概述：要在Springboot和Vue中实现文件的下载和上传，你需要分别在后端和前端进行操作。以下是具体的实现步骤：

1、后端（Springboot）：

首先，需要在pom.xml中添加依赖：

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

然后，创建一个控制器类，如FileController.java，并添加以下代码：

import org.springframework.core.io.Resource;
import org.springframework.core.io.UrlResource;
import org.springframework.http.HttpHeaders;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.multipart.MultipartFile;

import java.io.IOException;
import java.net.MalformedURLException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;

@RestController
public class FileController {

    private final Path fileStoragePath = Paths.get("uploads");

    @PostMapping("/upload")
    public ResponseEntity<String> uploadFile(@RequestParam("file") MultipartFile file) {
        try {
            if (!Files.exists(fileStoragePath)) {
                Files.createDirectories(fileStoragePath);
            }

            Path targetLocation = fileStoragePath.resolve(file.getOriginalFilename());
            Files.copy(file.getInputStream(), targetLocation, StandardCopyOption.REPLACE_EXISTING);

            return ResponseEntity.ok("文件上传成功");
        } catch (IOException e) {
            e.printStackTrace();
            return ResponseEntity.badRequest().body("文件上传失败");
        }
    }

    @GetMapping("/download")
    public ResponseEntity<Resource> downloadFile(@RequestParam("filename") String filename) {
        try {
            Path filePath = fileStoragePath.resolve(filename).normalize();
            Resource resource = new UrlResource(filePath.toUri());
            if (resource.exists()) {
                return ResponseEntity.ok()
                        .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + resource.getFilename() + "\"")
                        .body(resource);
            } else {
                return ResponseEntity.notFound().build();
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
            return ResponseEntity.badRequest().build();
        }
    }
}

2、前端（Vue）：

首先，安装axios库：

npm install axios --save

然后，在Vue组件中添加以下代码：

<template>
  <div>
    <input type="file" @change="uploadFile">
    <button @click="downloadFile">下载文件</button>
  </div>
</template>

<script>
import axios from 'axios';

export default {
  methods: {
    async uploadFile(event) {
      const file = event.target.files[0];
      const formData = new FormData();
      formData.append('file', file);

      try {
        await axios.post('/upload', formData, {
          headers: {
            'Content-Type': 'multipart/form-data'
          }
        });
        alert('文件上传成功');
      } catch (error) {
        alert('文件上传失败');
      }
    },
    async downloadFile() {
      try {
        const response = await axios.get('/download', {
          params: { filename: 'example.txt' },
          responseType: 'blob'
        });
        const url = window.URL.createObjectURL(new Blob([response.data]));
        const link = document.createElement('a');
        link.href = url;
        link.setAttribute('download', 'example.txt');
        document.body.appendChild(link);
        link.click();
      } catch (error) {
        alert('文件下载失败');
      }
    }
  }
};
</script>