I’ve went through all the answers for this question
download a file from Spring boot rest service, realizing that the differences are related to the return value. Basically, there are two type families.

1. Return type for Blocking Servlet

ResponseEntity<Resource>

• ResponseEntity<InputStreamResource>
• ResponseEntity<ByteArrayResource>
• ResponseEntity<FileSystemResource>

2. Return type for Async Servlet(Servlet >= 3.0)

• 2.1)DeferredResult<ResponseEntity<?>>
• 2.2)ResponseBodyEmitter or ResponseEntity<ResponseBodyEmitter>
• 2.3)SseEmitter or ResponseEntity<SseEmitter>
• 2.4)StreamingResponseBody or ResponseEntity<StreamingResponseBody>

Someone mentions that, for downloading big files, it might go OOM if we use ByteArrayResource.

IIUC, either FileSystemResource or StreamingResponseBody is friendly with big files, with http chunked encoding under the hood.

• FileSystemResource servlet container thread is blocked but can take advantage of Zero Copy. More efficient for memory.
• StreamingResponseBody servlet container thread is released. More efficient for CPU.

If that’s true, will it be the best choice to use DeferredResult<ResponseEntity<FileSystemResource>> instead, since it’s more efficient for both memory and CPU?

P.S. Let’s ignore CDN and some other tech like BitTorrent for downloading big files in real world.