前后端分离项目知识汇总（整合课程分类）

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>easyexcel</artifactId>
    <version>2.1.6</version>
</dependency>

@Service
public class EduSubjectServiceImpl extends ServiceImpl<EduSubjectMapper, EduSubject> implements EduSubjectService {
    //添加课程分类
    @Override
    public void saveSubject(MultipartFile file, EduSubjectService subjectService) {
        try {
            //文件输入流
            InputStream in = file.getInputStream();
            //调用方法进行读取
            EasyExcel.read(in, SubjectData.class, new SubjectExcelListener(subjectService)).sheet().doRead();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    @Override
    public List<OneSubject> getAllOneTwoSubject() {
        //查询所有一级分类 parentid=0
        QueryWrapper<EduSubject> wrapperOne = new QueryWrapper<>();
        wrapperOne.eq("parent_id", "0");
        List<EduSubject> oneSubjectList = baseMapper.selectList(wrapperOne);
//        this.list()也可以通过这种方式调用查询。
        //查询所有二级分类 parentid!=0
        QueryWrapper<EduSubject> wrapperTwo = new QueryWrapper<>();
        wrapperOne.ne("parent_id", "0");
        List<EduSubject> twoSubjectList = baseMapper.selectList(wrapperTwo);
        //创建list集合，用于存放最终封装的数据
        List<OneSubject> finalSubjectList = new ArrayList<>();
        //封装一级分类
        //查询出来所有的一级分类list集合遍历，得到每一级分类对象，获得每个一级分类对象值
        //封装到要求的list集合里面
        for (int i = 0; i < oneSubjectList.size(); i++) {
            EduSubject eduSubject = oneSubjectList.get(i);
            OneSubject oneSubject = new OneSubject();
//            oneSubject.setId(eduSubject.getId());
//            oneSubject.setTitle(eduSubject.getTitle());
            //把eduSubject值复制到对应的oneSubject对象里面，两个对象里面的属性相同对应的的自动赋值
            BeanUtils.copyProperties(eduSubject, oneSubject);
            //在一级分类循环遍历查询所有的二级分类
            //创建list集合封装每个一级分类的二级分类
            List<TwoSubject> twoFinalSubjectList = new ArrayList<>();
            //遍历二级分类list集合
            for (int j = 0; j < twoSubjectList.size(); j++) {
                EduSubject tSubject = twoSubjectList.get(j);
                //如果二级分类的parentid和一级分类的id一样，就把它加入到一级分类
                if (tSubject.getParentId().equals(eduSubject.getId())) {
                    TwoSubject twoSubject = new TwoSubject();
                    BeanUtils.copyProperties(tSubject, twoSubject);
                    twoFinalSubjectList.add(twoSubject);
                }
            }
            //把一级下面所有的二级分类放到一级分类里面
            oneSubject.setChildren(twoFinalSubjectList);
            finalSubjectList.add(oneSubject);
        }
        return finalSubjectList;
    }
}