mybatis_SQL映射(2)

文章摘录自：http://blog.csdn.net/y172158950/article/details/17258377

1. sql的重用：定义一个sql片段，可在任何SQL语句中重用该片段。

<sql id="personColumns"> name, sex, updateTime</sql>
<select id="selectPerson" parameterType="int" resultType="hashmap">
    select id, <include refid="personColumns"/> from person where id =#{id};
</select>

2. javabean别名：不用每次写包路径

<!-- In Config XML file，定义 -->
<typeAlias type=”com.someapp.model.User” alias=”User”/>
<!-- In SQL Mapping XML file，使用 -->
<select id=”selectUsers” parameterType=”int” resultType=”User”>
    select id, username, hashedPassword from some_table where id = #{id}
</select>

3. 表与实体列名不匹配的解决

a) SQL的别名

<select id=”selectUsers” parameterType=”int” resultType=”User”>
select user_id as "id", user_name as userName, hashed_password as hashedPassword from some_table where id = #{id}
</select>

b)定义外部的resultMap

<resultMap id="userResult" type="User">
    <id property="id" column="_id" />
    <result property="name" column="_name" />
    <result property="password" column="_password" />
</resultMap>

<select id="selectUser" parameterType="int" resultMap="userResult">
    select _id, _name, _password from _user where _id =#{id};
</select>

c) 异常及解决：

Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException:
### Error building SqlSession.
### The error may exist in com/yjq/entity/User.xml
### The error occurred while processing mapper_resultMap[userResult]
### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.lang.RuntimeException: Error parsing Mapper XML. Cause: org.apache.ibatis.builder.BuilderException: Error resolving class . Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'userResult'.  Cause: java.lang.ClassNotFoundException: Cannot find class: userResult
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:8)
    at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:32)
    at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:16)
    at com.yjq.db.DbFactory.getInstance(DbFactory.java:23)
    at com.yjq.dao.UserDao.selectUserById(UserDao.java:22)
    at com.yjq.dao.UserDao.main(UserDao.java:36)
Caused by: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.lang.RuntimeException: Error parsing Mapper XML. Cause: org.apache.ibatis.builder.BuilderException: Error resolving class . Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'userResult'.  Cause: java.lang.ClassNotFoundException: Cannot find class: userResult
    at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:85)
    at org.apache.ibatis.builder.xml.XMLConfigBuilder.parse(XMLConfigBuilder.java:69)
    at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:30)
    ... 4 more
Caused by: java.lang.RuntimeException: Error parsing Mapper XML. Cause: org.apache.ibatis.builder.BuilderException: Error resolving class . Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'userResult'.  Cause: java.lang.ClassNotFoundException: Cannot find class: userResult
    at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:97)
    at org.apache.ibatis.builder.xml.XMLMapperBuilder.parse(XMLMapperBuilder.java:73)
    at org.apache.ibatis.builder.xml.XMLConfigBuilder.mapperElement(XMLConfigBuilder.java:255)
    at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:83)
    ... 6 more
Caused by: org.apache.ibatis.builder.BuilderException: Error resolving class . Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'userResult'.  Cause: java.lang.ClassNotFoundException: Cannot find class: userResult
    at org.apache.ibatis.builder.BaseBuilder.resolveClass(BaseBuilder.java:69)
    at org.apache.ibatis.builder.xml.XMLStatementBuilder.parseStatementNode(XMLStatementBuilder.java:40)
    at org.apache.ibatis.builder.xml.XMLMapperBuilder.buildStatementFromContext(XMLMapperBuilder.java:105)
    at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:95)
    ... 9 more
Caused by: org.apache.ibatis.type.TypeException: Could not resolve type alias 'userResult'.  Cause: java.lang.ClassNotFoundException: Cannot find class: userResult
    at org.apache.ibatis.type.TypeAliasRegistry.resolveAlias(TypeAliasRegistry.java:92)
    at org.apache.ibatis.builder.BaseBuilder.resolveAlias(BaseBuilder.java:93)
    at org.apache.ibatis.builder.BaseBuilder.resolveClass(BaseBuilder.java:67)
    ... 12 more
Caused by: java.lang.ClassNotFoundException: Cannot find class: userResult
    at org.apache.ibatis.io.ClassLoaderWrapper.classForName(ClassLoaderWrapper.java:173)
    at org.apache.ibatis.io.ClassLoaderWrapper.classForName(ClassLoaderWrapper.java:72)
    at org.apache.ibatis.io.Resources.classForName(Resources.java:235)
    at org.apache.ibatis.type.TypeAliasRegistry.resolveAlias(TypeAliasRegistry.java:88)
    ... 14 more

<select id="selectUser" parameterType="int" resultType="userResult">  修改为resultMap