How to modify the response body in Spring Cloud Gateway just before the commit

Question

I'm using Spring Cloud Gateway with Spring 5, Spring Reactor and Netty for a project. For every request send to the gateway I want to do something just before the response is sent to the client. The best way I have found to do it is to add an action to the response with the beforeCommit method.

I first tried this approach :

        exchange.getResponse().beforeCommit(() -> {
            ServerHttpResponse response = exchange.getResponse();
            try {
                myActionDoneHere();
                response.setStatusCode(OK);
                return Mono.empty();
            } catch (Exception ex) {
                return Mono.error(new MyException(ex));
            }
        });

And tried to handle the exception in an exception handler :

public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
    if (isMyException(ex)) {
     exchange.getResponse().setStatusCode(HttpStatus.INTERNAL_SERVER_ERROR);
        exchange.getResponse().getHeaders().setContentLength(MSG.length());
        exchange.getResponse().getHeaders().setContentType(MediaType.APPLICATION_JSON_UTF8);
        return exchange.getResponse().writeWith(Mono.just(exchange.getResponse().bufferFactory().wrap(MSG.getBytes())));
    }

    return Mono.error(ex);
}

When I do that I have an exception when I try to modify the content length. If I understand well the situation. I can't modify the response anymore in my handler because the it as been already committed. So I tried an other solution and tried to modify the response in my action executed just before the commit :

exchange.getResponse().beforeCommit(() -> {
    ServerHttpResponse response = exchange.getResponse();
    try {
        myActionDoneHere();
        response.setStatusCode(OK);
        return Mono.empty();
    } catch (Exception ex) {
        response.setStatusCode(HttpStatus.INTERNAL_SERVER_ERROR);
        response.getHeaders().setContentLength(MSG.length());
        response.getHeaders().setContentType(MediaType.APPLICATION_JSON_UTF8);
        return exchange.getResponse().writeWith(Mono.just(exchange.getResponse().bufferFactory().wrap(MSG.getBytes())));
    }
});

This time I don't have any exception, I can modify the content length but I can't modify the body of the response.

So does someone know if it's possible to do so and How ?

Answer 1

I used the technique that was inside WebClientWriteResponseFilter

    if (failedAuthorization) {
        ServerWebExchangeUtils.setResponseStatus(exchange, HttpStatus.UNAUTHORIZED);
        ServerWebExchangeUtils.setAlreadyRouted(exchange);
        final Map<String, String> error = Map.of("error", "unauthorized");
        return chain.filter(exchange).then(Mono.defer(() -> {
            final ServerHttpResponse response = exchange.getResponse();
            response.getHeaders().add(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_UTF8.toString());
            return response.writeWith(new Jackson2JsonEncoder().encode(Mono.just(error),
                response.bufferFactory(),
                ResolvableType.forInstance(error),
                MediaType.APPLICATION_JSON_UTF8,
                Hints.from(Hints.LOG_PREFIX_HINT, exchange.getLogPrefix()))
            );
        }));
    }