;
32 }
33 public String getName() {
34 return name;
35 }
36 public void setName(String name) {
37 this.name = name;
38 }
39 public List<StudentBean> getStudent() {
40 return student;
41 }
42 public void setStudent(List<StudentBean> student) {
43 this.student = student;
44 }
45 @Override
46 public String toString() {
47 return "CoursesBean [id=" + id + ", name=" + name + ", student="
48 + student + "]";
49 }
50
51
52 }
StudentBean.java
1 package com.cy.mybatis.beans;
2
3 import java.io.Serializable;
4 import java.util.List;
5 /**
6 * manyTOmany
7 * @author acer
8 *
9 */
10 public class StudentBean implements Serializable{
11
12 private static final long serialVersionUID = 1L;
13
14 private Integer id;
15 private String name;
16 private List<CoursesBean> courses;
17 public StudentBean() {
18 super();
19 // TODO Auto-generated constructor stub
20 }
21 public StudentBean(Integer id, String name, List<CoursesBean> courses) {
22 super();
23 this.id = id;
24 this.name = name;
25 this.courses = courses;
26 }
27 public Integer getId() {
28 return id;
29 }
30 public void setId(Integer id) {
31 this.id = id;
32 }
33 public String getName() {
34 return name;
35 }
36 public void setName(String name) {
37 this.name = name;
38 }
39 public List<CoursesBean> getCourses() {
40 return courses;
41 }
42 public void setCourses(List<CoursesBean> courses) {
43 this.courses = courses;
44 }
45 @Override
46 public String toString() {
47 return "StudentBean [id=" + id + ", name=" + name + ", courses="
48 + courses + "]";
49 }
50
51
52 }
3) 定义接口
CoursesMapper.java
1 package com.cy.mybatis.mapper;
2
3 import com.cy.mybatis.beans.CoursesBean;
4
5 public interface CoursesMapper {
6
7 /**
8 * 根据id查询课程
9 * @param id
10 * @return
11 */
12 public CoursesBean findCouById(int id);
13
14 /**
15 * 要求查课时，将选课的学生一并查出
16 * @param id
17 * @return
18 */
19 public CoursesBean findCouAndStu(int id);
20
21
22
23
24 }
StudentMapper.java
1 package com.cy.mybatis.mapper;
2
3 import com.cy.mybatis.beans.StudentBean;
4
5 public interface StudentMapper {
6 /**
7 * 根据id值查询学生信息
8 * @param id
9 * @return
10 */
11 public StudentBean findStuById(int id);
12
13 /**
14 * 要求查询学生时，将学生选择的课程查出
15 * @param id
16 * @return
17 */
18 public StudentBean findStuAndCou(int id);
19
20 }
4) 定义xml文件。CoursesMapper.xml StudentMapper.xml
mybatis实际是对XML进行操作，我们所有的方法都直接定义在XML中，写个接口只是为了更好的符合我们3层的思想.XML中只要有方法，就可以使用，而调用的方式就是：namespace+方法名；
CoursesMapper.xml
1 <?xml version="1.0" encoding="UTF-8"?>
2 <!DOCTYPE mapper PUBLIC "-//mybatis.org/DTD Mapper 3.0" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
3 <mapper namespace="com.cy.mybatis.mapper.CoursesMapper">
4
5
6 <resultMap type="CoursesBean" id="coursesMap">
7 <!-- 在默认情况下，mybatis会自动在TypeAliasRegistry初始化的时候挂在很多jdk常用类，
8 所以javaType="java.lang.Integer"可以写成javaType="Integer"-->
9
10 <id property="id" column="id" javaType="java.lang.Integer"/>
11 <result property="name" column="courses_name" javaType="java.lang.String"/>
12 </resultMap>
13
14 <resultMap type="CoursesBean" id="couAndStu">
15 <id property="id" column="id" javaType="java.lang.Integer"/>
16 <result property="name" column="courses_name" javaType="java.lang.String"/>
17
18 <!-- 对于一个属性的类型是一个集合，就使用collection
19 对于一个属性的类型是一个类，就使用association -->
20 <collection property="student" column="id" select="findStudentByCourses"></collection>
21 </resultMap>
22
23
24
25 <select id="findCouById" resultMap="coursesMap">
26 select * from t_courses where id=#{id}
27 </select>
28
29
30
31 <!-- 有学生表，课程表这两张表都没有外键，我们就要使用第三张关联表。我们就要根据课程表的fk_cou_id，把学生的id值得到。
32 对于多对多那么这个学生的id值就不可能是一个值 。在数据库里就要使用in-->
33 <select id="findStudentByCourses" resultMap="com.cy.mybatis.mapper.StudentMapper.studentMap">
34 select * from t_student where id in (select fk_stu_id from t_stu_cou where fk_cou_id=#{id})
35 </select>
36
37 <select id="findCouAndStu" resultMap="couAndStu">
38 select * from t_courses where id=#{id}
39 </select>
40 </mapper>
StudentMapper.xml
1 <?xml version="1.0" encoding="UTF-8"?>
2 <!DOCTYPE mapper PUBLIC "-//mybatis.org/DTD Mapper 3.0" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
3 <mapper namespace="com.cy.mybatis.mapper.StudentMapper">
4
5 <resultMap type="StudentBean" id="studentMap">
