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

DROP TABLE IF EXISTS `student`; CREATE TABLE `student` (
 `id` int(11) NOT NULL auto_increment,
 `name` varchar(10) NOT NULL,
 `gender` char(1) NOT NULL,
 `major` varchar(20) NOT NULL,
 `grade` char(4) NOT NULL,
 `supervisor_id` int(11) NOT NULL,
 PRIMARY KEY  (`id`)
) 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` (
 `id` int(11) NOT NULL auto_increment,
 `name` varchar(10) NOT NULL,
 `gender` char(1) NOT NULL,
 `research_area` varchar(20) NOT NULL,
 PRIMARY KEY  (`id`)
) 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

public class Student {
private int id;  
private String name;     // 姓名
private String gender;   // 性别
private String major;    // 专业
private String grade;    // 年级
private Teacher supervisor; //指导教师  

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

public int getId() {
 return id;
}
public void setId(int id) {
 this.id = id;
}
public String getName() {
 return name;
}
public void setName(String name) {
 this.name = name;
}
public String getGender() {
 return gender;
}
public void setGender(String gender) {
 this.gender = gender;
}
public String getResearchArea() {
 return researchArea;
}
public void setResearchArea(String researchArea) {
 this.researchArea = researchArea;
}
public List<Student> getSupStudents() {
 return supStudents;
}
public void setSupStudents(List<Student> supStudents) {
 this.supStudents = supStudents;
}
}

mapper

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
"http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="net.zaodk.mybatis.mapper.StudentOperationMapper">
<!-- 定义java Bean的属性与数据库的列之间的映射 -->
<resultMap type="Student" id="studentResultMap">
 <id column="id" property="id" />
 <result column="name" property="name" />
 <result column="gender" property="gender" />
 <result column="major" property="major" />
 <result column="grade" property="grade"/>
 <!-- association处理has-a关系 -->
 <!--注意：这种方式javaType必须指定，表示supervisor的类型是Teacher，否则会报错 -->
 <association property="supervisor" javaType="Teacher">
  <!-- 教师自身的属性与数据库字段的映射 -->
  <id property="id" column="t_id"/>
  <result property="name" column="t_name"/>
  <result property="gender" column="t_gender"/>
  <result property="researchArea" column="research_area"/>
 </association>
</resultMap>

<!-- SQL语句中以"#{}"的形式引用参数 -->
<select id="getById" parameterType="int" resultMap="studentResultMap">
 SELECT st.id,st.name,st.gender,st.major,st.grade,t.id t_id,t.name t_name,
 t.gender t_gender,t.research_area  
 FROM student st, teacher t
 WHERE st.supervisor_id = t.id
 AND st.id=#{id}
</select> </mapper>

测试

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

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。

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
"http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="net.zaodk.mybatis.mapper.StudentOperationMapper">
<!-- 定义java Bean的属性与数据库的列之间的映射 -->
<resultMap type="Teacher" id="teacherResultMap">
 <id property="id" column="t_id"/>
  <result property="name" column="t_name"/>
  <result property="gender" column="t_gender"/>
  <result property="researchArea" column="research_area"/>
</resultMap>

<resultMap type="Student" id="studentResultMap">
 <id column="id" property="id" />
 <result column="name" property="name" />
 <result column="gender" property="gender" />
 <result column="major" property="major" />
 <result column="grade" property="grade"/>
 <!-- 引用teacherResultMap -->
 <association property="supervisor" resultMap="teacherResultMap"/>
</resultMap>

<!-- SQL语句中以"#{}"的形式引用参数 -->
<select id="getById" parameterType="int" resultMap="studentResultMap">
 SELECT st.id,st.name,st.gender,st.major,st.grade,t.id t_id,t.name t_name,
 t.gender t_gender,t.research_area  
 FROM student st, teacher t
 WHERE st.supervisor_id = t.id
 AND st.id=#{id}
</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)

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
"http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="net.zaodk.mybatis.mapper.StudentOperationMapper">
<!-- 定义java Bean的属性与数据库的列之间的映射 -->
<resultMap type="Teacher" id="supervisorResultMap">
 <id property="id" column="t_id"/>
  <result property="name" column="t_name"/>
  <result property="gender" column="t_gender"/>
  <result property="researchArea" column="research_area"/>
</resultMap>

<resultMap type="Student" id="studentResultMap">
 <id column="id" property="id" />
 <result column="name" property="name" />
 <result column="gender" property="gender" />
 <result column="major" property="major" />
 <result column="grade" property="grade"/>
 <!-- 引用teacherResultMap -->
 <association property="supervisor" column="supervisor_id" select="selectSupervisor"/>
</resultMap>

<!-- SQL语句中以"#{}"的形式引用参数 -->
<select id="getById" parameterType="int" resultMap="studentResultMap">
 select id,name,gender,major,grade,supervisor_id from student where id =#{id}
</select>

<select id="selectSupervisor" parameterType="int" resultMap="supervisorResultMap">
 select id,name,gender,research_area
 from teacher where id = #{id}
</select> </mapper>