Mybatis中的<association>用法

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这篇文章我们将来学习一些 association用法

表结构

1.  
   DROP TABLE IF EXISTS `student`; CREATE TABLE `student` (
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    `id` int(11) NOT NULL auto_increment,
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    `name` varchar(10) NOT NULL,
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    `gender` char(1) NOT NULL,
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    `major` varchar(20) NOT NULL,
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    `grade` char(4) NOT NULL,
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    `supervisor_id` int(11) NOT NULL,
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    PRIMARY KEY  (`id`)
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   ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of student -- ---------------------------- INSERT INTO `student` VALUES ('1', 'zhangsan', '男', '计算机科学与技术', '2011', '1'); -- ---------------------------- -- Table structure for teacher -- ---------------------------- DROP TABLE IF EXISTS `teacher`; CREATE TABLE `teacher` (
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     `id` int(11) NOT NULL auto_increment,
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     `name` varchar(10) NOT NULL,
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     `gender` char(1) NOT NULL,
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     `research_area` varchar(20) NOT NULL,
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     PRIMARY KEY  (`id`)
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    ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of teacher -- ---------------------------- INSERT INTO `teacher` VALUES ('1', 'laowu', '男', '软件工程');

对应的java bean

1.  
   public class Student {
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   private int id;  
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   private String name;     // 姓名
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   private String gender;   // 性别
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   private String major;    // 专业
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   private String grade;    // 年级
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   private Teacher supervisor; //指导教师  
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   public int getId() {
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     return id;
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    }
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    public void setId(int id) {
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     this.id = id;
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    }
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    public String getName() {
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     return name;
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    }
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    public void setName(String name) {
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     this.name = name;
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    }
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    public String getGender() {
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     return gender;
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    }
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    public void setGender(String gender) {
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     this.gender = gender;
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    }
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    public String getMajor() {
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     return major;
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    }
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    public void setMajor(String major) {
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     this.major = major;
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    }
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    public String getGrade() {
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     return grade;
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    }
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    public void setGrade(String grade) {
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     this.grade = grade;
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    }
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    public Teacher getSupervisor() {
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     return supervisor;
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    }
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    public void setSupervisor(Teacher supervisor) {
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     this.supervisor = supervisor;
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    }
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    }
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    public class Teacher {
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    private int id;    
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    private String name;               // 教师姓名
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    private String gender;        // 教师性别
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    private String researchArea;    // 研究领域
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    private List<Student> supStudents; // 指导学生
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    public int getId() {
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     return id;
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    }
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    public void setId(int id) {
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     this.id = id;
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    }
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    public String getName() {
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     return name;
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    }
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    public void setName(String name) {
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     this.name = name;
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    }
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    public String getGender() {
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     return gender;
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    }
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    public void setGender(String gender) {
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     this.gender = gender;
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    }
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    public String getResearchArea() {
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     return researchArea;
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    }
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    public void setResearchArea(String researchArea) {
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     this.researchArea = researchArea;
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    }
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    public List<Student> getSupStudents() {
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     return supStudents;
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    }
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    public void setSupStudents(List<Student> supStudents) {
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     this.supStudents = supStudents;
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    }
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    }

mapper

1.  
   <?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
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   "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="net.zaodk.mybatis.mapper.StudentOperationMapper">
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   <!-- 定义java Bean的属性与数据库的列之间的映射 -->
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   <resultMap type="Student" id="studentResultMap">
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    <id column="id" property="id" />
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    <result column="name" property="name" />
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    <result column="gender" property="gender" />
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    <result column="major" property="major" />
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    <result column="grade" property="grade"/>
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     <!-- association处理has-a关系 -->
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     <!--注意：这种方式javaType必须指定，表示supervisor的类型是Teacher，否则会报错 -->
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     <association property="supervisor" javaType="Teacher">
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      <!-- 教师自身的属性与数据库字段的映射 -->
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      <id property="id" column="t_id"/>
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      <result property="name" column="t_name"/>
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      <result property="gender" column="t_gender"/>
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      <result property="researchArea" column="research_area"/>
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     </association>
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    </resultMap>
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    <!-- SQL语句中以"#{}"的形式引用参数 -->
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    <select id="getById" parameterType="int" resultMap="studentResultMap">
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     SELECT st.id,st.name,st.gender,st.major,st.grade,t.id t_id,t.name t_name,
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     t.gender t_gender,t.research_area  
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     FROM student st, teacher t
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     WHERE st.supervisor_id = t.id
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     AND st.id=#{id}
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    </select> </mapper>

