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：
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@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" ?> <!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 > |
3. 核心配置
核心配置文件如下： mybatisCfg.xml
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<? 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 > |
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" ); 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(); } |
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
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@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" ?> <!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 > |
7. 编写测试类
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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(); } |
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
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。
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原文链接：https://juejin.cn/post/7176860138367057981
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