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      MyBatis 詳解（一對一，一對多，多對多）
      
                    
                    
                    
                    
      
                         ywasdfghjkl
                        
                        
                        2019-04-25
                        
                            
                        
                    
      
                    
      
                        
                        
      
                    
      
                    
                    
      
                        
      
                            
                        
      
                        
      
                            
                                
      
                                    
                                        
      

                                        
                                        

                                        
      1、什么是MyBatis?
      　　MyBatis 本是apache的一個開源項目iBatis, 2010年這個項目由apache software foundation 遷移到了google code，并且改名為MyBatis 。2013年11月遷移到Github。
      　　iBATIS一詞來源于“internet”和“abatis”的組合，是一個基于Java的持久層框架。iBATIS提供的持久層框架包括SQL Maps和Data Access Objects（DAO）。
      　　MyBatis 是支持普通 SQL查詢，存儲過程和高級映射的優(yōu)秀持久層框架。MyBatis 消除了幾乎所有的JDBC代碼和參數(shù)的手工設(shè)置以及結(jié)果集的檢索。MyBatis 使用簡單的 XML或注解用于配置和原始映射，將接口和 Java 的POJOs（Plain Ordinary Java Objects，普通的 Java對象）映射成數(shù)據(jù)庫中的記錄。
      PS:本文所有代碼下載鏈接：http://pan.baidu.com/s/1gf8CPQN 密碼：t2x9 
      　　再給大家推薦一個比較好的mybatis學(xué)習(xí)網(wǎng)站：www.
      2、MyBatis 入門實例基于xml配置
      　　①、創(chuàng)建MySQL數(shù)據(jù)庫：mybatisDemo和表：person
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      create database mybatisDemo;
      use mybatisDemo;
      create table person(pid int primary key AUTO_INCREMENT, pname varchar(50), page int);
      　　
      　　②、建立一個Java工程，并導(dǎo)入相應(yīng)的jar包
       　　
      　　相應(yīng)的 jar 包下載鏈接：http://pan.baidu.com/s/1skZM09Z  密碼：nkt6
       　　③、在 MyBatisDemo 工程中添加數(shù)據(jù)庫配置文件 mybatis-configuration.xml
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      <?xml version='1.0' encoding='UTF-8'?>
      <!DOCTYPE configuration PUBLIC '-////DTD Config 3.0//EN' 'http:///dtd/mybatis-3-config.dtd'>
      <configuration>
      <!-- 可以配置多個運行環(huán)境，但是每個 SqlSessionFactory 實例只能選擇一個運行環(huán)境   
        一、development:開發(fā)模式
         二、work：工作模式-->
       <environments default='development'>
       <!--id屬性必須和上面的default一樣  -->
          <environment id='development'>
          <!--事務(wù)管理器
              一、JDBC：這個配置直接簡單使用了 JDBC 的提交和回滾設(shè)置。它依賴于從數(shù)據(jù)源得到的連接來管理事務(wù)范圍
              二、MANAGED：這個配置幾乎沒做什么。它從來不提交或回滾一個連接。而它會讓容器來管理事務(wù)的整個生命周期
                  比如 spring 或 JEE 應(yīng)用服務(wù)器的上下文，默認(rèn)情況下，它會關(guān)閉連接。然而一些容器并不希望這樣，
                  因此如果你需要從連接中停止它，就可以將 closeConnection 屬性設(shè)置為 false，比如：
                  <transactionManager type='MANAGED'>
                      <property name='closeConnection' value='false'/>
                  </transactionManager>
            -->
            <transactionManager type='JDBC'/>
            <!--dataSource 元素使用標(biāo)準(zhǔn)的 JDBC 數(shù)據(jù)源接口來配置 JDBC 連接對象源  -->
            <dataSource type='POOLED'>
              <property name='driver' value='com.mysql.jdbc.Driver'/>
              <property name='url' value='jdbc:mysql://localhost:3306/mybatisdemo'/>
              <property name='username' value='root'/>
              <property name='password' value='root'/>
            </dataSource>
          </environment>
        </environments>
      </configuration>
      　　④、定義表所對應(yīng)的實體類
      　　
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      package com.ys.bean;
      public class Person {
          private int pid;
          private String pname;
          private int page;
           