6 <id property="id" column="id" javaType="java.lang.Integer"/>
7 <result property="name" column="student_name" javaType="java.lang.String"/>
8 </resultMap>
9
10 <resultMap type="StudentBean" id="studentAndCourses">
11 <id property="id" column="id" javaType="java.lang.Integer"/>
12 <result property="name" column="student_name" javaType="java.lang.String"/>
13 <collection property="courses" column="id" select="findCoursesByStudent"></collection>
14 </resultMap>
15 <select id="findStudentById" resultMap="studentMap">
16 select * from t_student where id = #{id}
17 </select>
18
19 <select id="findStuAndCou" resultMap="studentAndCourses">
20 select * from t_student where id = #{id}
21 </select>
22 <select id="findCoursesByStudent" resultMap="com.cy.mybatis.mapper.CoursesMapper.coursesMap">
23 select * from t_courses where id in (select fk_cou_id from t_stu_cou where fk_stu_id = #{id})
24 </select>
25
26 </mapper>
5) 测试
1 package com.cy.mybatis.service;
2
3 import org.apache.ibatis.session.SqlSession;
4
5 import com.cy.mybatis.beans.CoursesBean;
6 import com.cy.mybatis.beans.StudentBean;
7 import com.cy.mybatis.mapper.CoursesMapper;
8 import com.cy.mybatis.mapper.StudentMapper;
9 import com.cy.mybatis.tools.DBTools;
10
11
12
13 public class ManyToManyService {
14
15 public static void main(String[] args) {
16
17 findStudentByCourses();
18 findCoursesByStudent();
19 }
20
21
22
23 private static void findCoursesByStudent() {
24 SqlSession session = DBTools.getSession();
25 StudentMapper sm=session.getMapper(StudentMapper.class);
26 StudentBean sb=sm.findStuAndCou(1);
27 System.out.println(sb);
28
29 }
30
31
32
33 private static void findStudentByCourses() {
34 SqlSession session = DBTools.getSession();
35 CoursesMapper cm=session.getMapper(CoursesMapper.class);
36 CoursesBean cb=cm.findCouAndStu(2);
37 System.out.println(cb);
38 }
39
40 }
结果显示：
1 DEBUG 2016-02-27 09:56:53,852 org.apache.ibatis.transaction.jdbc.JdbcTransaction: Opening JDBC Connection 2 DEBUG 2016-02-27 09:56:54,070 org.apache.ibatis.datasource.pooled.PooledDataSource: Created connection 586269. 3 DEBUG 2016-02-27 09:56:54,070 org.apache.ibatis.transaction.jdbc.JdbcTransaction: Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@8f21d] 4 DEBUG 2016-02-27 09:56:54,070 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ==> Preparing: select * from t_courses where id=? 5 DEBUG 2016-02-27 09:56:54,105 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ==> Parameters: 2(Integer) 6 DEBUG 2016-02-27 09:56:54,136 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ====> Preparing: select * from t_student where id in (select fk_stu_id from t_stu_cou where fk_cou_id=?) 7 DEBUG 2016-02-27 09:56:54,136 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ====> Parameters: 2(Integer) 8 DEBUG 2016-02-27 09:56:54,136 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: <==== Total: 2 9 DEBUG 2016-02-27 09:56:54,136 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: <== Total: 1 10 CoursesBean [id=2, name=数学, student=[StudentBean [id=1, name=米兰, courses=null], StudentBean [id=4, name=睿懿, courses=null]]] 11 DEBUG 2016-02-27 09:56:54,136 org.apache.ibatis.transaction.jdbc.JdbcTransaction: Opening JDBC Connection 12 DEBUG 2016-02-27 09:56:54,136 org.apache.ibatis.datasource.pooled.PooledDataSource: Created connection 23881129. 13 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.transaction.jdbc.JdbcTransaction: Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@16c65a9] 14 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ==> Preparing: select * from t_student where id = ? 15 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ==> Parameters: 1(Integer) 16 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ====> Preparing: select * from t_courses where id in (select fk_cou_id from t_stu_cou where fk_stu_id = ?) 17 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: ====> Parameters: 1(Integer) 18 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: <==== Total: 2 19 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: <== Total: 1 20 StudentBean [id=1, name=米兰, courses=[CoursesBean [id=1, name=语文, student=null], CoursesBean [id=2, name=数学, student=null]]]