Mybatis中的association用法

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>