mybatis映射XML文件详解及实例

mybatis映射XML文件

一个简单的映射文件：

<?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.cnx.wxcar.mapper.CustomerMapper"></mapper>

当然这个文件中没有任何的元素

The Mapper XML files have only a few first class elements :

cache – Configuration of the cache for a given namespace. cache-ref – Reference to a cache configuration from another namespace. resultMap – The most complicated and powerful element that describes how to load your objects from the database result sets. sql – A reusable chunk of SQL that can be referenced by other statements. insert – A mapped INSERT statement. update – A mapped UPDATE statement. delete – A mapped DELETE statement. select – A mapped SELECT statement.

select

简单的例子：

<select id="selectPerson" parameterType="int" resultType="hashmap"> SELECT * FROM PERSON WHERE ID = #{id}</select>

select也有很多属性可以让你配置：

<select id="selectPerson" parameterType="int" parameterMap="deprecated" resultType="hashmap" resultMap="personResultMap" flushCache="false" useCache="true" timeout="10000" fetchSize="256" statementType="PREPARED" resultSetType="FORWARD_ONLY">

insert, update and delete

<insert id="insertAuthor" parameterType="domain.blog.Author" flushCache="true" statementType="PREPARED" keyProperty="" keyColumn="" useGeneratedKeys="" timeout="20"><update id="updateAuthor" parameterType="domain.blog.Author" flushCache="true" statementType="PREPARED" timeout="20"><delete id="deleteAuthor" parameterType="domain.blog.Author" flushCache="true" statementType="PREPARED" timeout="20">

语句：

<insert id="insertAuthor"> insert into Author (id,username,password,email,bio) values (#{id},#{username},#{password},#{email},#{bio})</insert><update id="updateAuthor"> update Author set  username = #{username},  password = #{password},  email = #{email},  bio = #{bio} where id = #{id}</update><delete id="deleteAuthor"> delete from Author where id = #{id}</delete>

f your database supports auto-generated key fields (e.g. mysql/' target='_blank'>mysql and SQL Server)，上面的插入语句可以写成：

<insert id="insertAuthor" useGeneratedKeys="true"  keyProperty="id"> insert into Author (username,password,email,bio) values (#{username},#{password},#{email},#{bio})</insert>

如果你的数据库还支持多条记录插入，可以使用下面这个语句：

<insert id="insertAuthor" useGeneratedKeys="true"  keyProperty="id"> insert into Author (username, password, email, bio) values <foreach item="item" collection="list" separator=",">  (#{item.username}, #{item.password}, #{item.email}, #{item.bio}) </foreach></insert>

sql

这个element可以定义一些sql代码的碎片，然后在多个语句中使用，降低耦合。比如：

<sql id="userColumns"> ${alias}.id,${alias}.username,${alias}.password </sql>

然后在下面的语句中使用：

<select id="selectUsers" resultType="map"> select  <include refid="userColumns"><property name="alias" value="t1"/></include>,  <include refid="userColumns"><property name="alias" value="t2"/></include> from some_table t1  cross join some_table t2</select>

Result Maps

官网给了个最最复杂的例子

大体意思呢就是一个博客系统有一个作者，很多博文，博文中有一个作者，很多评论，很多标签（包括了一对多，一对一）

<!-- Very Complex Statement --><select id="selectBlogDetails" resultMap="detailedBlogResultMap"> select    B.id as blog_id,    B.title as blog_title,    B.author_id as blog_author_id,    A.id as author_id,    A.username as author_username,    A.password as author_password,    A.email as author_email,    A.bio as author_bio,    A.favourite_section as author_favourite_section,    P.id as post_id,    P.blog_id as post_blog_id,    P.author_id as post_author_id,    P.created_on as post_created_on,    P.section as post_section,    P.subject as post_subject,    P.draft as draft,    P.body as post_body,    C.id as comment_id,    C.post_id as comment_post_id,    C.name as comment_name,    C.comment as comment_text,    T.id as tag_id,    T.name as tag_name from Blog B    left outer join Author A on B.author_id = A.id    left outer join Post P on B.id = P.blog_id    left outer join Comment C on P.id = C.post_id    left outer join Post_Tag PT on PT.post_id = P.id    left outer join Tag T on PT.tag_id = T.id where B.id = #{id}</select><!-- Very Complex Result Map --><resultMap id="detailedBlogResultMap" type="Blog"> <constructor>  <idArg column="blog_id" javaType="int"/> </constructor> <result property="title" column="blog_title"/> <association property="author" javaType="Author">  <id property="id" column="author_id"/>  <result property="username" column="author_username"/>  <result property="password" column="author_password"/>  <result property="email" column="author_email"/>  <result property="bio" column="author_bio"/>  <result property="favouriteSection" column="author_favourite_section"/> </association> <collection property="posts" ofType="Post">  <id property="id" column="post_id"/>  <result property="subject" column="post_subject"/>  <association property="author" javaType="Author"/>  <collection property="comments" ofType="Comment">   <id property="id" column="comment_id"/>  </collection>  <collection property="tags" ofType="Tag" >   <id property="id" column="tag_id"/>  </collection>  <discriminator javaType="int" column="draft">   <case value="1" resultType="DraftPost"/>  </discriminator> </collection></resultMap>

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