          public int getPid() {
              return pid;
          }
          public void setPid(int pid) {
              this.pid = pid;
          }
          public String getPname() {
              return pname;
          }
          public void setPname(String pname) {
              this.pname = pname;
          }
          public int getPage() {
              return page;
          }
          public void setPage(int page) {
              this.page = page;
          }
          @Override
          public String toString() {
              return 'Person [pid=' + pid + ', pname=' + pname + ', page=' + page
                      + ']';
          }
      }
      　　⑤、定義操作 person 表的sql映射文件personMapper.xml
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      <?xml version='1.0' encoding='UTF-8' ?>
      <!DOCTYPE mapper
        PUBLIC '-////DTD Mapper 3.0//EN'
        'http:///dtd/mybatis-3-mapper.dtd'>
      <mapper namespace='com.ys.bean.personMapper'>
        <!-- 根據(jù) pid 查詢 person 表中的數(shù)據(jù)
             id:唯一標(biāo)識符，此文件中的id值不能重復(fù)
             resultType:返回值類型，一條數(shù)據(jù)庫記錄也就對應(yīng)實體類的一個對象
             parameterType:參數(shù)類型，也就是查詢條件的類型
         -->
        <select id='selectPersonById'
                  resultType='com.ys.bean.Person' parameterType='int'>
          <!-- 這里和普通的sql 查詢語句差不多，對于只有一個查詢條件后面的 #{pid}表示占位符，里面不一定要寫pid,寫啥都可以，但是不要空著；如果有多個查詢條件，則要寫pojo類里面的屬性 -->
          select * from person where pid = #{pid}
        </select>
         
        <!-- 查詢person 表所有數(shù)據(jù) -->
        <select id='getAllPerson' resultType='com.ys.bean.Person'>
          select * from person
        </select>
         
        <!-- 根據(jù)id更新數(shù)據(jù) -->
        <update id='updatePersonById' parameterType='com.ys.bean.Person'>
          update person set pname=#{pname},page=#{page} where pid = #{pid}
        </update>
         
        <!-- 向 person 表插入一條數(shù)據(jù) -->
        <insert id='addPerson' parameterType='com.ys.bean.Person'>
          insert into person(pid,pname,page) values(#{pid},#{pname},#{page})
        </insert>
         
        <!-- 根據(jù) pid 刪除數(shù)據(jù) -->
        <delete id='deletePersonById' parameterType='Long'>
          delete from person where pid=#{pid}
        </delete>
         
      </mapper>
      　?、?strong>、向 mybatis-configuration.xml 配置文件中注冊 personMapper.xml 文件
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      <mappers>
             <!-- 注冊personMapper.xml文件， 
             personMapper.xml位于com.ys.bean這個包下，所以resource寫成com/ys/bean/personMapper.xml-->
             <mapper resource='com/ys/bean/personMapper.xml'/>
      </mappers>
      　　如下圖所示：
       　　⑦、創(chuàng)建測試類
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      package com.ys.test;
      import java.io.InputStream;
      import java.util.List;
      import org.apache.ibatis.session.SqlSession;
      import org.apache.ibatis.session.SqlSessionFactory;
      import org.apache.ibatis.session.SqlSessionFactoryBuilder;
      import org.junit.Before;
      import org.junit.Test;
      import com.ys.bean.Person;
      public class MyBatisTest {
          SqlSession session;
           
          @Before
          public void beforeLoadXML(){
              //加載 mybatis 配置文件
              InputStream inputStream = MyBatisTest.class.
                      getClassLoader().getResourceAsStream('mybatis-configuration.xml');
              //構(gòu)建sqlSession的工廠
              SqlSessionFactory sqlSessionFactory =
                      new SqlSessionFactoryBuilder().build(inputStream);
              //根據(jù) sqlSessionFactory 產(chǎn)生 session
              session = sqlSessionFactory.openSession();
          }
           
