Java 8 CompletionStage exceptionally rethrow exception

Question

Rethrowing an exception inside of exceptionally method of CompletionStage seems not to be allowed.

I need to check for a certain kind of exception and if not I need to rethrow it back :

Future<JsonNode> futureSite = someClient.getSite(siteId, queryParams);

CompletionStage<JsonNode> outcome = FutureConverters.toJava(futureSite);

return outcome.thenApplyAsync((siteJson) -> {
            Site site = Json.fromJson(siteJson, Site.class);
            try {
                return function.apply(site);
            } catch (RequestException e) {
                return e.result;
            }
        }, httpExecutionContext.current()).exceptionally(throwable -> {
            if(throwable instanceof SomeClientException) {
                if(((SomeClientException) throwable).httpStatusCode == 404) {
                   return entityNotFound("Site", siteId);
                }
            }


 // let JSON parsing failures and other errors bubble up, TODO play2.5


         throw throwable;
        });

throw throwable errors out saying unhandledException java.lang.Throwable

What interface would possibly allow to rethrow back the exception ? Or is there a better way around ?

Update :

I tried with advice from Holger as below, but I am still not sure at what point could I actually rethrow this :

BaseController.java : 
       protected class RequestException extends Exception {
            private static final long serialVersionUID = -2154871100650903869L;

            public Result result;

            public RequestException(Result result) {
                this.result = result;
            }
        }

        @FunctionalInterface
        protected interface RequestFunction<T, R> {
            R apply(T t) throws RequestException;
        }

    protected CompletionStage<Result> performWithSite(final Long siteId, RequestFunction<Site, Result> function) {
            QueryParams queryParams = QueryParams.create();
            Future<JsonNode> futureSite = someClient.getSite(siteId, queryParams);

            CompletionStage<JsonNode> outcome = FutureConverters.toJava(futureSite);

            return handleSpecific(
                    outcome.thenApplyAsync(siteJson -> {
                        Site site = Json.fromJson(siteJson, Site.class);
                        try {
                            return function.apply(site);
                        } catch (RequestException e) {
                            return e.result;
                        }
                    }, httpExecutionContext.current()),
                    throwable -> throwable instanceof SomeClientException
                            && ((SomeClientException)throwable).httpStatusCode == 404,
                    () -> entityNotFound("Site", siteId));
        } 

      protected Result entityNotFound(String entityName, String id) {
    // building our custom error model. 
  Error e = new Error(
                    Http.Status.NOT_FOUND,
                    ErrorCode.ENTITY_NOT_FOUND,
                    ErrorCodes.NOT_FOUND, new String[]{entityName, id});

            return notFound(e.asJson());
        }

So, the essence of above code is I need to reach out to someClient to check if the site exists and the client can throw a SomeClientException.

 Controller.java

       public Result destroy(Long siteId, Long productId){
            return performWithSite(siteId, site -> {
                productWriter.deleteProduct(siteId, productId);
                return noContent();
            }).toCompletableFuture().exceptionally(e -> {
              Logger.error(e+"exception");
               throw e.getCause(); 
              // above line errors out as unhandledException java.lang.throwable, I need the NotFoundException which is contained within the CompletionException to be thrown.
           });
        }

// In the above controller code, after I do the remote call to the client to check if the site exists, I need to go ahead and delete the product.

productWriter.deleteProduct(siteId, productId) might still throw a NotFoundException or something else which I need to rethrow it back ... so that any exception which is re-thrown from the controller code is picked up our custom exception handler in the call chain.

Here is my test case :

  "return 404 when deleting a nonexistent Product" {
      when(productDAO.findBySiteAndProductId(anyLong(), anyLong())) thenReturn null

      a[NotFoundException] should be thrownBy { controller.destroy(0L, 1L) }
    }

Answer 1

Afaik, there is no builtin solution. All function types you can pass to the chaining methods of CompletionStage are restricted to unchecked exceptions. You can built your own utility method:

public static <T> CompletionStage<T> handleSpecific(
    CompletionStage<T> previousStage, Predicate<Throwable> p, Supplier<T> s) {

    CompletableFuture<T> result = new CompletableFuture<>();
    previousStage.whenComplete((value,throwable)->{
        if(throwable == null) result.complete(value);
        else {
            Throwable t = throwable;
            if(t instanceof CompletionException) {
                t = t.getCause();
                if(t == null) t = throwable;
            }
            if(p.test(t)) result.complete(s.get());
            else result.completeExceptionally(throwable);
        }
    });
    return result;
}

This solution can be used like:

return handleSpecific(
    outcome.thenApplyAsync(siteJson -> {
        Site site = Json.fromJson(siteJson, Site.class);
        try {
            return function.apply(site);
        } catch (RequestException e) {
            return e.result;
        }
    }, httpExecutionContext.current()),
    throwable -> throwable instanceof SomeClientException
              && ((SomeClientException)throwable).httpStatusCode == 404,
    () -> entityNotFound("Site", siteId));