mybatisplus继承basemapper的方法找不到 mybatis baseresultmap

resultMap元素是mybatis中最重要最强大的元素，它的作用是告诉mybatis将从结果集中取出的数据转换成开发者自定义的对象

一、简单查询

上篇博客说到，mybatis查询会将数据表中的字段名与java对象的属性名进行匹配，匹配成功就将结果集保存到对象中，那如果java对象中的属性名与数据库表的字段名不一致怎么办呢？可以用resultMap元素来解决

1、在数据库mybatis中新建user2表，表中的字段名都加了前缀user2_

create table user2(
    -> user2_id int(11) not null primary key auto_increment,
    -> user2_name char(11) not null,
    -> user2_sex char(2),
    -> user2_age int(3));

接下来插入几条测试数据

insert into user2 values(1,'aa','M',23);
insert into user2 values(2,'bb','F',21);
insert into user2 values(3,'cc','M',13);
insert into user2 values(4,'dd','F',18);

2、新建一个User.java对象

package pojo;

public class User {

	private Integer id;
	private String name;
	private String sex;
	private Integer age;
	public User(){super();}
	public User(String name,String sex,Integer age){
		this.name = name;
		this.sex = sex;
		this.age = age;
	}
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	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 Integer getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	
}

3、修改映射文件UserMapper.xml，用resultMap元素指定User中属性名对应数据表中的字段名，如id对应user2_id。然后在查询元素select中增加一条属性resultMap指定刚才的resultMap的id，查询的时候直接查询user2表即可

<?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="pojo.UserMapper">
	<resultMap type="pojo.User" id="user2Result">
		<id property="id" column="user2_id"/>
		<result property="name" column="user2_name"/>
		<result property="sex" column="user2_sex"/>
		<result property="age" column="user2_age"/>
	</resultMap>
	<select id="selectUser2" resultMap="user2Result">
		SELECT * FROM USER2
	</select>
</mapper>

4、编写测试类ResultMapTest.java

package test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

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

import pojo.User;

public class ResultMapTest {

	public static void main(String[] args) throws IOException{
		//读取mybatis的配置文件
		InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
		//获取session
		SqlSession session = new SqlSessionFactoryBuilder().build(is).openSession();
		//查询全部，返回list集合
		List<User> userlist = session.selectList("pojo.UserMapper.selectUser2");
		for(User user:userlist){
			System.out.println(user.getId()+"-"+user.getName()+"-"+user.getSex()+"-"+user.getAge());
		}
		//提交事务
		session.commit();
		//关闭session
		session.close();
	}
}

5、另外附上maybatis的配置文件mybatis-config.xml，位于src目录下即可

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!-- 指定所有日志的具体实现 -->
	<settings>
		<setting name="logImpl" value="LOG4J"/>
	</settings>
	<!-- 环境配置 -->
	<environments default="mysql">
		<environment id="mysql">
			<!-- 指定事务管理类型 -->
			<transactionManager type="JDBC" />
			<!-- 配置数据源，POOLED是JDBC连接对象的数据源连接池的实现 -->
			<dataSource type="POOLED">
				<property name="driver" value="com.mysql.jdbc.Driver"/>
				<property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
				<property name="username" value="root"/>
				<property name="password" value="root"/>
			</dataSource>
		</environment>
	</environments>
	<!-- mappers告诉mybatis找持久化类的映射文件 -->
	<mappers>
		<mapper resource="pojo/UserMapper.xml"/>
	</mappers>
</configuration>

运行ResultMapTest.java类，可以看到输出

二、关联查询

在实际项目中我们需要用到多表查询，而不仅仅只是一个表。比如外键关联的两个表，查询其中一个时就需要把另一个也查询出来，这是就需要用resultMap来完成关联映射

1、首先创建两个表学生（student）和班级（clazz），一个学生对应一个班级，一个班级对应多个学生

create table clazz(
    -> id int primary key auto_increment,
    -> code char(12));

create table student(
    -> id int primary key auto_increment,
    -> name char(12),
    -> age int,
    -> clazz_id int,
    -> foreign key(clazz_id) references clazz(id));

插入一些测试数据

insert into clazz values(1,'G18001');
insert into clazz values(2,'G18002');
insert into student values(1,'wang',20,1);
insert into student values(2,'li',19,1);
insert into student values(3,'song',20,2);
insert into student values(4,'zhang',18,2);

2、编写对象类，src/pojo/Student.java，src/pojo/Clazz.java

package pojo;

import java.util.List;

public class Clazz {

	private int id;
	private String code;
	private List<Student> students;
	public List<Student> getStudents() {
		return students;
	}
	public void setStudents(List<Student> students) {
		this.students = students;
	}
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getCode() {
		return code;
	}
	public void setCode(String code) {
		this.code = code;
	}
	
