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。

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<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

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<!-- 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

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<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

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<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：

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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