测试

1.  
   public void testAssociation(){
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    SqlSession session = sqlSessionFactory.openSession();
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    StudentOperationMapper studentOperationMapper = session.getMapper(StudentOperationMapper.class);
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    Student student = studentOperationMapper.getById(1);
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    System.out.println(student.getName()+"的指导老师是:"+student.getSupervisor().getName());
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   }

myBatis association的两种形式

嵌套的resultMap

这种方法的本质就是把教师实体映射从association元素中提取出来，用一个resultMap元素表示。然后association元素再引用这个resultMap元素。

1.  
   <?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
2.  
   "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="net.zaodk.mybatis.mapper.StudentOperationMapper">
3.  
   <!-- 定义java Bean的属性与数据库的列之间的映射 -->
4.  
   <resultMap type="Teacher" id="teacherResultMap">
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    <id property="id" column="t_id"/>
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     <result property="name" column="t_name"/>
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     <result property="gender" column="t_gender"/>
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     <result property="researchArea" column="research_area"/>
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   </resultMap>
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    <resultMap type="Student" id="studentResultMap">
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     <id column="id" property="id" />
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     <result column="name" property="name" />
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     <result column="gender" property="gender" />
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     <result column="major" property="major" />
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     <result column="grade" property="grade"/>
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     <!-- 引用teacherResultMap -->
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     <association property="supervisor" resultMap="teacherResultMap"/>
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    </resultMap>
20.  
     
21.  
    <!-- SQL语句中以"#{}"的形式引用参数 -->
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    <select id="getById" parameterType="int" resultMap="studentResultMap">
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     SELECT st.id,st.name,st.gender,st.major,st.grade,t.id t_id,t.name t_name,
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     t.gender t_gender,t.research_area  
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     FROM student st, teacher t
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     WHERE st.supervisor_id = t.id
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     AND st.id=#{id}
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    </select> </mapper>

嵌套的select语句

这种方式是使用一条单独的select语句来加载关联的实体(本例中就是教师实体)，然后在association元素中引用此select语句(注：此方法会产生N+1问题，尽量不要用嵌套的select语句)

1.  
   <?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
2.  
   "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="net.zaodk.mybatis.mapper.StudentOperationMapper">
3.  
   <!-- 定义java Bean的属性与数据库的列之间的映射 -->
4.  
   <resultMap type="Teacher" id="supervisorResultMap">
5.  
    <id property="id" column="t_id"/>
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     <result property="name" column="t_name"/>
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     <result property="gender" column="t_gender"/>
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     <result property="researchArea" column="research_area"/>
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   </resultMap>
10.  
     
11.  
    <resultMap type="Student" id="studentResultMap">
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     <id column="id" property="id" />
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     <result column="name" property="name" />
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     <result column="gender" property="gender" />
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     <result column="major" property="major" />
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     <result column="grade" property="grade"/>
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     <!-- 引用teacherResultMap -->
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     <association property="supervisor" column="supervisor_id" select="selectSupervisor"/>
19.  
    </resultMap>
20.  
     
21.  
    <!-- SQL语句中以"#{}"的形式引用参数 -->
22.  
    <select id="getById" parameterType="int" resultMap="studentResultMap">
23.  
     select id,name,gender,major,grade,supervisor_id from student where id =#{id}
24.  
    </select>
25.  
     
26.  
    <select id="selectSupervisor" parameterType="int" resultMap="supervisorResultMap">
27.  
     select id,name,gender,research_area
28.  
     from teacher where id = #{id}
29.  
    </select> </mapper