解决mybatis 中collection嵌套collection引发的bug

我就废话不多说了，大家还是直接看代码吧~

<resultMap id="ParentMap" type="org.example.mybatis.Parent">
 <id column="Id" jdbcType="VARCHAR" property="id" />
 <result column="Name" jdbcType="VARCHAR" property="name" />
 <result column="SurName" jdbcType="VARCHAR" property="surName" />

<collection property="children"
   javaType="ArrayList" ofType="org.example.mybatis.Child"
   resultMap="ChildMap" columnPrefix="c_"/>  

</resultMap>

<resultMap id="ChildMap" type="org.example.mybatis.Child">
 <id column="Id" jdbcType="VARCHAR" property="id" />
 <result column="ParentId" jdbcType="VARCHAR" property="parentId" />
 <result column="Name" jdbcType="VARCHAR" property="name" />
 <result column="SurName" jdbcType="VARCHAR" property="surName" />
 <result column="Age" jdbcType="INTEGER" property="age" />

<collection property="toys"
   javaType="ArrayList" ofType="org.example.mybatis.Toy"
   resultMap="ToyMap" columnPrefix="t_"/>  

</resultMap>

<resultMap id="ToyMap" type="org.example.mybatis.Toy">
 <id column="Id" jdbcType="VARCHAR" property="id" />
 <result column="ChildId" jdbcType="VARCHAR" property="childId" />
 <result column="Name" jdbcType="VARCHAR" property="name" />
 <result column="Color" jdbcType="VARCHAR" property="color" />
</resultMap>

<sql id="Parent_Column_List">
 p.Id, p.Name, p.SurName,
</sql>

<sql id="Child_Column_List">
 c.Id as c_Id, c.ParentId as c_ParentId, c.Name as c_Name, c.SurName as c_Surname, c.Age as c_Age,
</sql>

<sql id="Toy_Column_List">
 t.Id as t_Id, t.Name as t_Name, t.Color as t_Color
</sql>

<select id="getParent" parameterType="java.lang.String" resultMap="ParentMap" >
 select
 <include refid="Parent_Column_List"/>
 <include refid="Child_Column_List" />
 <include refid="Toy_Column_List" />
 from Parent p

left outer join Child c on p.Id = c.ParentId
 left outer join Toy t on c.Id = t.ChildId
 where p.id = #{id,jdbcType=VARCHAR}
</select>

表面来看没有任何问题 实际 查询的child对象中的toys一直是空

类关系介绍：

Parent类有属性ArrayList<Child> children

Child类有属性ArrayList<Toy> toys

Toy是一个普通的类

原因在于：

<collection property="toys"
   javaType="ArrayList" ofType="org.example.mybatis.Toy"
   resultMap="ToyMap" columnPrefix="t_"/>

columnPrefix配置的是t_实际mybatis处理后是 c_t_

解决办法：

只需要修改 sql 修改前缀为 c_t_ 即可

<sql id="Toy_Column_List">
 t.Id as c_t_Id, t.Name as c_t_Name, t.Color as c_t_Color
</sql>

补充知识：mybatis 嵌套的结果集不能被安全的转为自定义ResultHandler 的解决办法

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.executor.ExecutorException: Mapped Statements with nested result mappings cannot be safely used with a custom ResultHandler. Use safeResultHandlerEnabled=false setting to bypass this check.

问题描述

session.select("dao.ArticleMapper.selectAll", null, new RowBounds(1, 2),resultHandler);

会报不安全, 查询Configuration 源码发现里面有一个常量是

public Configuration() {
 this.safeRowBoundsEnabled = false;
 this.safeResultHandlerEnabled = true;//意思是不允许自定义ResultHand 处理器,
 this.mapUnderscoreToCamelCase = false;
 this.aggressiveLazyLoading = true;

解决办法

 public static SqlSession getsqlSession(){
 SqlSession session = sqlSessionFactory.openSession(ExecutorType.REUSE);
 Configuration configuration = session.getConfiguration(); //反射得到configuration ,然后
 configuration.setSafeResultHandlerEnabled(false); // 设置为false
 return session;
}

这样就可以了。

来源：https://blog.csdn.net/AndyMocan/article/details/83059459