比如一個班級有多個學生

班級表

CREATE TABLE `class` (
  `CLASS_ID` int(11) NOT NULL AUTO_INCREMENT,
  `CLASS_NAME` varchar(255) DEFAULT NULL,
  `CLASS_YEAR` varchar(255) DEFAULT NULL,
  `TEACHER_ID` int(255) DEFAULT NULL,
  PRIMARY KEY (`CLASS_ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

學生表

CREATE TABLE `student` (
  `student_id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `sex` varchar(255) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `class_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`student_id`),
  KEY `class_id` (`class_id`),
  CONSTRAINT `class_id` FOREIGN KEY (`class_id`) REFERENCES `class` (`CLASS_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
 

對應的實體類

package com.guahao.entity;

import java.util.List;

public class ClassEntity {
	private int classid;
	private String className;
	private String classYear;
	private Teacher teacher;
	private List<Student> students;
	public int getClassid() {
		return classid;
	}
	public void setClassid(int classid) {
		this.classid = classid;
	}
	public String getClassName() {
		return className;
	}
	public void setClassName(String className) {
		this.className = className;
	}
	public String getClassYear() {
		return classYear;
	}
	public void setClassYear(String classYear) {
		this.classYear = classYear;
	}
	public Teacher getTeacher() {
		return teacher;
	}
	public void setTeacher(Teacher teacher) {
		this.teacher = teacher;
	}
	public List<Student> getStudents() {
		return students;
	}
	public void setStudents(List<Student> students) {
		this.students = students;
	}
	@Override
	public String toString() {
		return "ClassEntity [classid=" + classid + ", className=" + className + ", classYear=" + classYear
				+ ", teacher=" + teacher + ", students=" + students + "]";
	}
	
	
	
	
}
 

package com.guahao.entity;

public class Student {
	private int studentId;
	private String name;
	private String sex;
	private int age;
	public int getStudentId() {
		return studentId;
	}
	public void setStudentId(int studentId) {
		this.studentId = studentId;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getSex() {
		return sex;
	}
	public void setSex(String sex) {
		this.sex = sex;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	@Override
	public String toString() {
		return "Student [studentId=" + studentId + ", name=" + name + ", sex=" + sex + ", age=" + age + "]";
	}
	
}
 

classDao.xml

聚集元素用來處理“一對多”的關係。需要指定對映的Java實體類的屬性，屬性的javaType（一般為ArrayList）；列表中物件的型別ofType（Java實體類）；對應的資料庫表的列名稱；（需要注意的是，這裡的查詢student的select語句條件必須是外來鍵class_id）。

<?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="com.guahao.dao.ClassDao">
	<resultMap type="com.guahao.entity.ClassEntity" id="classResultMap">
		<id property="classid" column="CLASS_ID" />
		<result property="className" column="CLASS_NAME" />
		<result property="classYear" column="CLASS_YEAR" />
		<association property="teacher" column="TEACHER_ID" select="com.guahao.dao.TeacherDao.getTeacher" />
		<collection property="students" column="class_id" 
		javaType="ArrayList" ofType="com.guahao.entity.Student" select="com.guahao.dao.StudentDao.getStudentByClassId">
		</collection>
	</resultMap>
	<select id="getClass" parameterType="java.lang.Integer" resultMap="classResultMap">
		SELECT
		*
		FROM CLASS CT
		WHERE CT.CLASS_ID = #{id};
	</select>
</mapper>

studentDao.xml

<?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="com.guahao.dao.StudentDao">
	<resultMap type="com.guahao.entity.Student" id="ResultMap">
		<id property="studentId" column="STUDENT_ID" />
		<result property="name" column="NAME" />
		<result property="sex" column="sex" />
		<result property="age" column="age" />
		
	</resultMap>
	<select id="getStudentByClassId" parameterType="java.lang.Integer" resultMap="ResultMap">
		SELECT
		*
		FROM Student CT
		WHERE CT.CLASS_ID = #{id};
	</select>
</mapper>

測試

@ContextConfiguration(locations = {"classpath:spring-mybatis.xml","classpath:spring-servlet.xml"})
@RunWith(SpringJUnit4ClassRunner.class)
public class Tt {
	@Resource
	private ClassDao classDao;
	@Resource
	private TeacherDao teacherDao;
	@Test
	public void getClassName(){
		ClassEntity classs = classDao.getClass(1);
		System.out.println(classs);
	}
	
}

結果

ClassEntity [classid=1, className=一班, classYear=三年級, teacher=Teacher [teacherId=1, teacherName=打發, teacherSex=男], students=[Student [studentId=1, name=張三, sex=男, age=12], Student [studentId=2, name=李四, sex=男, age=13]]]