【MyBatis】ResultMap
转载：https://www.cnblogs.com/yangchongxing/p/10486854.html
支持的 JDBC 类型
为了未来的参考,MyBatis 通过包含的 jdbcType 枚举型,支持下面的 JDBC 类型。
注意：在指定jdbcType时类型是列出的这些，并且必须是大写
| BIT | FLOAT | CHAR | TIMESTAMP | OTHER | UNDEFINED |
| TINYINT | REAL | VARCHAR | BINARY | BLOB | NVARCHAR |
| SMALLINT | DOUBLE | LONGVARCHAR | VARBINARY | CLOB | NCHAR |
| INTEGER | NUMERIC | DATE | LONGVARBINARY | BOOLEAN | NCLOB |
| BIGINT | DECIMAL | TIME | NULL | CURSOR | ARRAY |
目录
====================================================================
====================================================================
constructor - 用于在实例化类时，注入结果到构造方法中
idArg - ID 参数;标记出作为 ID 的结果可以帮助提高整体性能
arg - 将被注入到构造方法的一个普通结果
id – 一个 ID 结果;标记出作为 ID 的结果可以帮助提高整体性能
result – 注入到字段或 JavaBean 属性的普通结果
association – 一个复杂类型的关联;许多结果将包装成这种类型
嵌套结果映射 – 关联可以指定为一个 resultMap 元素，或者引用一个
collection – 一个复杂类型的集合
嵌套结果映射 – 集合可以指定为一个 resultMap 元素，或者引用一个
discriminator – 使用结果值来决定使用哪个 resultMap
case – 基于某些值的结果映射
嵌套结果映射 – 一个 case 也是一个映射它本身的结果,因此可以包含很多相 同的元素，或者它可以参照一个外部的 resultMap。
1、基本内容 id 和 result
属性 property，column，javaType，jdbcType，typeHandler
注意：jdbcType="VARCHAR" 上表列出的类型，必须大写
<id property="id" column="post_id"/>
<result property="subject" column="post_subject" jdbcType="VARCHAR"/>
2、构造方法 constructor
由顺序决定选择那个构造器
构造器，注意：id 不能用 int 而要用 Integer 对象
public Blog(Integer id, String title, String content) {
this.id = id;
this.title = title;
this.content = content;
}
映射
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int"/>
<arg column = "title" javaType = "String"/>
<arg column = "content" javaType = "String"/>
</constructor>
</resultMap>
由参数名决定选择那个构造器，与顺序无关
方式一、构造器添加 @Param 注解
public Blog(@Param("id") Integer id, @Param("title") String title, @Param("content") String content) {
this.id = id;
this.title = title;
this.content = content;
}
映射顺序随意
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "content" javaType = "String" name = "content"/>
<arg column = "title" javaType = "String" name = "title"/>
</constructor>
</resultMap>
方式二、使用 '-parameters' 编译选项并启用 useActualParamName 选项（默认开启）来编译项目
<settings>
<setting name="useActualParamName" value="true"/>
</settings>
useActualParamName 允许使用方法签名中的名称作为语句参数名称。 为了使用该特性，你的工程必须采用Java 8编译，并且加上-parameters选项。（从3.4.1开始）默认值是true
命令行：javac -parameters 文件
eclipse：Window->Preferences->Java->Compiler 下勾选 Store information about method parameters(usable via reflection)
public Blog(Integer id, String title, String content) {
this.id = id;
this.title = title;
this.content = content;
}
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
</resultMap>
3、关联 association
用到的实体
public class Blog {
private int id;
private String title;
private String content;
protected Author author;
}
public class Author {
protected int id;
protected String name;
}
①嵌套查询
映射
public interface BlogMapper {
public Blog selectBlog(int id);
}
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<association property="author" column="author_id" javaType="Author" select="selectAuthor"/>
</resultMap>
<!-- 下面的参数通过上面 association 的 column 字段指定 -->
<select id="selectBlog" resultMap="blogResultMap" >
select id,title,content,author_id from blog where id = #{id}
</select>
<select id="selectAuthor" resultType="Author">
select id,name from author where id = #{id}
</select>
嵌套的 select author 参数 是通过 association 的 column 传递进去
②嵌套结果
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- column 不需要 -->
<association property="author" javaType="Author">
<id property="id" column="author_id"/>
<result property="name" column="author_name"/>
</association>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap" >
select b.id, b.title, b.content, b.author_id, a.name as author_name
from blog b
left outer join author a on b.author_id = a.id
where b.id = #{id}
</select>
引用外部结果映射
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- 外部结果映射来映射关联 -->
<association property="author" javaType="Author" resultMap="authorResultMap"/>
</resultMap> <resultMap id="authorResultMap" type="Author">
<id property="id" column="author_id"/>
<result property="name" column="author_name"/>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap" >
select b.id, b.title, b.content, b.author_id, a.name as author_name
from blog b
left outer join author a on b.author_id = a.id
where b.id = #{id}
</select>
使用 columnPrefix 共用外部结果映射
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- 外部结果映射 -->
<association property="author" javaType="Author" resultMap="authorResultMap"/>
<association property="ycxAuthor" javaType="Author" resultMap="authorResultMap" columnPrefix="ycx_"/>
</resultMap>
<resultMap id="authorResultMap" type="Author">
<id property="id" column="author_id"/>
<result property="name" column="author_name"/>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap" >
select b.id, b.title, b.content, b.author_id, a.name as author_name, ycx.id as ycx_author_id, ycx.name as ycx_author_name
from blog b
left outer join author a on b.author_id = a.id
left outer join author ycx on b.author_id = ycx.id
where b.id = #{id}
</select>
4、集合 collection
实体
public class Blog {
private int id;
private String title;
private String content;
protected Author author;
protected List<Post> posts;
}
public class Author {
protected int id;
protected String name;
}
public class Post {
protected int id;
protected String subject;
protected String body;
}
①嵌套查询
注意：javaType="ArrayList" 是 Blog 属性的的类型为 ArrayList，新字段 ofType="Post" 是集合 ArrayList 包含类型为 Post。javaType 可以省略 MyBatis 可以推算出来。
