@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

上課時，壹哥講解的關聯(lián)查詢是通過查詢書籍信息，并同時對書籍類型查詢。即在查詢Book對象時i，同時查詢出BookType對象。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>
        <!--        單個對象的關聯(lián)，javaType是指實體類的類型-->
        <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">
<!--            事務管理器-->
            <transactionManager type="JDBC"></transactionManager>
<!--            使用mybatis自帶連接池-->
            <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. 測試代碼

接著我們對上面的配置進行測試。

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();
    }

學生按照我講的內(nèi)容，測試沒有問題。在后續(xù)的預習練習中，要求實現(xiàn)在BookType中添加List屬性books，在查詢BookType對象同時將該類型的Book對象集合查出。小張同學有了如下實現(xiàn)思路。

5. 修改實體類

@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. 編寫測試類

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();
    }

然后就出現(xiàn)了一開始提到的異常：

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從入門到精通, author=千鋒, bookDesc=很不錯的Java書籍, 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從入門到精通, author=千鋒, bookDesc=很不錯的Java書籍, createTime=2022-05-27, type=null, imgPath=174cc662fccc4a38b73ece6880d8c07e)' Cause: java.lang.IllegalArgumentException: argument type mismatch

三. 異常分析

上面的 異常提示 ， 是說在 BookType類中的books屬性設置有問題 。 我們來仔細查看一下代碼，發(fā)現(xiàn)是因為直接 復制了之前的關系配置， 在配置文件中 使用javaType 節(jié)點 ， 但正確的 應該是 使用ofType。如下圖所示：