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：

<?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

<?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&amp;characterEncoding=utf-8&amp;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。

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

@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

<?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

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。