细节决定成败：探究Mybatis中javaType和ofType的区别

开发运维 2023-07-14 法医手机阅读

一. 背景描述

今天给学生讲解了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

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; } 复制代码 2.BookMapper.xml映射文件

上课时，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：



        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

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核心配置
核心配置文件如下：mybatisCfg.xml

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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();
}
}
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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。

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; } 复制代码

添加映射文件BookTypeMapper.xml

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}

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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();
}
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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
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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：