细节决定成败：探究Mybatis中javaType和ofType的区别

桃扇骨 2024-04-01 14:07 90阅读 0赞

## 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>
            
            <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">
    
                <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<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：

![ccb4f2313728479eac04c63f796722d0.png][]

## 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：

![055938a32c974f5f902ed1acf3006c4e.png][]

## 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。

[ccb4f2313728479eac04c63f796722d0.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/01/21e8a7fe38064dffb3f1a7a7ec6b9b00.png
[055938a32c974f5f902ed1acf3006c4e.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/01/90683b36d022478caa86da958f630ee9.png