org.apache.ibatis.exceptions.PersistenceException: ### Error building SqlSession. ### The error may

Mapper.xml错误查找与分析。

在写mybatis的demo时 报错。

报错代码

org.apache.ibatis.exceptions.PersistenceException: 
### Error building SqlSession.
### The error may exist in com/scarecrow/mapper/OrdersMapper.xml
### The error occurred while processing mapper_resultMap[orders]_association[user]
### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Result Maps collection already contains value for com.scarecrow.mapper.OrdersMapper.orders
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26)
    at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:82)
    at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:66)
    at com.scarecrow.test.TestMybatis2.run(TestMybatis2.java:24)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
...

---

错误很明显出在OrdersMapper.xml中，

附上OrdersMapper.xml

<?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.scarecrow.mapper.OrdersMapper">

    <resultMap type="Orders" id="orders">
        <result column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="number" property="number"/>
        <result column="createtime" property="createtime"/>

        <association property="user" javaType="User">
            <id column="user_id" property="id"/>
            <result column="username" property="username"/>
        </association>

    </resultMap>

    <select id="selectOrders"  resultMap="orders">
        select o.id,o.user_id,o.number,o.createtime,u.username 
        from orders o 
        LEFT JOIN user u 
        on o.user_id=u.id
    </select>



</mapper>

原因分析：

问题就出在

<resultMap type="Orders" id="orders">

<select id="selectOrders"  resultMap="orders">

由于我在sqlMapConfig.xml文件中配置了<typeAliases>标签，能够批量别名定义，扫描整个包下的类，别名为类名（大小写不敏感）。

则com.scarecrow.javabean下的Orders类的别名 为 orders 以及 Orders。

所以在resultMap不能再使用 orders 或 Orders 否则相当于使用了resultType自动映射，而sql查询字段名和javabean属性名并不一致 导致报错。

附上sqlMapConfig.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <typeAliases>
    <package name="com.scarecrow.javabean"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <!-- 使用jdbc事务管理 -->
            <transactionManager type="JDBC" />
            <!-- 数据库连接池 -->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver" />
                <property name="url"
                    value="jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf-8" />
                <property name="username" value="root" />
                <property name="password" value="123" />
            </dataSource>
        </environment>
    </environments>


    <mappers>
<!--        <mapper class="com.scarecrow.mapper.UserMapper"/> -->
        <package name="com.scarecrow.mapper"/>
    </mappers>
</configuration>

解决方案：

1. 将<resultMap type="Orders" id="orders">中的orders改成除Orders以外的其他命名。并修改<select>中的resultMap

2. 在sqlMapConfig.xml 中

<typeAliases>
    <package name="com.scarecrow.javabean"/>
    </typeAliases>

使用<typeAlias alias="Orders" type="com.scarecrow.javabean.Orders" />进行单个别名定义。