mybatis的一对一，一对多的crud操作

于 2016-12-28 23:40:44 发布
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本文链接：https://blog.youkuaiyun.com/linyeban/article/details/53915045
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本文详细介绍了如何在MyBatis中进行一对一和一对多的关联查询。首先，通过创建表和数据，定义实体类，配置sql映射文件classMapper.xml，以及编写工具类MyBatisUtil来实现一对一关联查询。接着，讨论了一对多关联查询的需求，创建学生表，修改映射文件，并编写测试代码。文章总结了MyBatis中association和collection标签的使用，提供了相关资源链接及源代码下载地址。

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一、一对一关联

 1.1、提出需求

　　根据班级id查询班级信息(带老师的信息)

1.2、创建表和数据

　　创建一张教师表和班级表，这里我们假设一个老师只负责教一个班，那么老师和班级之间的关系就是一种一对一的关系。

CREATE TABLE teacher(
    t_id INT PRIMARY KEY AUTO_INCREMENT, 
    t_name VARCHAR(20)
);
CREATE TABLE class(
    c_id INT PRIMARY KEY AUTO_INCREMENT, 
    c_name VARCHAR(20), 
    teacher_id INT
);
ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);    

INSERT INTO teacher(t_name) VALUES('teacher1');
INSERT INTO teacher(t_name) VALUES('teacher2');

INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1);
INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);
表之间的关系如下：

1.3、定义实体类

　　1、Teacher类，Teacher类是teacher表对应的实体类。

package me.gacl.domain;

/**
 * @author gacl
 * 定义teacher表对应的实体类
 */
public class Teacher {

    //定义实体类的属性，与teacher表中的字段对应
    private int id;            //id===>t_id
    private String name;    //name===>t_name

    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;
    }

    @Override
    public String toString() {
        return "Teacher [id=" + id + ", name=" + name + "]";
    }
}

　　2、Classes类，Classes类是class表对应的实体类

package me.gacl.domain;

/**
 * @author gacl
 * 定义class表对应的实体类
 */
public class Classes {

    //定义实体类的属性，与class表中的字段对应
    private int id;            //id===>c_id
    private String name;    //name===>c_name
    
    /**
     * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
     * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
     */
    private Teacher teacher;

    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 Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    @Override
    public String toString() {
        return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";
    }
}

1.4、定义sql映射文件classMapper.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.lin.dao.ClassesMapper">

	<!-- 根据班级id查询班级信息(带老师的信息) ##1. 联表查询 SELECT * FROM class c,teacher t WHERE 
		c.teacher_id=t.t_id AND c.c_id=1; ##2. 执行两次查询 SELECT * FROM class WHERE c_id=1; 
		//teacher_id=1 SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id -->

	<!-- 方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 封装联表查询的数据(去除重复的数据) select * from class 
		c, teacher t where c.teacher_id=t.t_id and c.c_id=1 -->
	<select id="findById" parameterType="int" resultMap="ClassResultMap">
		select *
		from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
	</select>
	<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
	<resultMap type="com.lin.bean.Classes" id="ClassResultMap">
		<id property="id" column="c_id" />
		<result property="name" column="c_name" />
		<association property="teacher" javaType="com.lin.bean.Teacher">
			<id property="id" column="t_id" />
			<result property="name" column="t_name" />
		</association>
	</resultMap>
	<insert id="insertClass" parameterType="com.lin.bean.Classes"
		useGeneratedKeys="true" keyProperty="id">
		insert into class(c_name,teacher_id) value(#{name},#{teacher.id})
	</insert>
	<select id="getTeacher" parameterType="int" resultType="com.lin.bean.Teacher">
		SELECT
		t_id id, t_name name FROM teacher WHERE t_id=#{id}
	</select>
	<select id="getClass4" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and  c.c_id=#{id}	</select>
</mapper>
在conf.xml文件中注册classMapper.xml 
<mappers>
        <!-- 注册classMapper.xml文件， 
        classMapper.xml位于me.gacl.mapping这个包下，所以resource写成me/gacl/mapping/classMapper.xml-->
        <mapper resource="me/gacl/mapping/classMapper.xml"/>
</mappers>

1.5、编写工具MyBatisUtil

package com.lin.Util;

import java.io.IOException;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class MyBatisUtil {
	
	  public static  SqlSession getSession() {
		SqlSessionFactory sessionFactory = null;
		String resource = "configuration.xml";
		try {
			sessionFactory = new SqlSessionFactoryBuilder().build(Resources
					.getResourceAsReader(resource));
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		return sessionFactory.openSession();
	}

}


1.6、编写单元测试代码

package com.lin.test;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import com.lin.Util.MyBatisUtil;
import com.lin.bean.Classes;
import com.lin.bean.Teacher;
import com.lin.dao.ClassesMapper;

public class MyTest {
	@Test
	public void testinstertClass() {
		SqlSession sqlSession = MyBatisUtil.getSession();
		ClassesMapper ClassesM = sqlSession.getMapper(ClassesMapper.class);
		Classes classes=new Classes();
		
		Teacher teacher=new Teacher();
		teacher.setId(2);
		teacher.setName("teacher1");
		classes.setName("linbany2");
		classes.setTeacher(teacher);
		
		ClassesM.insertClass(classes);
		sqlSession.commit();
	}
		
	@Test
	public void testGetClass2() {
		SqlSession sqlSession = MyBatisUtil.getSession();
		ClassesMapper ClassesM = sqlSession.getMapper(ClassesMapper.class);
		Classes clazz=ClassesM.findById(1);		
		System.out.println(clazz);// 打印结果：Classes [id=1, name=class_a,
		sqlSession.commit();							// teacher=Teacher [id=1, name=teacher1]]
	}
	
