Mybatis中的association用法

時間 2019-11-21

原文原文鏈接

這篇文章咱們未來學習一些 association 用法html

表結構

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', '計算機科學與技術', '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', '軟件工程');

對應的java bean

public class Student { 

 private int id;   

 private String name;     // 姓名 

 private String gender;   // 性別 

 private String major;    // 專業 

 private String grade;    // 年級 

 private Teacher supervisor; //指導教師  



 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;               // 教師姓名 

 private String gender;        // 教師性別 

 private String researchArea;    // 研究領域 

 private List<Student> supStudents; // 指導學生 



 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"> 

 <!-- 定義java Bean的屬性與數據庫的列之間的映射 --> 

 <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處理has-a關係 --> 
  <!--注意：這種方式javaType必須指定，表示supervisor的類型是Teacher，不然會報錯 --> 

  <association property="supervisor" javaType="Teacher"> 

   <!-- 教師自身的屬性與數據庫字段的映射 --> 

   <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語句中以"#{}"的形式引用參數 --> 

 <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>

測試

public void testAssociation(){ 

  SqlSession session = sqlSessionFactory.openSession(); 

  StudentOperationMapper studentOperationMapper = session.getMapper(StudentOperationMapper.class); 

  Student student = studentOperationMapper.getById(1); 

  System.out.println(student.getName()+"的指導老師是:"+student.getSupervisor().getName()); 

 }

myBatis association的兩種形式

嵌套的resultMap

這種方法的本質就是把教師實體映射從association元素中提取出來，用一個resultMap元素表示。而後association元素再引用這個resultMap元素。java

<?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"> 

 <!-- 定義java Bean的屬性與數據庫的列之間的映射 --> 

 <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語句中以"#{}"的形式引用參數 --> 

 <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語句

這種方式是使用一條單獨的select語句來加載關聯的實體(本例中就是教師實體)，而後在association元素中引用此select語句(注：此方法會產生N+1問題，儘可能不要用嵌套的select語句)sql

<?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"> 

 <!-- 定義java Bean的屬性與數據庫的列之間的映射 --> 

 <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語句中以"#{}"的形式引用參數 --> 

 <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>