Spring Webfilter bypass for Mono.empty() in Spring Boot 2.4.1

When applying webfilter, I found that it cannot handle Mono.empty() returned from controllers The below codes (switchIfEmpty) make empty messages return to Mono.empty() directly

## https://github.com/spring-projects/spring-framework/blob/master/spring-web/src/main/java/org/springframework/http/codec/EncoderHttpMessageWriter.java

if (inputStream instanceof Mono) {
    return body
        .singleOrEmpty()
        .switchIfEmpty(Mono.defer(() -> {
            message.getHeaders().setContentLength(0);
            return message.setComplete().then(Mono.empty());
        }))
        .flatMap(buffer -> {
            Hints.touchDataBuffer(buffer, hints, logger);
            message.getHeaders().setContentLength(buffer.readableByteCount());
            return message.writeWith(Mono.just(buffer)
                    .doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release));
        })
        .doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
}

As you know, webfilter can wrap or rewrite the message when controller returns body with data. In case of empty, a developer cannot wrap additional data through webfilter even if it is an empty because empty message is returned directly.

So, I think it would be better to create new function to handle message or to reuse writeWith regardless of value received from controller

Comment From: rstoyanchev

Result handling, including writing the response body (empty or not) happens within DispatcherHandler and a WebFilter on the outbound side is after that. So I don't expect there should be any difference whether the controller returned Mono.empty() or some actual data.

Comment From: liuzhongkai

The controller returns empty, you can add a default callback

Comment From: rstoyanchev

A WebFilter cannot change the response regardless of what the controller returns. This is expected behavior. I understand what you want to do but it's not possible nor supported currently in a cross-cutting way.

The controller returns empty, you can add a default callback

Add a callback to what?

Comment From: timewizhan

Thanks for the reply.

Um... I currently applied WebFilter to change the response with WebServerExchange. For example, when ServerHttpResponse is manipulated in WebFilter#filter, the databuffer for the response is overwritten by the "writeWith" function. In case that the controller has a body, the below codes is executed

 message.writeWith(Mono.just(buffer)
                    .doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release));

Perhaps, is it to misuse the webfilter??

Otherwise, I think it would be better a chance to overwrap returned data than to bypass Mono.empty directly

Comment From: rstoyanchev

The body cannot be set twice and I'm not sure what exactly your WebFilter does. Can you show some example code?

Comment From: spring-projects-issues

If you would like us to look at this issue, please provide the requested information. If the information is not provided within the next 7 days this issue will be closed.

Comment From: spring-projects-issues

Closing due to lack of requested feedback. If you would like us to look at this issue, please provide the requested information and we will re-open the issue.