【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() + "\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>

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