mybatis学习笔记3-mapper

Mybatis中，DAO层的代码只需要定义接口，其他的事情由Mybatis框架帮忙搞定。一个例子：

package example.dao;

import java.util.List;

import org.apache.ibatis.annotations.Param;

import example.model.User;

public interface UserMapper {
	public User findById(int id);
	public User findByName(String name);
	public void addUser(User user);
	public List<User> findUsers(@Param("start")int start, @Param("size")int size);
        public List<User> findUsers1(int limit);
}

定义了几个方法，方法名称和mapper.xml中的sql定义的id相同。例如findById，在mapper.xml中定义了一个：

<select id="findById" parameterType="HashMap" resultMap="usermap">
	select 
		a.id as user_id,
		a.name as user_name,
		a.sex as user_sex,
		a.age as user_age
	from T_User a where a.id=#{id}
</select>

需要注意的是，如果方法定义只有一个参数的，在mapper.xml文件中定义sql时可以直接使用参数名作为sql语句中的占位符。如果方法定义有多个参数的，要么使用@Param注解指定mapper.xml中的占位符名称，要么在mapper.xml中使用param1, param2等作为占位符。下面是mapper.xml的完整定义：

<?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">
	<!--这块等于dao接口的实现  namespace必须和接口的类路径一样-->
<mapper namespace="example.dao.UserMapper">
	<resultMap type="User" id="usermap">
		<id property="id" column="user_id" />
		<result column="user_name" property="name" typeHandler="example.dao.typehandler.SimpleTypeHandler"/>
		<result property ="sex" column="user_sex" />
		<result property="age" column="user_age" />
		<!-- 
		<discriminator javaType="byte" column="user_sex">
			<case value="1" resultType="example.model.GameUser">
				<result property="wins" column="wins" />
			</case>
			<case value="0" resultType="example.model.SingUser">
				<result property="sings" column="sings" />
			</case>
		</discriminator>
		-->
	</resultMap>
	<resultMap type="User" id="userMapWithAlbum">
		<id property="id" column="user_id" />
		<result property="name" column="user_name" />
		<result property ="sex" column="user_sex" />
		<result property="age" column="user_age" />
		<collection property="albums" column="user_id" ofType="Album" select="selectAlbumForUser" />
	</resultMap>
	
	<select id="findById" parameterType="HashMap" resultMap="usermap">
		select 
			a.id as user_id,
			a.name as user_name,
			a.sex as user_sex,
			a.age as user_age
		from T_User a where a.id=#{id}
    </select>
    
    <!-- 这里的#{start}和#{size}，如果代码中没有用@Param指定，就需要使用#{param1},#{param2} -->
    <select id="findUsers" parameterType="HashMap" resultMap="usermap">
    	select 
			a.id as user_id,
			a.name as user_name,
			a.sex as user_sex,
			a.age as user_age
		from T_User a limit #{start},#{size}
    </select>
    
    <select id="findUsers1" parameterType="HashMap" resultMap="usermap">
    	select 
			a.id as user_id,
			a.name as user_name,
			a.sex as user_sex,
			a.age as user_age
		from T_User a limit #{limit}
    </select>
    
    <select id="selectAlbumForUser" resultType="Album">
    	select * from T_Album where userId=#{user_id}
    </select>
    
    <select id="findByName" parameterType="HashMap" resultType="User" resultMap="usermap">
		select 
			a.id as user_id,
			a.name as user_name,
			a.sex as user_sex,
			a.age as user_age
		from T_User a where a.name=#{name}
    </select>
     
     <insert id="addUser" parameterType="User" useGeneratedKeys="true" keyProperty="id">
     	insert into T_User (name,sex,age) values(#{name},#{sex},#{age})
     </insert>
</mapper>