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解決mybatis 中collection嵌套collection引發(fā)的bug

更新時間：2020年12月01日 11:57:31 作者：AndyMocan

這篇文章主要介紹了解決mybatis 中collection嵌套collection引發(fā)的bug，具有很好的參考價值，希望對大家有所幫助。一起跟隨小編過來看看吧

我就廢話不多說了，大家還是直接看代碼吧~

<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>

表面來看沒有任何問題實際查詢的child對象中的toys一直是空

類關(guān)系介紹：

Parent類有屬性ArrayList<Child> children

Child類有屬性ArrayList<Toy> toys

Toy是一個普通的類

原因在于：

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

columnPrefix配置的是t_實際mybatis處理后是 c_t_

解決辦法：

只需要修改 sql 修改前綴為 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>

補充知識：mybatis 嵌套的結(jié)果集不能被安全的轉(zhuǎn)為自定義ResultHandler 的解決辦法

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.

問題描述

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

會報不安全, 查詢Configuration 源碼發(fā)現(xiàn)里面有一個常量是

public Configuration() {
  this.safeRowBoundsEnabled = false;
  this.safeResultHandlerEnabled = true;//意思是不允許自定義ResultHand 處理器, 
  this.mapUnderscoreToCamelCase = false;
  this.aggressiveLazyLoading = true;

解決辦法

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

這樣就可以了。

以上這篇解決mybatis 中collection嵌套collection引發(fā)的bug就是小編分享給大家的全部內(nèi)容了，希望能給大家一個參考，也希望大家多多支持腳本之家。

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