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MyBatis级联操作 MyBatis使用级联操作解决lombok构造方法识别失败问题

viewts 人气:0

先解决一下idea无法识别lombok构造方法的问题,解决方案是在idea的插件中下载并安装lombok插件。

MyBatis级联操作,列举最简单的student-classes(学生与班级)的关系表:

create table if not exists student (
  id int primary key auto_increment,
  name varchar(20) not null comment '学生姓名',
  cid int not null comment '班级id'
);
create table if not exists classes (
  id int primary key auto_increment,
  name varchar(20) not null comment '班级名'
);

接下来,创建关系表对应的实体类:

@Data
public class Student {
  private long id;
  private String name;
  private Classes classes;
}
@Data
public class Classes {
  private long id;
  private String name;
  private List<Student> students;
}

在repository包下新建StudentRepository接口:

public interface StudentRepository {
  public Student findById(long id);
}

然后创建对应的mapper文件StudentRepository.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.wts.repository.StudentRepository">
  
  <resultMap id="studentMap" type="com.wts.entity.Student">
    <id column="id" property="id"></id>
    <result column="name" property="name"></result>
    <association property="classes" javaType="com.wts.entity.Classes">
      <id column="cid" property="id"></id>
      <result column="cname" property="name"></result>
    </association>
  </resultMap>
  
  <select id="findById" parameterType="long" resultMap="studentMap">
    select s.id,s.name,c.id as cid,c.name as cname from student s,classes c where s.id = #{id} and s.cid = c.id
  </select>
</mapper>

注意这里有几个限制:

1.命名空间,xml文件的namespace必须是对应接口的全类名

2.Statement标签的id必须与接口方法相同,其中parameterType为参数,resultType为返回类型,复杂类型用resultMap

3.复杂类型resultMap中多对一用association,一堆多用集合collection

MyBatis执行sql返回的结果集会和关系对象映射起来,注意列与字段的对应关系。

然后将mapper引入:

<mappers>
  <mapper resource="com/wts/repository/StudentRepository.xml"></mapper>
</mappers>

编写测试方法:

@Test
public void test03() {
  InputStream inputStream = AppTest.class.getClassLoader().getResourceAsStream("config.xml");
  SqlSessionFactory sqlSessionFactory = (new SqlSessionFactoryBuilder()).build(inputStream);
  try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
    // 级联查询
    StudentRepository studentRepository = sqlSession.getMapper(StudentRepository.class);
    System.out.println(studentRepository.findById(1L));
  }
}

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