	
}

1.7、MyBatis一对一关联查询总结

　　MyBatis中使用association标签来解决一对一的关联查询，association标签可用的属性如下：

  • property:对象属性的名称
  • javaType:对象属性的类型
  • column:所对应的外键字段名称
  • select:使用另一个查询封装的结果

二、一对多关联

2.1、提出需求

　　根据classId查询对应的班级信息,包括学生,老师

2.2、创建表和数据

　　在上面的一对一关联查询演示中，我们已经创建了班级表和教师表，因此这里再创建一张学生表

CREATE TABLE student(
    s_id INT PRIMARY KEY AUTO_INCREMENT, 
    s_name VARCHAR(20), 
    class_id INT
);
INSERT INTO student(s_name, class_id) VALUES('student_A', 1);
INSERT INTO student(s_name, class_id) VALUES('student_B', 1);
INSERT INTO student(s_name, class_id) VALUES('student_C', 1);
INSERT INTO student(s_name, class_id) VALUES('student_D', 2);
INSERT INTO student(s_name, class_id) VALUES('student_E', 2);
INSERT INTO student(s_name, class_id) VALUES('student_F', 2);




2.3、定义实体类

　　1、Student类

package me.gacl.domain;

/**
 * @author gacl
 * 定义student表所对应的实体类
 */
public class Student {

    //定义属性，和student表中的字段对应
    private int id;            //id===>s_id
    private String name;    //name===>s_name
    
    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;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + "]";
    }
}

2、修改Classes类，添加一个List<Student> students属性，使用一个List<Student>集合属性表示班级拥有的学生，如下： 

package me.gacl.domain;

import java.util.List;

/**
 * @author gacl
 * 定义class表对应的实体类
 */
public class Classes {

    //定义实体类的属性，与class表中的字段对应
    private int id;            //id===>c_id
    private String name;    //name===>c_name
    
    /**
     * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
     * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
     */
    private Teacher teacher;
    //使用一个List<Student>集合属性表示班级拥有的学生
    private List<Student> students;

    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 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 "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
                + ", students=" + students + "]";
    }
}

2.4、修改sql映射文件classMapper.xml

　　添加如下的SQL映射信息

<?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.lin.dao.ClassesMapper">

	<!-- 根据班级id查询班级信息(带老师的信息) ##1. 联表查询 SELECT * FROM class c,teacher t WHERE 
		c.teacher_id=t.t_id AND c.c_id=1; ##2. 执行两次查询 SELECT * FROM class WHERE c_id=1; 
		//teacher_id=1 SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id -->

	<!-- 方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 封装联表查询的数据(去除重复的数据) select * from class 
		c, teacher t where c.teacher_id=t.t_id and c.c_id=1 -->
	<select id="findById" parameterType="int" resultMap="ClassResultMap">
		select *
		from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
	</select>
	<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
	<resultMap type="com.lin.bean.Classes" id="ClassResultMap">
		<id property="id" column="c_id" />
		<result property="name" column="c_name" />
		<association property="teacher" javaType="com.lin.bean.Teacher">
			<id property="id" column="t_id" />
			<result property="name" column="t_name" />
		</association>
		<!-- ofType指定students集合中的对象类型 -->
		<collection property="students" ofType="com.lin.bean.Student">
			<id property="id" column="s_id" />
			<result property="name" column="s_name" />
		</collection>
	</resultMap>
	<insert id="insertClass" parameterType="com.lin.bean.Classes"
		useGeneratedKeys="true" keyProperty="id">
		insert into class(c_name,teacher_id) value(#{name},#{teacher.id})
	</insert>
	<select id="getTeacher" parameterType="int" resultType="com.lin.bean.Teacher">
		SELECT
		t_id id, t_name name FROM teacher WHERE t_id=#{id}
	</select>
	<select id="getClass4" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and  c.c_id=#{id}	</select>
</mapper>

2.5、编写单元测试代码


package com.lin.test;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import com.lin.Util.MyBatisUtil;
import com.lin.bean.Classes;
import com.lin.bean.Teacher;
import com.lin.dao.ClassesMapper;

public class MyTest {
	@Test
	public void testinstertClass() {
		SqlSession sqlSession = MyBatisUtil.getSession();
		ClassesMapper ClassesM = sqlSession.getMapper(ClassesMapper.class);
		Classes classes=new Classes();
		
		Teacher teacher=new Teacher();
		teacher.setId(2);
		teacher.setName("teacher1");
		classes.setName("linbany2");
		classes.setTeacher(teacher);
		
		ClassesM.insertClass(classes);
		sqlSession.commit();
	}
		
	@Test
	public void testGetClass2() {
		SqlSession sqlSession = MyBatisUtil.getSession();
		ClassesMapper ClassesM = sqlSession.getMapper(ClassesMapper.class);
		Classes clazz=ClassesM.findById(1);		
		System.out.println(clazz);// 打印结果：Classes [id=1, name=class_a,
		sqlSession.commit();							// teacher=Teacher [id=1, name=teacher1]]
	}
	
	@Test
	public void testGetClass4(){
		SqlSession sqlSession = MyBatisUtil.getSession();
		ClassesMapper ClassesM = sqlSession.getMapper(ClassesMapper.class);
		Classes clazz=ClassesM.getClass4(1);
		System.out.print(clazz);
		sqlSession.commit();

	}
	
}

  //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]

2.6、MyBatis一对多关联查询总结

　　MyBatis中使用collection标签来解决一对多的关联查询，ofType属性指定集合中元素的对象类型。

参考博客：

http://www.cnblogs.com/xdp-gacl/p/4264440.html

http://www.cnblogs.com/liunanjava/p/4919752.html

源代码下载地址：

http://download.youkuaiyun.com/detail/linyeban/9724042


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