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首先给大家推荐几个网页：

 没事看看 - MyBatis工具：www.mybatis.tk

 入门

  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     @Override46     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 stub20     }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     @Override46     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 id10      * @return11      */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 id17      * @return18      */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      * @return10      */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 id16      * @return17      */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collection19                       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: 110 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 Connection12 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: 219 DEBUG 2016-02-27 09:56:54,152 org.apache.ibatis.logging.jdbc.BaseJdbcLogger: <==      Total: 120 StudentBean [id=1, name=米兰, courses=[CoursesBean [id=1, name=语文, student=null], CoursesBean [id=2, name=数学, student=null]]]

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