Streaming and Downloading Files with Spring Boot

In today's digital world, handling files is a common part of many applications. Whether it's uploading user profile pictures, generating reports, or serving media content, effectively streaming and downloading files is an essential aspect of any modern application. In this article, we will explore how to achieve this functionality using Spring Boot, a powerful framework for building Java applications.

Streaming Files

Streaming files involves sending the content of a file incrementally as it becomes available, allowing clients to start processing or displaying the content even before the entire file is transmitted. With Spring Boot, this can be achieved by utilizing the 'StreamingResponseBody' class, which allows for streaming large files efficiently.

To begin, let's assume we have a file stored in our server's file system that we want to stream to clients upon request. We can define a Spring REST controller and expose an API endpoint to stream the file as follows:

@RestController
public class FileController {

    @GetMapping("/files/{fileName}")
    public ResponseEntity<StreamingResponseBody> streamFile(@PathVariable String fileName) {
        File fileToStream = new File("path/to/files/" + fileName);
      
        if (fileToStream.exists()) {
            StreamingResponseBody responseBody = outputStream -> {
                try (InputStream inputStream = new FileInputStream(fileToStream)) {
                    byte[] buffer = new byte[1024];
                    int bytesRead;
                    while ((bytesRead = inputStream.read(buffer)) != -1) {
                        outputStream.write(buffer, 0, bytesRead);
                    }
                }
            };
            return ResponseEntity.ok()
                    .contentType(MediaType.APPLICATION_OCTET_STREAM)
                    .body(responseBody);
        } else {
            return ResponseEntity.notFound().build();
        }
    }
}

In the above code snippet, the 'streamFile' method fetches the desired file, creates an instance of 'StreamingResponseBody', and defines its behavior during streaming. The file's content is read in chunks and written to the output stream, which gets sent to the client incrementally. The 'MediaType.APPLICATION_OCTET_STREAM' is used to indicate to the client that the response contains binary data.

By calling the API endpoint GET /files/{fileName}, clients can effectively stream the file's content and process it as needed.

Downloading Files

Downloading files involves making files available for clients to save to their local device. Spring Boot offers an elegant way to enable file downloads using the 'ResponseEntity' class.

Let's modify our previous example to allow clients to download the file rather than just streaming it:

@RestController
public class FileController {

    @GetMapping("/files/{fileName}")
    public ResponseEntity<Resource> downloadFile(@PathVariable String fileName) throws IOException {
        File fileToDownload = new File("path/to/files/" + fileName);
      
        if (fileToDownload.exists()) {
            Resource fileResource = new FileSystemResource(fileToDownload);
            return ResponseEntity.ok()
                    .contentType(MediaType.APPLICATION_OCTET_STREAM)
                    .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + fileResource.getFilename() + "\"")
                    .body(fileResource);
        } else {
            return ResponseEntity.notFound().build();
        }
    }
}

In the updated code snippet, the 'downloadFile' method utilizes the 'Resource' interface, which abstracts file resources for downloading purposes. By creating a 'FileSystemResource' instance using the file to download, we can generate a 'ResponseEntity' with the appropriate headers (content type and content disposition). The 'attachment' value in the 'CONTENT_DISPOSITION' header prompts the browser to download the file instead of rendering it.

By accessing the URL GET /files/{fileName}, clients can download the file and save it locally on their device.

Conclusion

Handling file streaming and downloading is a crucial feature for many applications, and Spring Boot makes it easier to implement. By using 'StreamingResponseBody' for efficient file streaming and 'ResponseEntity' with 'Resource' for file downloading, developers can streamline the process and provide a seamless user experience. Whether it's delivering media files, generating dynamic reports, or managing user uploads, Spring Boot offers the tools to handle file-related operations effectively.