          //根據(jù) pid 查詢 person 表中的數(shù)據(jù)
          @Test
          public void testSelectById(){
              //這個字符串有 personMapper.xml 文件中 兩個部分構(gòu)成
              //<mapper namespace='com.ys.bean.personMapper'> 的 namespace 的值
              //<select id='selectPersonById' > id 值
              String statement = 'com.ys.bean.personMapper'+'.selectPersonById';
              Person p = session.selectOne(statement, 1);
              System.out.println(p);
              session.close();
          }
           
          //查詢person 表所有數(shù)據(jù)
          @Test
          public void testGetAllPerson(){
              String statement = 'com.ys.bean.personMapper.getAllPerson';
              List<Person> listPerson = session.selectList(statement);
              System.out.println(listPerson);
              session.close();
          }
           
          //根據(jù)id更新數(shù)據(jù) 
          @Test
          public void updateById(){
              String statement = 'com.ys.bean.personMapper.updatePersonById';
              Person p = new Person();
              p.setPid(1);
              p.setPname('aaa');
              p.setPage(11);
              session.update(statement, p);
              session.commit();
              session.close();
          }
           
          //向 person 表插入一條數(shù)據(jù)
          @Test
          public void addPerson(){
              String statement = 'com.ys.bean.personMapper.addPerson';
              Person p = new Person();
              //由于我們設(shè)置了主鍵的自增長機制，故這里不需要手動設(shè)置 pid 的值
              //p.setPid(1);
              p.setPname('add');
              p.setPage(11);
              session.insert(statement, p);
              session.commit();
              session.close();
          }
           
          //根據(jù) pid 刪除person 表中的數(shù)據(jù)
          @Test
          public void deletePersonById(){
              String statement = 'com.ys.bean.personMapper.deletePersonById';
              session.delete(statement, 1);
              session.commit();
              session.close();
               
          }
           
      }
      3、MyBatis 入門實例注解配置
       　　①、上面的前面四步都是一樣的，但是第五步不一樣，我們不需要創(chuàng)建 personMapper.xml 文件，首先在 src 目錄下創(chuàng)建 personMapper.java 文件
      　　
      　　　內(nèi)容如下：
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      package com.ys.annocation;
      import org.apache.ibatis.annotations.Delete;
      import org.apache.ibatis.annotations.Insert;
      import org.apache.ibatis.annotations.Select;
      import org.apache.ibatis.annotations.Update;
      import com.ys.bean.Person;
      public interface PersonMapper {
           
          @Insert('insert into person(pid,pname,page) values(#{pid},#{pname},#{page})')
          public int add(Person person); 
           
          @Select('select * from person where pid = #{pid}')
          public Person getPerson(int pid);
           
          @Update('update person set pname=#{pname},page=#{page} where pid = #{pid}')
          public int updatePerson(Person preson);
           
          @Delete('delete from person where pid=#{pid}')
          public int deletePerson(int pid);
      }
      　?、?、向 mybatis-configuration.xml 配置文件中注冊 personMapper.xml 文件
       　　③、編寫測試類
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      @Test
      public void testAnnocation(){
          PersonMapper mapper = session.getMapper(PersonMapper.class);
          Person p = new Person();
          p.setPid(7);
          p.setPname('abc');
          p.setPage(11);
          //調(diào)用增加方法
          mapper.add(p);
          //調(diào)用查詢方法
          Person p1 = mapper.getPerson(3);
          System.out.println(p1);
          //調(diào)用更新方法
          p.setPage(100);
          mapper.updatePerson(p);
          //調(diào)用刪除方法
          mapper.deletePerson(7);
          session.commit();
          session.close();
      }
      4、MyBatis 入門實例  一對一  基于xml配置
      　　這里我們以老師和班級為例，假設(shè)一般班級只能擁有有一個老師，一個老師只能帶一個班級。
      　　①、創(chuàng)建實體類
      　　
      　　Teacher.java
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      package one.to.one;
      public class Teacher {
          private int tid;
          private String tname;
          private Classes classes;
           
          public int getTid() {
              return tid;
          }
          public void setTid(int tid) {
              this.tid = tid;
          }
          public String getTname() {
              return tname;
          }
          public void setTname(String tname) {
              this.tname = tname;
          }
          public Classes getClasses() {
              return classes;
          }
          public void setClasses(Classes classes) {
              this.classes = classes;
          }
          @Override
          public String toString() {
              return 'Teacher [tid=' + tid + ', tname=' + tname + ', classes=' + classes + ']';
          }
           
