细节决定成败：探究Mybatis中javaType和ofType的区别

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：

"1.0" encoding="UTF-8"?>
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>
        
        <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

"1.0" encoding="UTF-8" ?>
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">
            <transactionManager type="JDBC">transactionManager>
            <dataSource type="POOLED">
                <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。

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 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 books;
}

6. 添加映射文件BookTypeMapper.xml

"1.0" encoding="UTF-8"?>
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。