The Problem: You’re building a Spring Boot REST API that needs to handle multiple file uploads and downloads without zipping the files. The client application requires processing individual files separately. You’ve attempted to use @RequestParam("documents") MultipartFile[] documents for uploads and @GetMapping for downloads, but you’re unsure of the best approach for efficient and scalable multi-file handling in a RESTful context.
Understanding the “Why” (The Root Cause): HTTP is fundamentally designed for single-file transfers. While you can upload multiple files in a single request using MultipartFile[], returning multiple files directly in a single HTTP response is inefficient and problematic. Browsers don’t inherently handle multiple files within one response. This necessitates a different approach for downloads. Simply attempting to return a Resource[] will not work reliably. Furthermore, processing large files directly in memory (file.getBytes()) is memory-intensive and can lead to performance issues, especially with multiple files.
Step-by-Step Guide: The recommended approach involves asynchronous processing and a change in how you manage downloads. We will implement a system using a “prepare” endpoint for initiating the download process and a polling endpoint to retrieve files one by one.
Step 1: Stream Uploads: Modify your upload endpoint to process files individually using streams instead of loading them fully into memory:
@PostMapping("/files/batch-upload")
public ResponseEntity<?> handleMultipleFiles(@RequestParam("documents") MultipartFile[] documents) {
for (MultipartFile file : documents) {
try (InputStream stream = file.getInputStream()) {
// Process each file using the stream
processFileStream(stream, file.getOriginalFilename(), file.getContentType());
} catch (IOException e) {
// Handle exceptions appropriately, perhaps log and return an error response
return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body("Error processing file: " + e.getMessage());
}
}
return ResponseEntity.ok().build();
}
Step 2: Implement Asynchronous Download Session Management: Create a service (DownloadSessionService) to manage download sessions. This will handle tracking which files have been sent and maintaining the session state. A unique session ID (UUID) will be used to identify each download session.
Step 3: Create Prepare and Next File Endpoints: Create two endpoints: one to prepare a download session and another to retrieve the next file in the session:
@PostMapping("/files/prepare-batch")
public ResponseEntity<BatchDownloadResponse> prepareBatch(@RequestBody List<String> fileIds) {
String sessionId = UUID.randomUUID().toString();
downloadSessionService.createSession(sessionId, fileIds);
return ResponseEntity.ok(new BatchDownloadResponse(sessionId, fileIds.size()));
}
@GetMapping("/files/batch/{sessionId}/next")
public ResponseEntity<Resource> getNextFile(@PathVariable String sessionId) {
return downloadSessionService.getNextFile(sessionId);
}
BatchDownloadResponse is a custom class that would contain the sessionId and the total number of files. The getNextFile method in DownloadSessionService should return the next file in the session, or a null/empty response if the session is complete or invalid. Implement appropriate session expiration logic (e.g., after 30 minutes).
Step 4: Client-Side Polling: The client application will now need to:
- Call the
/files/prepare-batch endpoint with the list of desired file IDs to initiate a download session.
- Poll the
/files/batch/{sessionId}/next endpoint repeatedly until all files have been retrieved.
- Handle potential errors (e.g., session expiration, file not found).
Step 5: Configure Multipart File Limits (Spring Boot): Increase the maximum file size and request size limits in your application.properties file to avoid hitting default limits:
spring.servlet.multipart.max-file-size=50MB
spring.servlet.multipart.max-request-size=200MB
Common Pitfalls & What to Check Next:
- Error Handling: Implement robust error handling for both upload and download processes. Handle
IOExceptions during file processing, invalid session IDs, and file not found errors. Return informative error messages to the client.
- Session Expiration: Configure a reasonable session expiration time to prevent resource exhaustion. Consider adding session cleanup mechanisms.
- File Storage: Ensure your chosen file storage mechanism (e.g., filesystem, cloud storage) is appropriately configured and can handle the expected volume of files.
- Concurrency: If you expect a high volume of concurrent requests, consider using a queuing system (e.g., RabbitMQ, Kafka) to manage file processing and downloads asynchronously.
- Security: Protect your download endpoints with appropriate authentication and authorization mechanisms. Use short-lived, signed tokens for download URLs if you need finer-grained control.
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