           
      }
      　　Classes.java
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      package one.to.one;
      public class Classes {
          private int cid;
          private String cname;
          private Teacher teacher;
           
          public int getCid() {
              return cid;
          }
          public void setCid(int cid) {
              this.cid = cid;
          }
          public String getCname() {
              return cname;
          }
          public void setCname(String cname) {
              this.cname = cname;
          }
          public Teacher getTeacher() {
              return teacher;
          }
          public void setTeacher(Teacher teacher) {
              this.teacher = teacher;
          }
          @Override
          public String toString() {
              return 'Classes [cid=' + cid + ', cname=' + cname + ', teacher=' + teacher + ']';
          }
           
      }
      　　②、在數(shù)據(jù)庫中根據(jù)實體類創(chuàng)建相應(yīng)的數(shù)據(jù)表
      　?、?、定義操作 Classes 表的sql映射文件classesMapper.xml　
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      <?xml version='1.0' encoding='UTF-8' ?>
      <!DOCTYPE mapper
        PUBLIC '-////DTD Mapper 3.0//EN'
        'http:///dtd/mybatis-3-mapper.dtd'>
      <mapper namespace='one.to.one.classesMapper'>
          <!-- 
               方式一：嵌套結(jié)果：使用嵌套結(jié)果映射來處理重復(fù)的聯(lián)合結(jié)果的子集
                       封裝聯(lián)表查詢的數(shù)據(jù)(去除重復(fù)的數(shù)據(jù))
               select * from classes c, teacher t where c.tid=t.tid and c.tid=#{tid}
           -->
          <select id='getClasses' resultMap='getClassesMap' parameterType='int'>
              select * from classes c ,teacher t 
                  where c.tid=t.tid and c.tid=#{tid}
          </select>
          <resultMap type='one.to.one.Classes' id='getClassesMap'>
              <id column='cid' property='cid'/>
              <result column='cname' property='cname'/>
              <association property='teacher' javaType='one.to.one.Teacher'>
                  <id column='tid' property='tid'></id>
                  <result column='tname' property='tname'/>
              </association>
          </resultMap>
          <!-- 
               方式一：嵌套結(jié)果：使用嵌套結(jié)果映射來處理重復(fù)的聯(lián)合結(jié)果的子集
                       封裝聯(lián)表查詢的數(shù)據(jù)(去除重復(fù)的數(shù)據(jù))
               select * from teacher t,classes c where t.cid = c.cid and t.cid=#{cid}
           -->
          <select id='getTeacher' resultMap='getTeacherMap' parameterType='int'>
              select * from teacher t,classes c
                  where t.cid = c.cid and t.cid=#{cid}
          </select>
          <resultMap type='one.to.one.Teacher' id='getTeacherMap'>
              <id column='tid' property='tid'/>
              <result column='tname' property='tname'/>
              <association property='classes' javaType='one.to.one.Classes'>
                  <id column='cid' property='cid'/>
                  <result column='cname' property='cname'/>
              </association>
          </resultMap>
           
