mybatis之resultmap的使用

野性酷女 2022-05-20 10:10 201阅读 0赞

# mybatis之resultmap的使用 #

mybatis在查询的时候,可以自定义resultmap的返回类型，实现复杂的查找。

下面通过一个具体的例子看看是如何实现的。

首先有这样三个表：user表，interests表，和userinterests表

user表中存放用户

![70][]

interests表中存放兴趣

![70 1][]

userinteres中存放用户与兴趣的对应关系。

![70 2][]

我们的目标是，输入用户的ID，将对应的用户和其所有兴趣都查找出来。这设计到一对多查询，可以用resultmap来实现。

1、首先，编写两个实体对应的类**User.class** 和 **Interests.class**

package com.xiao.mybatis.bean;
    
    import java.util.List;
    
    public class User {
        private int id;
        private String name;
        private int age;
        private String sex;
        private String school;
    //    private List<Post> posts;
        private List<Interests> interests;
    
        public List<Interests> getInterests() {
            return interests;
        }
    
        public void setInterests(List<Interests> interests) {
            this.interests = interests;
        }
    
        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 String getSex() {
            return sex;
        }
    
        public void setSex(String sex) {
            this.sex = sex;
        }
    
        public String getSchool() {
            return school;
        }
    
        public void setSchool(String school) {
            this.school = school;
        }
    
    //    public List<Post> getPosts() {
    //        return posts;
    //    }
    //
    //    public void setPosts(List<Post> posts) {
    //        this.posts = posts;
    //    }
    
        @Override
        public String toString(){
            return "User <id=" + id + ", name=" + name + ", sex=" + sex + ", age=" + age + ", interests=" + interests + ">";
        }
    
    }

package com.xiao.mybatis.bean;
    
    public class Interests {
        private int id;
        private String 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 "Interests [id=" + id + ", name=" + name + "]";
        }
    
    }

2、然后写查询对应的Dao接口 IUser

package com.xiao.mybatis.dao;
    
    import com.xiao.mybatis.bean.Post;
    import com.xiao.mybatis.bean.User;
    
    import java.util.List;
    
    public interface IUser {
         User findUserById(int userId);
    }

3、之后在mybatis配置文件mybatis-local-config.xml中，为两个类创建别名。

**注意：alias标签中的是别名，type标签中的是类所在的资源路径。**

4、创建跟IUser dao文件对应的Mapper映射文件User.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.xiao.mybatis.dao.IUser">
    
        <resultMap id="UserWithInterestsMap" type="User">
            <id property="id" column="id"/>
            <result property="name" column="name"/>
            <result property="sex" column="sex"/>
            <result property="age" column="age"/>
            <result property="school" column="school"/>
            <collection property="interests" column="interestid" ofType="com.xiao.mybatis.bean.Interests">
                <id property="id" column="interestid"/>
                <result property="name" column="interestname"/>
            </collection>
        </resultMap>
    
        <select id="findUserById" resultMap="UserWithInterestsMap">
            		select a.*,c.* from user as a right join userinterests as b on a.id = b.userid right join interests as c on b.interestid = c.interestid where id = #{id}
        </select>
    
    
    
    </mapper>

**注意：**

*  <mapper namespace="com.xiao.mybatis.dao.IUser"> 中要填写dao文件所在的资源路径。
 *  <resultMap></resultMap>标签内就是我们定义的复杂的返回类型。
 *  <resultMap id="UserWithInterestsMap" type="User"> 中，type的值要写在mybatis配置文件中定义的别名。
 *  <id property="id" column="id"/>  
            <result property="name" column="name"/>，id对应主键属性，result对应其他属性。
 *  <collection property="interests" column="interestid" ofType="com.xiao.mybatis.bean.Interests">  
            </collection>，<collection>标签是实现一对多的关键，它把Interests以集合的形式传入resultMap中，这样一侧的返回值中，就可以包含多个Interests。

5、在mybatis配置文件下添加这个映射文件

6、测试

package com.xiao.mybatis.bean;
    
    import com.sun.org.apache.bcel.internal.generic.IUSHR;
    import com.xiao.mybatis.dao.IUser;
    import org.apache.ibatis.session.SqlSession;
    import org.apache.ibatis.session.SqlSessionFactory;
    import org.apache.ibatis.io.Resources;
    import org.apache.ibatis.session.SqlSessionFactoryBuilder;
    
    import java.io.Reader;
    import java.util.List;
    
    public class DemoTest {
        private static SqlSessionFactory sqlSessionFactory;
        static{
            try{
                Reader reader = Resources.getResourceAsReader("mybatis-local-config.xml");
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
            }catch (Exception e){
                e.printStackTrace();
            }
        }
    
    
     public static SqlSessionFactory getSession(){
            return sqlSessionFactory;
     }
    
    
        public static void main(String[] args) {
            SqlSession session = sqlSessionFactory.openSession();
            try{
                int userid=5;
                IUser userMapper = session.getMapper(IUser.class);
                User user = userMapper.findUserById(userid);
                System.out.println("记录为"+user.toString());
    
    
    
    
    
    //            User user = session.selectOne("com.xiao.mybatis.bean.UserMapper.getUser",1);
    //            System.out.println("username:"+user.getName());
    //            List<Post> posts = user.getPosts();
    //            for(Post p : posts) {
    //                System.out.println("Title:" + p.getTitle());
    //                System.out.println("Content:" + p.getContent());
    //            }
            }finally {
                session.close();
            }
        }
    
    
    
    
    
    }

7、结果

![70 3][]

参考资料：[Mybatis中Mapper映射文件详解][Mybatis_Mapper]

[Mybatis表关联一对多][Mybatis]

[70]: /images/20220520/7481dbe70daa46b2a074d958cdf86a4b.png
[70 1]: /images/20220520/0673c8bf42384d0c93ef97cd49d988c4.png
[70 2]: /images/20220520/90054883a8ff4eb99e63713c03d5ce28.png
[70 3]: /images/20220520/08d19c7832d44a03828b77ef9f015e16.png
[Mybatis_Mapper]: https://blog.csdn.net/majinggogogo/article/details/72123185
[Mybatis]: https://www.yiibai.com/mybatis/mybatis-one2many.html