详解Mybatis中javaType和ofType的区别-Java开发

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：