           
          <!-- 
               方式二：嵌套查詢：通過執(zhí)行另外一個SQL映射語句來返回預(yù)期的復(fù)雜類型
               SELECT * FROM classes WHERE cid=1;
               SELECT * FROM teacher WHERE tid=1   //1 是上一個查詢得到的tid的值
               property:別名(屬性名)    column：列名 -->
                <!-- 把teacher的字段設(shè)置進去 -->
          <select id='getClasses2' resultMap='getClassesMap2'>
              select * from classes c where c.cid = #{cid}
          </select>
          <resultMap type='one.to.one.Classes' id='getClassesMap2'>
              <id column='cid' property='cid'/>
              <result column='cname' property='cname'/>
              <collection property='teacher' column='tid' select='getTeacherCollection'>
              </collection>
          </resultMap>
          <select id='getTeacherCollection' resultType='one.to.one.Teacher'>
              select tid tid,tname tname from teacher where tid=#{tid}
          </select>
         
      </mapper>
      　　說明：我們這里一對一的關(guān)聯(lián)操作，有兩種方式：
      　　　　1、使用嵌套結(jié)果映射來處理重復(fù)的聯(lián)合結(jié)果的子集
      　　　　2、通過執(zhí)行另外一個SQL映射語句來返回預(yù)期的復(fù)雜類型
      　　　　相關(guān)屬性解釋：
      　　　　
      　?、堋?strong>向 mybatis-configuration.xml 配置文件中注冊 classesMapper.xml 文件
      　　
      　　⑤、編寫測試類
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      package one.to.one;
      import java.io.InputStream;
      import org.apache.ibatis.session.SqlSession;
      import org.apache.ibatis.session.SqlSessionFactory;
      import org.apache.ibatis.session.SqlSessionFactoryBuilder;
      import org.junit.Before;
      import org.junit.Test;
      import com.ys.test.MyBatisTest;
      public class OneToOneTest {
      SqlSession session;
           
          @Before
          public void beforeLoadXML(){
              //加載 mybatis 配置文件
              InputStream inputStream = MyBatisTest.class.
                      getClassLoader().getResourceAsStream('mybatis-configuration.xml');
              //構(gòu)建sqlSession的工廠
              SqlSessionFactory sqlSessionFactory =
                      new SqlSessionFactoryBuilder().build(inputStream);
              //根據(jù) sqlSessionFactory 產(chǎn)生 session
              session = sqlSessionFactory.openSession();
          }
           
          //一對一嵌套結(jié)果方式：根據(jù)教師id查詢班級信息
          @Test
          public void testGetClasses(){
              String statement = 'one.to.one.classesMapper.getClasses';
              Classes c = session.selectOne(statement, 1);
              System.out.println(c);
          }
           
          //一對一嵌套結(jié)果方式：根據(jù)班級id查詢教師信息
          @Test
          public void testGetTeacher(){
              String statement = 'one.to.one.classesMapper.getTeacher';
              Teacher t = session.selectOne(statement, 1);
              System.out.println(t);
          }
           
          //一對一嵌套查詢方式：根據(jù)教師id查詢班級信息
          @Test
          public void testGetClasses2(){
              String statement = 'one.to.one.classesMapper.getClasses2';
              Classes c = session.selectOne(statement, 1);
              System.out.println(c);
          }
           
           
      }
      4、MyBatis 入門實例  一對多，多對一  基于xml配置
      　　這里我們以班級和學(xué)生為例，一個班級里面對應(yīng)多個學(xué)生，這是一對多；反過來，多個學(xué)生對應(yīng)一個班級，這是多對一
      　?、佟⒔W(xué)生和班級的實體類
      　　Student.java
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      package one.to.many;
      public class Student {
          private int sid;
          private String sname;
          private Classes classes;
          public int getSid() {
              return sid;
          }
          public void setSid(int sid) {
              this.sid = sid;
          }
          public String getSname() {
              return sname;
          }
          public void setSname(String sname) {
              this.sname = sname;
          }
          public Classes getClasses() {
              return classes;
          }
          public void setClasses(Classes classes) {
              this.classes = classes;
          }
          @Override
          public String toString() {
              return 'Student [sid=' + sid + ', sname=' + sname + ', classes=' + classes + ']';
          }
           