}

package pojo;

public class Student {

	private int id;
	private String name;
	private int age;
	private Clazz clazz;
	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 int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public Clazz getClazz() {
		return clazz;
	}
	public void setClazz(Clazz clazz) {
		this.clazz = clazz;
	}
}

3、编写映射文件ClazzMapper.xml和StudentMapper.xml，与java对象位于同一级目录pojo下。

ClazzMapper.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="pojo.ClazzMapper">
	<resultMap type="pojo.Clazz" id="clazzMap">
		<id property="id" column="id"/>
		<result property="code" column="code"/>
		<!-- 学生集合 -->
		<collection property="students" javaType="ArrayList"
		column="id" ofType="pojo.Student" select="selectStudent"/>
	</resultMap>
	<!-- 根据班级id查询学生 -->
	<select id="selectStudent" resultType="pojo.Student">
		SELECT*FROM STUDENT WHERE clazz_id=#{id}
	</select>
	<!-- 查询班级信息 -->
	<select id="selectClazz" resultMap="clazzMap">
		SELECT*FROM CLAZZ
	</select>
</mapper>

StudentMapper.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="pojo.StudentMapper">
	<resultMap type="pojo.Student" id="studentMap">
		<id property="id" column="id"/>
		<result property="name" column="name"/>
		<result property="age" column="age"/>
		<!-- 关系映射 -->
		<association property="clazz" column="clazz_id" javaType="pojo.Clazz" select="selectClazz"/>
	</resultMap>
	<!-- 根据id查询班级 -->
	<select id="selectClazz" resultType="pojo.Clazz">
		SELECT*FROM CLAZZ WHERE id=#{id}
	</select>
	<!-- 查询所有学生信息 -->
	<select id="selectStudent" resultMap="studentMap">
		SELECT*FROM STUDENT
	</select>
</mapper>

其中association和collection元素的属性详解如下：

property：对象类型的属性名，如Student.java中的clazz属性名

column：属性名在数据表中对应的字段名

javaType：该属性所对应的java类型，clazz对应是Clazz类类型，students对应的是集合类型

select：表示执行一条查询语句，其值对应下面的查询元素的id

ofType：表示集合ArrayList中的类型

4、在mybatis-config.xml配置文件中添加映射

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!-- 指定所有日志的具体实现 -->
	<settings>
		<setting name="logImpl" value="LOG4J"/>
	</settings>
	<!-- 环境配置 -->
	<environments default="mysql">
		<environment id="mysql">
			<!-- 指定事务管理类型 -->
			<transactionManager type="JDBC" />
			<!-- 配置数据源，POOLED是JDBC连接对象的数据源连接池的实现 -->
			<dataSource type="POOLED">
				<property name="driver" value="com.mysql.jdbc.Driver"/>
				<property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
				<property name="username" value="root"/>
				<property name="password" value="root"/>
			</dataSource>
		</environment>
	</environments>
	<!-- mappers告诉mybatis找持久化类的映射文件 -->
	<mappers>
		<mapper resource="pojo/UserMapper.xml"/>
		<mapper resource="pojo/StudentMapper.xml"/>
		<mapper resource="pojo/ClazzMapper.xml"/>
	</mappers>
</configuration>

5、编写src/test/StudentTest.java测试类，测试查询学生的同时查询其对应的班级

package test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

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

import pojo.Student;

public class StudentTest {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		//读取映射文件
		InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
		//获取 session
		SqlSession session = new SqlSessionFactoryBuilder().build(is).openSession();
		//查询学生信息
		List<Student> students = session.selectList("pojo.StudentMapper.selectStudent");
		for(Student s:students){
			System.out.println(s.getId()+"-"+s.getName()+"-"+s.getAge()+"-"+s.getClazz().getCode());
		}
		//提交事务
		session.commit();
		//关系session
		session.close();
	}

}

运行结果如下

6、编写src/test/ClazzTest.java测试类，测试查询班级的同时查询出班级内所有的学生信息

package test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

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

import pojo.Clazz;
import pojo.Student;

public class ClazzTest {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		//读取映射文件
		InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
		//获取 session
		SqlSession session = new SqlSessionFactoryBuilder().build(is).openSession();
		//查询班级信息
		List<Clazz> clazzs = session.selectList("pojo.ClazzMapper.selectClazz");
		for(Clazz clazz:clazzs){
			System.out.println("班级："+clazz.getCode());
			//获取班级中的学生信息
			List<Student> students = clazz.getStudents();
			for(Student student:students){
				System.out.println(student.getId()+"-"+student.getName()+"-"+student.getAge());
			}
		}
	}

}

运行结果如下