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- 嵌套查询 column 是要传入子查询的值 -->
<association property="author" javaType="Author" column="author_id" select="selectAuthor"/>
<collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="selectPost"/>
</resultMap>
<resultMap id="authorResultMap" type="Author">
<id property="id" column="author_id"/>
<result property="name" column="author_name"/>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap">
select * from blog where id = #{id}
</select>
<select id="selectAuthor" resultType="Author">
select * from author where id = #{id}
</select>
<select id="selectPost" resultType="Post">
select * from post where blog_id = #{id}
</select>
②嵌套结果
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- 嵌套结果 -->
<association property="author" javaType="Author">
<id property="id" column="author_id"/>
<result property="name" column="author_name"/>
</association>
<collection property="posts" javaType="ArrayList" ofType="Post">
<id property="id" column="post_id"/>
<result property="subject" column="subject"/>
<result property="body" column="body"/>
</collection>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap">
select b.id, b.title, b.content,
a.id as author_id, a.name as author_name,
p.id as post_id, p.subject as post_subject, p.body as post_body
from blog b
left outer join author a on b.author_id = a.id
left outer join post p on p.blog_id = b.id
where b.id = #{id}
</select>
引用外部结果映射
<resultMap id="authorResultMap" type="Author">
<id property="id" column="author_id"/>
<result property="name" column="author_name"/>
</resultMap>
<resultMap id="postResultMap" type="Post">
<id property="id" column="post_id"/>
<result property="subject" column="post_subject"/>
<result property="body" column="post_body"/>
</resultMap>
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- 外部结果映射 -->
<association property="author" javaType="Author" resultMap="authorResultMap"/>
<collection property="posts" javaType="ArrayList" ofType="Post" resultMap="postResultMap"/>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap">
select b.id, b.title, b.content,
a.id as author_id, a.name as author_name,
p.id as post_id, p.subject as post_subject, p.body as post_body
from blog b
left outer join author a on b.author_id = a.id
left outer join post p on p.blog_id = b.id
where b.id = #{id}
</select>
使用 columnPrefix 共用外部结果映射
<resultMap id="authorResultMap" type="Author">
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<resultMap id="postResultMap" type="Post">
<id property="id" column="id"/>
<result property="subject" column="subject"/>
<result property="body" column="body"/>
</resultMap>
<resultMap id="blogResultMap" type="Blog">
<constructor>
<idArg column = "id" javaType = "int" name = "id"/>
<arg column = "title" javaType = "String" name = "title"/>
<arg column = "content" javaType = "String" name = "content"/>
</constructor>
<!-- 外部结果映射 -->
<association property="author" javaType="Author" resultMap="authorResultMap" columnPrefix="author_"/>
<collection property="posts" javaType="ArrayList" ofType="Post" resultMap="postResultMap" columnPrefix="post_"/>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap">
select b.id, b.title, b.content,
a.id as author_id, a.name as author_name,
p.id as post_id, p.subject as post_subject, p.body as post_body
from blog b
left outer join author a on b.author_id = a.id
left outer join post p on p.blog_id = b.id
where b.id = #{id}
</select>
5、鉴别器 discriminator
实体
public class Blog {
protected int id;
protected String title;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
@Override
public String toString() {
return "Blog [id=" + id + ", title=" + title + "]";
}
}
public class BlogContent extends Blog{
protected String content;
public String getContent() {
return content;
}
public void setContent(String content) {
this.content = content;
}
@Override
public String toString() {
return super.toString() + "\r\n" + "BlogContent [content=" + content + "]";
}
}
public class BlogImage extends Blog {
protected String path;
public String getPath() {
return path;
}
public void setPath(String path) {
this.path = path;
}
@Override
public String toString() {
return super.toString() + "\r\n" + "BlogImage [path=" + path + "]";
}
}
简洁方式
<resultMap id="blogResultMap" type="Blog">
<id property="id" column="id"/>
<result property="title" column="title"/>
<discriminator javaType="int" column="type">
<case value="1" resultType="BlogContent">
<result property="content" column="content"/>
</case>
<case value="2" resultType="BlogImage">
<result property="path" column="path"/>
</case>
</discriminator>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap">
select * from blog where id = #{id}
</select>
外部结果映射
<resultMap id="blogContentResultMap" type="BlogContent">
<result property="content" column="content"/>
</resultMap>
<resultMap id="blogImageResultMap" type="BlogImage">
<result property="path" column="path"/>
</resultMap>
<resultMap id="blogResultMap" type="Blog">
<id property="id" column="id"/>
<result property="title" column="title"/>
<discriminator javaType="int" column="type">
<case value="1" resultMap="blogContentResultMap"/>
<case value="2" resultMap="blogImageResultMap"/>
</discriminator>
</resultMap>
<select id="selectBlog" resultMap="blogResultMap">
select * from blog where id = #{id}
</select>