How to read the body of a HttpRequest in Java 11?

In a test, I'd like to look inside the body of a HttpRequest. I'd like to get the body as a string. It seems that the only way to do that, is to subscribe to the BodyPublisher but how does that work?

This is an interesting question. Where do you get your HttpRequest from? The easiest way would be to obtain the body directly from the code that creates the HttpRequest. If that's not possible then the next thing would be to clone that request and wraps its body publisher in your own implementation of BodyPublisher before sending the request through the HttpClient. It should be relatively easy (if tedious) to write a subclass of HttpRequest that wraps an other instance of HttpRequest and delegates every calls to the wrapped instance, but overrides HttpRequest::bodyPublisher to do something like:

return request.bodyPublisher().map(this::wrapBodyPublisher);

Otherwise, you might also try to subscribe to the request body publisher and obtain the body bytes from it - but be aware that not all implementations of BodyPublisher may support multiple subscribers (whether concurrent or sequential).

To illustrate my suggestion above: something like below may work, depending on the concrete implementation of the body publisher, and provided that you can guard against concurrent subscriptions to the body publisher. That is - in a controlled test environment where you know all the parties, then it might be workable. Don't use anything this in production:

public class HttpRequestBody {

    // adapt Flow.Subscriber<List<ByteBuffer>> to Flow.Subscriber<ByteBuffer>
    static final class StringSubscriber implements Flow.Subscriber<ByteBuffer> {
        final BodySubscriber<String> wrapped;
        StringSubscriber(BodySubscriber<String> wrapped) {
            this.wrapped = wrapped;
        }
        @Override
        public void onSubscribe(Flow.Subscription subscription) {
            wrapped.onSubscribe(subscription);
        }
        @Override
        public void onNext(ByteBuffer item) { wrapped.onNext(List.of(item)); }
        @Override
        public void onError(Throwable throwable) { wrapped.onError(throwable); }
        @Override
        public void onComplete() { wrapped.onComplete(); }
    }

    public static void main(String[] args) throws Exception {
        var request = HttpRequest.newBuilder(new URI("http://example.com/blah"))
                .POST(BodyPublishers.ofString("Lorem ipsum dolor sit amet"))
                .build();

        // you must be very sure that nobody else is concurrently 
        // subscribed to the body publisher when executing this code,
        // otherwise one of the subscribers is likely to fail.
        String reqbody = request.bodyPublisher().map(p -> {
            var bodySubscriber = BodySubscribers.ofString(StandardCharsets.UTF_8);
            var flowSubscriber = new StringSubscriber(bodySubscriber);
            p.subscribe(flowSubscriber);
            return bodySubscriber.getBody().toCompletableFuture().join();
        }).get();
        System.out.println(reqbody);
    }

}