springboot如何实现文件上传和下载，给我编写一个简单的例子实现

Spring Boot实现文件上传和下载通常会用到MultipartFile接口和FileSystemResource类。这里是一个简单的例子来说明如何处理这两个操作：

1. **文件上传**:

首先，你需要在控制器方法上添加`@RequestParam("file") MultipartFile file`参数来接收上传的文件。然后，你可以将文件保存到服务器的某个位置。

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.multipart.MultipartFile;

import java.io.File;
import java.io.IOException;

@RestController
public class FileUploadController {

    @PostMapping("/upload")
    public String handleFileUpload(@RequestParam("file") MultipartFile file) throws IOException {
        if (file.isEmpty()) {
            return "No file uploaded";
        }

        // 指定上传目录
        String targetPath = "/path/to/upload/folder/" + file.getOriginalFilename();
        File targetFile = new File(targetPath);
        
        try {
            file.transferTo(targetFile);
            return "File uploaded successfully: " + targetFile.getAbsolutePath();
        } catch (Exception e) {
            throw new RuntimeException("Error uploading file", e);
        }
    }
}

2. **文件下载**:

对于文件下载，可以使用`ResponseEntity`返回一个`LinkedResource`，它从指定的文件系统路径读取内容。

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.util.UriComponentsBuilder;

import java.io.File;
import java.net.URI;

@GetMapping("/download/{filename}")
public ResponseEntity<InputStreamResource> downloadFile(@PathVariable String filename) {
    File file = new File("/path/to/download/folder/" + filename);
    
    if (!file.exists()) {
        return ResponseEntity.notFound().build();
    }

    InputStream inputStream = new FileInputStream(file);
    HttpHeaders headers = new HttpHeaders();
    headers.setContentDispositionFormData("attachment", filename);

    return ResponseEntity.ok()
            .headers(headers)
            .body(new InputStreamResource(inputStream));
}