BookService.java

@GET
@Path("{isbn}")
@Produces(MediaType.APPLICATION_JSON)
public void bookAndComment(@Suspended final AsyncResponse asyncResponse, @PathParam("isbn") String isbn) {
    //Calling previous defined functions
    Observable<JsonObject> bookInfo = getBookInfo(isbn);
    Observable<JsonArray> comments = getComments(isbn);

    Observable.zip(bookInfo, comments, (JsonObject book, JsonArray bookcomments) ->
                    Json.createObjectBuilder().add("book", book).add("comments", bookcomments).build()
                  )
                  .subscribe(new Subscriber<JsonObject>() {
                        @Override
                        public void onCompleted() {
                        }
                
                        @Override
                        public void onError(Throwable e) {
                            asyncResponse.resume(e);
                        }

                        @Override
                        public void onNext(JsonObject jsonObject) {
                            asyncResponse.resume(jsonObject);
                        }
                    });
}