一. 背景描述
今天，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，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;
}
复制代码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：
<?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.qf.day7.dao.BookDAO">
<resultMap id="booksMap" type="com.qf.day7.entity.Books">
<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">
SELECT
b.id,
b.`name`,
b.author,
b.book_desc,
b.create_time,
b.img_path,
t.id type_id,
t.`name` type_name
FROM
books AS b
INNER JOIN book_type AS t ON b.type_id = t.id
</select>
</mapper>
复制代码3. 核心配置
核心配置文件如下：mybatisCfg.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.qf.day7.entity"/>
</typeAliases>
<environments default="development">
<environment id="development">
<!-- 事务管理器-->
<transactionManager type="JDBC"></transactionManager>
<!-- 使用mybatis自带连接池-->
<dataSource type="POOLED">
<!-- jdbc四要素-->
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url"
value="jdbc:mysql://localhost:3306/books?useUnicode=true&characterEncoding=utf-8&useSSL=false"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="mapper/BookMapper.xml"></mapper>
<mapper resource="mapper/BookTypeMapper.xml"></mapper>
</mappers>
</configuration>
复制代码4. 测试代码
接着我们对上面的配置进行测试。
public class BookDAOTest {
private SqlSessionFactory factory;
@Before
public void setUp() throws Exception {
final InputStream inputStream = Resources.getResourceAsStream("mybatisCfg.xml");
factory = new SqlSessionFactoryBuilder().build(inputStream);
}
@Test
public void findAll() {
final SqlSession session = factory.openSession();
final BookDAO bookDAO = session.getMapper(BookDAO.class);
final List<Book> list = bookDAO.findAll();
list.stream().forEach(System.out::println);
session.close();
}
}
复制代码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;
}
复制代码6. 添加映射文件BookTypeMapper.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.qf.day7.dao.BookTypeDAO">
<resultMap id="bookTypeMap" type="com.qf.day7.entity.BookType">
<id column="id" property="id"></id>
<result column="name" property="name"></result>
<collection property="books" javaType="com.qf.day7.entity.Book">
<id property="id" column="book_id"></id>
<result property="name" column="book_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>
</collection>
</resultMap>
<select id="findById" resultMap="bookTypeMap">
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 = #{typeId}
</select>
</mapper>
复制代码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());
}
session.close();
}
复制代码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
### The error may exist in mapper/BookTypeMapper.xml
### The error may involve com.qf.day7.dao.BookTypeDAO.findById
### The error occurred while handling results
### 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
复制代码三. 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。
