一. 背景描述
今天，壹哥给学生讲解了Mybatis框架，学习了基础的ORM框架操作及多对一的查询。在练习的时候，小张同学突然举手求助，说在做预习作业使用一对多查询时，遇到了ReflectionException异常。
二. 情景再现
1. 实体类
为了给大家讲清楚这个异常的产生原因，壹哥先列出今天案例中涉及到的两张表：书籍表和书籍类型表。这两张表中存在着简单的多对一关系，实体类如下：
-
@Data
-
@NoArgsConstructor
-
@AllArgsConstructor
-
@Builder
-
public
class
Book {
-
private Integer id;
-
private String name;
-
private String author;
-
private String bookDesc;
-
private String createTime;
-
private BookType type;
-
private String imgPath;
-
}
-
-
@Data
-
@NoArgsConstructor
-
@AllArgsConstructor
-
@Builder
-
public
class
BookType {
-
private Integer id;
-
private String name;
-
}
2.BookMapper.xml映射文件
上课时，壹哥讲解的关联查询是通过查询书籍信息，并同时对书籍类型查询。即在查询Book对象时i，同时查询出BookType对象。BookMapper.xml映射文件如下：
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
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<mapper namespace="com.qf.day7.dao.BookDAO">
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<resultMap id="booksMap" type="com.qf.day7.entity.Books">
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<id property="id" column="id">
</id>
-
<result property="name" column="name">
</result>
-
<result property="author" column="author">
</result>
-
<result property="bookDesc" column="book_desc">
</result>
-
<result property="createTime" column="create_time">
</result>
-
<result property="imgPath" column="img_path">
</result>
-
<!-- 单个对象的关联，javaType是指实体类的类型-->
-
<association property="type" javaType="com.qf.day7.entity.BookType">
-
<id property="id" column="type_id">
</id>
-
<result property="name" column="type_name">
</result>
-
</association>
-
</resultMap>
-
-
<select id="findAll" resultMap="booksMap">
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SELECT
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b.id,
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b.`name`,
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b.author,
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b.book_desc,
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b.create_time,
-
b.img_path,
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t.id type_id,
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t.`name` type_name
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FROM
-
books AS b
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INNER JOIN book_type AS t ON b.type_id = t.id
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</select>
-
</mapper>
3. 核心配置
核心配置文件如下：mybatisCfg.xml
-
<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE configuration
-
PUBLIC
"-//mybatis.org//DTD Config 3.0//EN"
-
"http://mybatis.org/dtd/mybatis-3-config.dtd">
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<configuration>
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<typeAliases>
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<package name="com.qf.day7.entity"/>
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</typeAliases>
-
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<environments default="development">
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<environment id="development">
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<!-- 事务管理器-->
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<transactionManager type="JDBC">
</transactionManager>
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<!-- 使用mybatis自带连接池-->
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<dataSource type="POOLED">
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<!-- jdbc四要素-->
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<property name="driver" value="com.mysql.jdbc.Driver"/>
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<property name="url"
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value=
"jdbc:mysql://localhost:3306/books?useUnicode=true&characterEncoding=utf-8&useSSL=false"/>
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<property name="username" value="root"/>
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<property name="password" value="root"/>
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</dataSource>
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</environment>
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</environments>
-
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<mappers>
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<mapper resource="mapper/BookMapper.xml">
</mapper>
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<mapper resource="mapper/BookTypeMapper.xml">
</mapper>
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</mappers>
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</configuration>
4. 测试代码
接着我们对上面的配置进行测试。
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public
class
BookDAOTest {
-
private SqlSessionFactory factory;
-
-
@Before
-
public
void
setUp
()
throws Exception {
-
final
InputStream
inputStream
= Resources.getResourceAsStream(
"mybatisCfg.xml");
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factory =
new
SqlSessionFactoryBuilder().build(inputStream);
-
}
-
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@Test
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public
void
findAll
() {
-
final
SqlSession
session
= factory.openSession();
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final
BookDAO
bookDAO
= session.getMapper(BookDAO.class);
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final List<Book> list = bookDAO.findAll();
