开启懒加载配置

config.xml

    <settings>
        <!-- 配置log4j日志信息 -->
        <setting name="logImpl" value="LOG4J"></setting>
        
        <!-- 懒加载 配置 -->
        <setting name="lazyLoadingEnabled" value="true"></setting>
        <setting name="aggressiveLazyLoading" value="true"></setting>
    </settings>

实体bean

BorrowBean

public class BorrowBean {
    private int id;
    private Date date;
    private int num;
    
    // 书籍
    private BookBean book;
    // 用户
    private UserBean user;
    // ... get set 方法
}

BooKBean

public class BookBean {
    private int id;
    private String name;
    private String picture;
    private String author;
    private String press;
    private Date time;
    private int price;
    private int stock;
    private String describe;
    // ... get set 方法
}

UserBean

public class UserBean {
    private int id;
    private String name;
    private String pwd;
    
    // 一个用户借多本图书
    private List<BookBean> booklist;
    // ... get set 方法
}

配置 Mapper.xml

BorrowMapper.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.project.dao.IBorrowDao">
    <select id="findByBookId" parameterType="int" resultMap="borrowResultMap">
        SELECT * FROM t_ub WHERE ub_b_id = #{id};
    </select>
    <!-- 将查询结果字段，映射成BorrowBean 属性 -->
    <resultMap id="borrowResultMap" type="BorrowBean">
        <!-- 主键字段 -->
        <id property="id" column="ub_id" />
        <!-- 其它字段 -->
        <result property="date" column="ub_date"/>
        <result property="num" column="ub_num"/>
        
        <!-- 
        懒加载  --通过 ub_u_iu 用 select 属性指定的 sql 查询数据库，
        将结果封装成UsreBean对象，赋给 user
        注意：column指定的字段类型要与指定sql接收参数一样
        JavaType要与sql语句返回的实体bean要一样
         -->
        <association property="user" javaType="UserBean" column="ub_u_id" 
          select="com.project.UserMapper.findById">
        </association>
    </resultMap>
</mapper>