感谢@how2j的教学

1.两张表：

分类表.PNG

产品表.PNG

2.在之前学习的基础上，新建一个Product实体类：

package com.how2java.pojo;

public class Product {
    private int id;
    private String name;
    private Float price;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Float getPrice() {
        return price;
    }
    public void setPrice(Float price) {
        this.price = price;
    }
    @Override
    public String toString() {
        return "Product [id="+id+", name="+name+", price="+price+"]";
    }

}

注意最下面的那个复写toString的方法

3.修改Category实体类，提供products的集合

package com.how2java.pojo;

import java.util.List;

public class Category {
    private int id;
    private String name;
    List<Product> products;
    public int getId() {
        return id;  
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<Product> getProducts() {
        return products;
    }
    public void setProducts(List<Product> products) {
        this.products = products;
    }
    @Override
    public String toString() {
        return "Category [id="+id+", name="+name+"]";
    }
}

4.修改Category.xml文件

源码如下：

<?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.how2java.pojo">
        <resultMap type="Category" id="categoryBean">
            <id column="cid" property="id"/>
            <result column="cname" property="name"/>
            
            <collection property="products" ofType="Product">
                <id column="pid" property="id"/>
                <result column="pname" property="name"/>
                <result column="price" property="price"/>
            </collection>
        </resultMap>
        
        <select id="listCategory" resultMap="categoryBean">
            select c.*, p.*, c.id 'cid', p.id 'pid', c.name 'cname', p.name 'pname' from category_ c left join product_ p on c.id = p.cid
        </select>
    </mapper>

5.修改TestMybatis.java

package com.how2java.pojo;

import java.io.IOException;
import java.io.InputStream;

import java.util.List;


import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;


public class TestMybatis { 
    public static void main(String[] args) throws IOException {
        String resource="mybatis-config.xml";
        InputStream inputStream=Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession=sqlSessionFactory.openSession();
        
        List<Category> cs=sqlSession.selectList("listCategory");
        for(Category c:cs) {
            System.out.println(c);
            List<Product> ps=c.getProducts();
            for(Product p:ps) {
                System.out.println("\t"+p);
            }
        }
}
}

6.测试结果：

Category [id=1, name=category1]
Product [id=1, name=product a, price=88.88]
Product [id=2, name=product b, price=88.88]
Product [id=3, name=product c, price=88.88]
Category [id=2, name=category2]
Product [id=4, name=product x, price=88.88]
Product [id=5, name=product y, price=88.88]
Product [id=6, name=product z, price=88.88]