When mapping query results in MyBatis, you can choose betwean resultType and resultMap. Only one can be used in a single select. With resultType, MyBatis relies on column-to-property name matching (and aliases) to populate a target class. With resultMap, you describe exact how each column should be mapped, and you can define object graphs (one-to-one and one-to-many) via association and collection. MyBatis internally represents every mapping as a ResultMap; resultType is a convenient shorthand for straightforward cases.

Domain model

Post.java

package org.example.mybatis.domain;

import java.util.List;

public class Post {
    private long postId;
    private String headline;
    private String body;
    private String author;
    private List<Comment> comments;

    public long getPostId() { return postId; }
    public void setPostId(long postId) { this.postId = postId; }

    public String getHeadline() { return headline; }
    public void setHeadline(String headline) { this.headline = headline; }

    public String getBody() { return body; }
    public void setBody(String body) { this.body = body; }

    public String getAuthor() { return author; }
    public void setAuthor(String author) { this.author = author; }

    public List<Comment> getComments() { return comments; }
    public void setComments(List<Comment> comments) { this.comments = comments; }
}

Comment.java

package org.example.mybatis.domain;

import java.util.Date;

public class Comment {
    private long commentId;
    private String message;
    private Date createdAt;
    private Post post; // owning Post

    public long getCommentId() { return commentId; }
    public void setCommentId(long commentId) { this.commentId = commentId; }

    public String getMessage() { return message; }
    public void setMessage(String message) { this.message = message; }

    public Date getCreatedAt() { return createdAt; }
    public void setCreatedAt(Date createdAt) { this.createdAt = createdAt; }

    public Post getPost() { return post; }
    public void setPost(Post post) { this.post = post; }
}

Assume relational tables:

• posts(id, title, content, author)
• comments(id, text, created_at, post_id)

Basic mapping with resultType

<!-- aliases -->
<typeAliases>
  <typeAlias alias="Post" type="org.example.mybatis.domain.Post"/>
  <typeAlias alias="Comment" type="org.example.mybatis.domain.Comment"/>
</typeAliases>

<!-- simple mapping using resultType -->
<select id="getPost" parameterType="long" resultType="Post">
  select
    p.id      as postId,
    p.title   as headline,
    p.content as body,
    p.author  as author
  from posts p
  where p.id = #{value}
</select>

With resultType, MyBatis uses the column labels (or aliases) to match bean properties. Here aliases map id→postId, title→headline, content→body.

Explicit ResultMap for the same query

<resultMap id="PostMap" type="Post">
  <id     column="id"      property="postId"/>
  <result column="title"   property="headline"/>
  <result column="content" property="body"/>
  <result column="author"  property="author"/>
</resultMap>

<select id="getPostByMap" parameterType="long" resultMap="PostMap">
  select id, title, content, author from posts where id = #{value}
</select>

Defining the id element helps MyBatis identify unique rows, which matters when assembling collections from joined or nested results.

One-to-one: Comment → Post via association

A Comment references its owning Post. Use association with a nested select.

<resultMap id="CommentMap" type="Comment">
  <id     column="id"          property="commentId"/>
  <result column="text"        property="message"/>
  <result column="created_at"  property="createdAt"/>
  <association property="post"
               column="post_id"
               select="getPostByMap"
               javaType="Post"/>
</resultMap>

<select id="getComment" parameterType="long" resultMap="CommentMap">
  select c.id, c.text, c.created_at, c.post_id
  from comments c
  where c.id = #{value}
</select>

• property: target property on the result type (Comment.post)
• select: the nested statement that loads the associated object
• column: the current row’s value past to the nested select as the parameter object
• javaType: the type of the associated property

The nested select receives a simple parameter (the post_id value). Inside getPostByMap, reference it with #{value} or #{_parameter}.

One-to-many: Post → Comments via collection

Load a Post and its Comments with a nested select. Use ofType for collection element type.

<resultMap id="PostWithComments" type="Post">
  <id     column="id"      property="postId"/>
  <result column="title"   property="headline"/>
  <result column="content" property="body"/>
  <result column="author"  property="author"/>
  <collection property="comments"
              ofType="Comment"
              column="id"
              select="findCommentsForPost"/>
</resultMap>

<select id="findPostWithComments" parameterType="long" resultMap="PostWithComments">
  select p.id, p.title, p.content, p.author from posts p where p.id = #{value}
</select>

<select id="findCommentsForPost" parameterType="long" resultMap="CommentMap">
  select id, text, created_at, post_id
  from comments
  where post_id = #{value}
</select>

• collection maps a property that is a List/Set
• ofType is the element type of the collection
• column passes the outer row’s id into the nested select

Mapper interfaces and sample tests

PostMapper.java

package org.example.mybatis.mapper;

import org.example.mybatis.domain.Post;

public interface PostMapper {
    Post getPost(long id);
    Post getPostByMap(long id);
    Post findPostWithComments(long id);
}

CommentMapper.java

package org.example.mybatis.mapper;

import java.util.List;
import org.example.mybatis.domain.Comment;

public interface CommentMapper {
    Comment getComment(long id);
    List<Comment> findCommentsForPost(long postId);
}

JUnit examples

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.example.mybatis.domain.Comment;
import org.example.mybatis.domain.Post;
import org.example.mybatis.mapper.CommentMapper;
import org.example.mybatis.mapper.PostMapper;
import org.junit.Test;

import java.util.List;

public class MyBatisResultMapTest {

    private final SqlSessionFactory sqlSessionFactory = MyBatisBootstrap.build();

    @Test
    public void loadCommentsByPost() {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            CommentMapper mapper = session.getMapper(CommentMapper.class);
            List<Comment> list = mapper.findCommentsForPost(10L);
            for (Comment c : list) {
                System.out.printf("%d %s%n", c.getCommentId(), c.getMessage());
            }
        }
    }

    @Test
    public void loadPostGraph() {
        try (SqlSession session = sqlSessionFactory.openSession()) {
            PostMapper mapper = session.getMapper(PostMapper.class);
            Post post = mapper.findPostWithComments(10L);
            if (post.getComments() != null) {
                System.out.println("comments=" + post.getComments().size());
            }
        }
    }
}

Notes on automatic mapping

• With resultType, MyBatis maps columns to properties by name. Aliases help align column names to bean properties.
• With resultMap, you can be explicit and compose graphs. MyBatis still performs automatic mapping for any properties not explicit listed if auto-mapping is enabled.
• When using nested selects, the value from column is passed as the paramter object. For simple types, reference it with #{value} or #{_parameter}.