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 :

1. cache – configuration of the cache for a given namespace.
2. cache-ref – reference to a cache configuration from another namespace.
3. resultmap – the most complicated and powerful element that describes how to load your objects from the database result sets.
4. sql – a reusable chunk of sql that can be referenced by other statements.
5. insert – a mapped insert statement.
6. update – a mapped update statement.
7. delete – a mapped delete statement.
8. 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 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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