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【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`

====================================================================

====================================================================

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() + "
" + "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() + "
" + "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>

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原文地址：https://www.cnblogs.com/yangchongxing/p/10486854.html