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list.stream().forEach(System.out::println);
-
session.close();
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}
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}
学生按照我讲的内容，测试没有问题。在后续的预习练习中，要求实现在BookType中添加List属性books，在查询BookType对象同时将该类型的Book对象集合查出。小张同学有了如下实现思路。
5. 修改实体类
-
@Data
-
@NoArgsConstructor
-
@AllArgsConstructor
-
@Builder
-
public
class
Book {
-
private Integer id;
-
private String name;
-
private String author;
-
private String bookDesc;
-
private String createTime;
-
private String imgPath;
-
}
-
-
@Data
-
@NoArgsConstructor
-
@AllArgsConstructor
-
@Builder
-
public
class
BookType {
-
private Integer id;
-
private String name;
-
private List<Book> books;
-
}
6. 添加映射文件BookTypeMapper.xml
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
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<mapper namespace="com.qf.day7.dao.BookTypeDAO">
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<resultMap id="bookTypeMap" type="com.qf.day7.entity.BookType">
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<id column="id" property="id">
</id>
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<result column="name" property="name">
</result>
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<collection property="books" javaType="com.qf.day7.entity.Book">
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<id property="id" column="book_id">
</id>
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<result property="name" column="book_name">
</result>
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<result property="author" column="author">
</result>
-
<result property="bookDesc" column="book_desc">
</result>
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<result property="createTime" column="create_time">
</result>
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<result property="imgPath" column="img_path">
</result>
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</collection>
-
</resultMap>
-
-
<select id="findById" resultMap="bookTypeMap">
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SELECT
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b.id book_id,
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b.`name` book_name,
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b.author,
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b.book_desc,
-
b.create_time,
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b.img_path,
-
t.id,
-
t.`name`
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FROM
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books AS b
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INNER JOIN book_type AS t ON b.type_id = t.id
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where t.id = #{typeId}
-
</select>
-
</mapper>
7. 编写测试类
-
public
class
BookTypeDAOTest {
-
private SqlSessionFactory factory;
-
@Before
-
public
void
setUp
()
throws Exception {
-
final
InputStream
inputStream
= Resources.getResourceAsStream(
"mybatisCfg.xml");
-
factory =
new
SqlSessionFactoryBuilder().build(inputStream);
-
}
-
-
@Test
-
public
void
findById
() {
-
final
SqlSession
session
= factory.openSession();
-
final
BookTypeDAO
bookTypeDAO
= session.getMapper(BookTypeDAO.class);
-
BookType
bookType
= bookTypeDAO.findById(
1);
-
for (Book book : bookType.getBooks()) {
-
System.out.println(book.getName());
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}
-
session.close();
-
}
然后就出现了一开始提到的异常：
-
org.apache.ibatis.exceptions.PersistenceException:
-
### Error querying database. Cause: org.apache.ibatis.reflection.ReflectionException: Could not set property
'books' of
'class com.qf.day7.entity.BookType' with value
'Book(id=1, name=Java从入门到精通, author=千锋, bookDesc=很不错的Java书籍, createTime=2022-05-27, type=null, imgPath=174cc662fccc4a38b73ece6880d8c07e)' Cause: java.lang.IllegalArgumentException: argument type mismatch
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### The error may exist in mapper/BookTypeMapper.xml
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### The error may involve com.qf.day7.dao.BookTypeDAO.findById
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### The error occurred
while handling results
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### SQL: SELECT b.id book_id, b.`name` book_name, b.author, b.book_desc, b.create_time, b.img_path, t.id, t.`name` FROM books AS b INNER JOIN book_type AS t ON b.type_id = t.id where t.id = ?
-
### Cause: org.apache.ibatis.reflection.ReflectionException: Could not set property
'books' of
'class com.qf.day7.entity.BookType' with value
'Book(id=1, name=Java从入门到精通, author=千锋, bookDesc=很不错的Java书籍, createTime=2022-05-27, type=null, imgPath=174cc662fccc4a38b73ece6880d8c07e)' Cause: java.lang.IllegalArgumentException: argument type mismatch
三. 异常分析
上面的异常提示，是说在BookType类中的books属性设置有问题。我们来仔细查看一下代码，发现是因为直接复制了之前的关系配置，在配置文件中使用javaType节点，但正确的应该是使用ofType。如下图所示：
四. 解析
那么为什么有的关系配置要使用javaType，而有的地方又要使用ofType呢？
这我们就不得不说说Mybatis的底层原理了！在关联映射中，如果是单个的JavaBean对象，那么可以使用javaType；而如果是集合类型，则需要写ofType。以下是Mybatis的官方文档原文：
五. 结尾
虽然上面的代码中只是因为一个单词的不同，却造成了不小的错误。我们的程序是严格的，小问题就可能会耽误你很久的时间。就比如我们的小张同学，在求助壹哥之前已经找bug找了一个小时......最后壹哥一眼就给他看出了问题所在，他都无语凝噎了.....
现在你明白javaType和ofType用法上的区别了吗？如果你还有其他什么问题，可以在评论区留言或私信哦！关注Java架构栈，干货天天都不断。
转载：https://blog.csdn.net/syc000666/article/details/127663921
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