前言：

文件上传下载功能是Web应用中的常见需求，从简单的用户头像上传到大型文件的传输与共享，都需要可靠的文件处理机制。

1. Apache Commons FileUpload

SpringBoot内部已集成文件上传功能，但在某些场景下，可能需要Apache Commons FileUpload提供的更多高级特性。

添加依赖：

<dependency>
    <groupId>commons-fileupload</groupId>
    <artifactId>commons-fileupload</artifactId>
    <version>1.5</version>
</dependency>

配置Bean：

@Bean
public CommonsMultipartResolver multipartResolver() {
    CommonsMultipartResolver resolver = new CommonsMultipartResolver();
    resolver.setDefaultEncoding("UTF-8");
    resolver.setMaxUploadSize(10485760); // 10MB
    resolver.setMaxUploadSizePerFile(2097152); // 2MB
    return resolver;
}

使用：

@RestController
@RequestMapping("/api/commons")
public class CommonsFileUploadController {
    
    @PostMapping("/upload")
    public String handleFileUpload(HttpServletRequest request) throws Exception {
        // 判断是否包含文件上传
        boolean isMultipart = ServletFileUpload.isMultipartContent(request);
        if (!isMultipart) {
            return "不是有效的文件上传请求";
        }
        
        // 创建上传器
        DiskFileItemFactory factory = new DiskFileItemFactory();
        ServletFileUpload upload = new ServletFileUpload(factory);
        
        // 解析请求
        List<FileItem> items = upload.parseRequest(request);
        
        for (FileItem item : items) {
            if (!item.isFormField()) {
                // 处理文件项
                String fileName = FilenameUtils.getName(item.getName());
                File uploadedFile = new File("uploads/" + fileName);
                item.write(uploadedFile);
                return "文件上传成功: " + fileName;
            }
        }
        
        return "未找到要上传的文件";
    }
}

2. MultipartFile接口

SpringBoot内置的MultipartFile接口是处理文件上传的基础工具，简单易用且功能完善。

配置方式：

在application.properties或application.yml中配置上传参数

spring:
  servlet:
    multipart:
      max-file-size: 10MB
      max-request-size: 10MB
      enabled: true

使用示例：

@RestController
@RequestMapping("/api/files")
public class FileUploadController {
    
    @PostMapping("/upload")
    public String uploadFile(@RequestParam("file") MultipartFile file) {
        if (file.isEmpty()) {
            return "请选择要上传的文件";
        }
        
        try {
            // 获取文件名
            String fileName = file.getOriginalFilename();
            // 获取文件类型
            String contentType = file.getContentType();
            // 获取文件内容
            byte[] bytes = file.getBytes();
            
            // 创建存储路径
            Path path = Paths.get("uploads/" + fileName);
            Files.createDirectories(path.getParent());
            
            // 保存文件
            Files.write(path, bytes);
            
            return "文件上传成功: " + fileName;
        } catch (IOException e) {
            e.printStackTrace();
            return "文件上传失败: " + e.getMessage();
        }
    }
}

3.Spring Resource抽象

Spring的Resource抽象提供了对不同来源资源的统一访问方式，非常适合文件下载场景。

@RestController
@RequestMapping("/api/resources")
public class ResourceDownloadController {
    
    @GetMapping("/download/{fileName:.+}")
    public ResponseEntity<Resource> downloadFile(@PathVariable String fileName) {
        try {
            // 创建文件资源
            Path filePath = Paths.get("uploads/" + fileName);
            Resource resource = new UrlResource(filePath.toUri());
            
            // 检查资源是否存在
            if (resource.exists()) {
                return ResponseEntity.ok()
                    .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename="" + resource.getFilename() + """)
                    .contentType(MediaType.APPLICATION_OCTET_STREAM)
                    .body(resource);
            } else {
                return ResponseEntity.notFound().build();
            }
        } catch (Exception e) {
            return ResponseEntity.internalServerError().build();
        }
    }
}

4.基于ResponseEntity的下载响应

SpringBoot中，ResponseEntity类型可以精确控制HTTP响应，为文件下载提供完善的HTTP头信息。

@RestController
@RequestMapping("/api/download")
public class FileDownloadController {
    
    @GetMapping("/file/{fileName:.+}")
    public ResponseEntity<byte[]> downloadFile(@PathVariable String fileName) {
        try {
            Path filePath = Paths.get("uploads/" + fileName);
            byte[] data = Files.readAllBytes(filePath);
            
            HttpHeaders headers = new HttpHeaders();
            headers.setContentDisposition(ContentDisposition.attachment()
                    .filename(fileName, StandardCharsets.UTF_8).build());
            headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);
            headers.setContentLength(data.length);
            
            return new ResponseEntity<>(data, headers, HttpStatus.OK);
        } catch (IOException e) {
            return ResponseEntity.notFound().build();
        }
    }
    
