Can I use StreamingResponseBody with a generic ResponseEntity

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Summary

The issue arises when attempting to use generic ResponseEntity<?> with StreamingResponseBody in a Spring MVC application. The goal is to return either a stream or a synchronous value, but using ResponseEntity<?> with StreamingResponseBody results in an HttpMessageNotWritableException.

Root Cause

The root cause of the issue is that ResponseEntity<?> does not know how to handle StreamingResponseBody because:

  • StreamingResponseBody is a functional interface that requires a specific handler to write the response body
  • The HttpMessageConverter mechanism in Spring MVC does not support StreamingResponseBody out of the box
  • When using ResponseEntity<?>, Spring MVC attempts to find a suitable HttpMessageConverter to write the response body, but none is found for StreamingResponseBody

Why This Happens in Real Systems

This issue occurs in real systems when:

  • Developers need to return different types of responses (e.g., streams, strings, objects) from a single endpoint
  • The response type is determined dynamically based on the request or other factors
  • The ResponseEntity<?> type is used to accommodate multiple response types, but StreamingResponseBody is not properly handled

Real-World Impact

The impact of this issue includes:

  • HttpMessageNotWritableException is thrown when attempting to return a StreamingResponseBody from a method with a ResponseEntity<?> return type
  • The client receives an error response instead of the expected stream or data
  • Developers must find alternative solutions to handle StreamingResponseBody with ResponseEntity<?>

Example or Code

@RestController
public class WebDavController {
    @RequestMapping("/{filename}")
    public ResponseEntity request(HttpServletRequest request, HttpServletResponse response, @PathVariable String filename) {
        //...
        case "GET":
            return handleGet(request, response, filename);
        //...
    }

    private ResponseEntity handleGet(HttpServletRequest request, HttpServletResponse response, @PathVariable String filename) {
        StreamingResponseBody responseBody = outputStream -> Files.copy(filename, outputStream);
        return ResponseEntity.ok()
               .contentType(MediaType.APPLICATION_OCTET_STREAM)
               .body(responseBody);
    }
}

How Senior Engineers Fix It

To resolve this issue, senior engineers can:

  • Use a ResponseEntity<StreamingResponseBody> return type for methods that return streams
  • Create a custom HttpMessageConverter to handle StreamingResponseBody
  • Use a library or framework that provides built-in support for StreamingResponseBody with ResponseEntity<?>

Why Juniors Miss It

Junior engineers may miss this issue because:

  • They may not fully understand the implications of using ResponseEntity<?> with StreamingResponseBody
  • They may not be familiar with the HttpMessageConverter mechanism in Spring MVC
  • They may not have experience with handling streams and synchronous responses in a single endpoint