      }
      　　　　Classes.java
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      package one.to.many;
      import java.util.Set;
      public class Classes {
          private int cid;
          private String cname;
          private Set<Student> students;
           
          public int getCid() {
              return cid;
          }
          public void setCid(int cid) {
              this.cid = cid;
          }
          public String getCname() {
              return cname;
          }
          public void setCname(String cname) {
              this.cname = cname;
          }
          public Set<Student> getStudents() {
              return students;
          }
          public void setStudents(Set<Student> students) {
              this.students = students;
          }
          @Override
          public String toString() {
              return 'Classes [cid=' + cid + ', cname=' + cname + ', students=' + students + ']';
          }
      }
      　　②、在數(shù)據(jù)庫中根據(jù)實體類創(chuàng)建相應(yīng)的數(shù)據(jù)表
      　　③、多對一：定義操作 Classes 表的sql映射文件classesMapper.xml　
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      <?xml version='1.0' encoding='UTF-8' ?>
      <!DOCTYPE mapper
        PUBLIC '-////DTD Mapper 3.0//EN'
        'http:///dtd/mybatis-3-mapper.dtd'>
      <mapper namespace='one.to.many.classesMapper'>
          <select id='getClasses' resultMap='getClassesMap'>
              select * from classes c,student s where s.cid=c.cid and c.cid=#{cid}
          </select>
          <resultMap type='one.to.many.Classes' id='getClassesMap'>
              <id column='cid' property='cid'></id>
              <result column='cname' property='cname'/>
              <collection property='students' ofType='one.to.many.Student'>
                  <id column='sid' property='sid'/>
                  <result column='sname' property='sname'/>
              </collection>
          </resultMap>
           
      </mapper>
      　?、?、一對多：定義操作 Student 表的sql映射文件studentMapper.xml
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      <?xml version='1.0' encoding='UTF-8' ?>
      <!DOCTYPE mapper
        PUBLIC '-////DTD Mapper 3.0//EN'
        'http:///dtd/mybatis-3-mapper.dtd'>
      <mapper namespace='many.to.one.studentMapper'>
          <select id='getStudents' resultMap='getStudentMap'>
              select * from classes c,student s where s.cid=c.cid and s.sid=#{sid}
          </select>
          <resultMap type='one.to.many.Student' id='getStudentMap'>
              <id column='sid' property='sid'></id>
              <result column='sname' property='sname'/>
              <association property='classes' javaType='one.to.many.Classes'>
                  <id column='cid' property='cid'/>
                  <result column='cname' property='cname'/>
              </association>
          </resultMap>
           
      </mapper>
      　　⑤、向 mybatis-configuration.xml 配置文件中注冊 classesMapper.xml 、studentMapper.xml文件
      　　⑥、編寫測試類
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      package one.to.many;
      import java.io.InputStream;
      import org.apache.ibatis.session.SqlSession;
      import org.apache.ibatis.session.SqlSessionFactory;
      import org.apache.ibatis.session.SqlSessionFactoryBuilder;
      import org.junit.Before;
      import org.junit.Test;
      import com.ys.test.MyBatisTest;
      public class OneToManyTest {
      SqlSession session;
           
          @Before
          public void beforeLoadXML(){
              //加載 mybatis 配置文件
              InputStream inputStream = MyBatisTest.class.
                      getClassLoader().getResourceAsStream('mybatis-configuration.xml');
              //構(gòu)建sqlSession的工廠
              SqlSessionFactory sqlSessionFactory =
                      new SqlSessionFactoryBuilder().build(inputStream);
              //根據(jù) sqlSessionFactory 產(chǎn)生 session
              session = sqlSessionFactory.openSession();
          }
           
          //一對多嵌套結(jié)果方式：根據(jù)班級id查詢班級所有的學(xué)生信息
          @Test
          public void testGetClasses(){
              String statement = 'one.to.many.classesMapper.getClasses';
              Classes c = session.selectOne(statement, 1);
              System.out.println(c);
              System.out.println(c.getStudents().size());
          }
           
           
          //多對一嵌套結(jié)果方式：根據(jù)學(xué)生id查詢班級信息
          @Test
          public void testGetStudents(){
              String statement = 'many.to.one.studentMapper.getStudents';
              Student s = session.selectOne(statement, 1);
              System.out.println(s);
              System.out.println(s.getClasses());
          }
           