    @GetMapping("/dynamic-pdf")
    public ResponseEntity<byte[]> generatePdf() {
        // 假设这里生成了PDF文件的字节数组
        byte[] pdfContent = generatePdfService.createPdf();
        
        HttpHeaders headers = new HttpHeaders();
        headers.setContentDisposition(ContentDisposition.attachment()
                .filename("generated-report.pdf").build());
        headers.setContentType(MediaType.APPLICATION_PDF);
        
        return new ResponseEntity<>(pdfContent, headers, HttpStatus.OK);
    }
}

5. 基于Servlet的原生文件操作

在某些情况下，可能需要使用原生Servlet API进行更底层的文件操作。

@WebServlet("/servlet/download")
public class FileDownloadServlet extends HttpServlet {
    
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        
        String fileName = request.getParameter("file");
        if (fileName == null || fileName.isEmpty()) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST);
            return;
        }
        
        File file = new File("uploads/" + fileName);
        if (!file.exists()) {
            response.sendError(HttpServletResponse.SC_NOT_FOUND);
            return;
        }
        
        // 设置响应头
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename="" + fileName + """);
        response.setContentLength((int) file.length());
        
        // 写入文件内容
        try (FileInputStream input = new FileInputStream(file);
             ServletOutputStream output = response.getOutputStream()) {
            
            byte[] buffer = new byte[4096];
            int bytesRead;
            
            while ((bytesRead = input.read(buffer)) != -1) {
                output.write(buffer, 0, bytesRead);
            }
        }
    }
}

要在SpringBoot中启用Servlet：

@SpringBootApplication
@ServletComponentScan  // 扫描Servlet组件
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

6.云存储服务SDK集成

对于生产环境，将文件存储在专业的存储服务中通常是更好的选择，如AWS S3、阿里云OSS、七牛云等。

配置方式（以阿里云OSS为例）

<dependency>
    <groupId>com.aliyun.oss</groupId>
    <artifactId>aliyun-sdk-oss</artifactId>
    <version>3.15.1</version>
</dependency>

配置客户端：

@Configuration
public class OssConfig {
    
    @Value("${aliyun.oss.endpoint}")
    private String endpoint;
    
    @Value("${aliyun.oss.accessKeyId}")
    private String accessKeyId;
    
    @Value("${aliyun.oss.accessKeySecret}")
    private String accessKeySecret;
    
    @Value("${aliyun.oss.bucketName}")
    private String bucketName;
    
    @Bean
    public OSS ossClient() {
        return new OSSClientBuilder().build(endpoint, accessKeyId, accessKeySecret);
    }
    
    @Bean
    public String bucketName() {
        return bucketName;
    }
}

使用：

@RestController
@RequestMapping("/api/oss")
public class OssFileController {
    
    @Autowired
    private OSS ossClient;
    
    @Value("${aliyun.oss.bucketName}")
    private String bucketName;
    
    @PostMapping("/upload")
    public String uploadToOss(@RequestParam("file") MultipartFile file) {
        if (file.isEmpty()) {
            return "请选择文件";
        }
        
        try {
            String fileName = UUID.randomUUID().toString() + "-" + file.getOriginalFilename();
            
            // 上传文件流
            ossClient.putObject(bucketName, fileName, file.getInputStream());
            
            // 生成访问URL
            Date expiration = new Date(System.currentTimeMillis() + 3600 * 1000);
            URL url = ossClient.generatePresignedUrl(bucketName, fileName, expiration);
            
            return "文件上传成功，访问URL: " + url.toString();
        } catch (Exception e) {
            e.printStackTrace();
            return "文件上传失败: " + e.getMessage();
        }
    }
    
    @GetMapping("/download")
    public ResponseEntity<byte[]> downloadFromOss(@RequestParam("key") String objectKey) {
        try {
            // 获取文件
            OSSObject ossObject = ossClient.getObject(bucketName, objectKey);
            
            // 读取文件内容
            try (InputStream is = ossObject.getObjectContent()) {
                byte[] content = IOUtils.toByteArray(is);
                
                HttpHeaders headers = new HttpHeaders();
                headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);
                headers.setContentDisposition(ContentDisposition.attachment()
                        .filename(objectKey).build());
                
                return new ResponseEntity<>(content, headers, HttpStatus.OK);
            }
        } catch (Exception e) {
            e.printStackTrace();
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).build();
        }
    }
}

总结

无论选择哪种方案，都应根据应用的具体需求、预期用户数量和文件大小来设计文件处理系统。