           
      }
      5、MyBatis 入門實例  多對多  基于xml配置
      　　這里我們以 users 表和 groups 表為例，一個 users 可能加入多個 groups，而一個 groups 可能包含多個 users，故構(gòu)成 多對多 的關(guān)聯(lián)
      　　①、在數(shù)據(jù)庫中建立相應(yīng)的表
      　　users 表
      　　
      　　groups 表
      　　
      　　兩者之間的關(guān)聯(lián)表users_groups表
      　　
      　　②、建立對應(yīng)的實體類
      　　Users.java
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      package many.to.many;
      import java.util.Set;
      public class Users {
          private int uid;
          private String uname;
          private Set<Groups> groups;
          public int getUid() {
              return uid;
          }
          public void setUid(int uid) {
              this.uid = uid;
          }
          public String getUname() {
              return uname;
          }
          public void setUname(String uname) {
              this.uname = uname;
          }
          public Set<Groups> getGroups() {
              return groups;
          }
          public void setGroups(Set<Groups> groups) {
              this.groups = groups;
          }
          @Override
          public String toString() {
              return 'User [uid=' + uid + ', uname=' + uname + ', groups=' + groups + ']';
          }
           
      }
      　　　　Groups.java
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      package many.to.many;
      import java.util.Set;
      public class Groups {
          private int gid;
          private String gname;
          private Set<Users> users;
          public int getGid() {
              return gid;
          }
          public void setGid(int gid) {
              this.gid = gid;
          }
          public String getGname() {
              return gname;
          }
          public void setGname(String gname) {
              this.gname = gname;
          }
          public Set<Users> getUsers() {
              return users;
          }
          public void setUsers(Set<Users> users) {
              this.users = users;
          }
          @Override
          public String toString() {
              return 'Group [gid=' + gid + ', gname=' + gname + ', users=' + users + ']';
          }
           
      }
      　　　　Users_Groups.java
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      package many.to.many;
      public class Users_Groups {
          private Users user;
          private Groups group;
          public Users getUser() {
              return user;
          }
          public void setUser(Users user) {
              this.user = user;
          }
          public Groups getGroup() {
              return group;
          }
          public void setGroup(Groups group) {
              this.group = group;
          }
      }
      　?、?、多對多：定義操作 sql映射文件userMapper.xml
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      <?xml version='1.0' encoding='UTF-8' ?>
      <!DOCTYPE mapper
        PUBLIC '-////DTD Mapper 3.0//EN'
        'http:///dtd/mybatis-3-mapper.dtd'>
      <mapper namespace='many.to.many.userMapper'>
          <!-- 給一個用戶 id，查看該用戶下的所有用戶組信息 -->
          <select id='getUsers' resultMap='getGroupMap'>
              select g.gid,g.gname from users_groups ug,groups g 
                  where ug.group_id=g.gid and ug.user_id=#{uid}
          </select>
          <resultMap type='many.to.many.Groups' id='getGroupMap'>
              <id column='gid' property='gid'/>
              <result column='gname' property='gname'/>
              <collection property='users' ofType='many.to.many.Users'>
                  <id column='uid' property='uid'/>
              <result column='uname' property='uname'/>
              </collection>
          </resultMap>
           
      </mapper>
      　?、?、向 mybatis-configuration.xml 配置文件中注冊 userMapper.xml文件
      　　⑥、編寫測試類
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      //多對多：根據(jù)根據(jù)用戶 id 查詢所有的用戶組信息
          @Test
          public void testGetGroups(){
              String statement = 'many.to.many.userMapper.getUsers';
              List<Groups> listGroup = session.selectList(statement,1);
              for(Groups g : listGroup){
                  System.out.println(g.toString());
              }
          }
      
                                          		
                                
      
                                  
                        
      
                    
      



                
      
                
      
                    
                    
                        
      
                            
                                
        
                                    		
                        
        
                            
                
        
                
        
                
                
                
        
                
        


                

                
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