[resteasy-dev] Extending Resteasy async support (server-side)

[Stephane Epardaud]

Hi,

I'm working on trying to make Resteasy and Vertx integrate better, and
while writing a prototype I noticed several things which would need
support from Resteasy.

The first thing which I need RE to support is that I want to be able to
return async types from my resource methods. In JAX-RS 2.0 async
resource methods are supported but require a peculiar style, such as
taking a "@Suspended AsyncResponse asyncResponse" parameter, returning
void, and resuming with asyncResponse.resume(entityOrException).

I know JAX-RS 2.1 supports CompletionStage<Response>. This is great, but
it doesn't define a pluggable API to support other types such as rxJava.
We should really make that be pluggable, so that I can use rx.Single or
other async frameworks.

ATM I have a prototype which supports CompletionStage and rx.Single,
without modifying RE, but it's using CDI validators to intercept the
returned value and do the plugging, which is a terrible hack. I couldn't
find a way to do this properly with RE because I can't intercept the
returned value before it's sent to the marshallers, by which time it's
too late to mark the response as async and register on the listeners.

Now, because the return type is CompletionStage<X> (or rx.Single<X>
doesn't matter), the ResourceMethodInvoker.getGenericReturnType()
returns a very unhelpful type, which I can't used to look up the proper
marshallers, so again this is something that has to be taken into account.

Sometimes I will declare a return type of CompletionStage<Object>
because Java doesn't have Union Types and I can't declare it as
CompletionStage<Response | MyEntity>, and in that case I have to look up
marshallers based on the entity instance's class rather than the
declared type. I suppose the same applies to non-async return types. Is
it the spec that fixes this looking up of marshallers based on the
static return type and not the entity's actual type?

Now, I haven't said much about things like Observable or Flowable (async
types that emit more than one value), because it's not yet clear in my
mind how that would work with RE. In my proto I do support returning
such flows and based on how the method is annotated, RE will pick one
strategy:

- collect all values in a java.util.List and when all done proceed,
- send them as HTTP Chunked: marshall and send each value as they are
made available
- send them as server-sent events but that's a bit weird when it's not
plain text

Since JAX-RS 2.1 supports server-sent events, that would also be a
strategy that makes sense.

I have contacted the CDI team to discuss async injection, because I also
want to be able to inject instances of X in my resources where there
only exists providers of type CompletionStage<X>, which requires a new
CDI API to support async resource instantiation/injection. If/when that
gets added to CDI, RE will need to be updated to support this async
injection.

Similarly I also have a problem with interceptors, because sometimes my
RE interceptors require resolving of async types before they can go on
with the request. For example for authentication or sessions my
interceptors have to wait for vert.x to resolve async values before I
can check them and proceed with the requests. The wait the interceptor
API is written doesn't let me suspend interception. It's be great if it
could. Alternately CDI interceptors have an InvocationContext parameter
given to the interceptor on which you have to call proceed() in order to
proceed. This allows me to delay proceeding until I have all my async
values resolved and only then proceed with the call to the next
interceptor. That'd work too.

And I guess the same applies to Marshallers. I have Template entity
objects that I need to marshall and that is done via an async API in
vertx, so again I can't entirely marshall the response when the
marshaller is called.

Last, and least interestingly, I've always been bitten by the fact that
ExceptionMappers are global and I can't override them per resource.
Sometimes I have UI/HTML resources that need exception mappers that
forward to UI pages, and API resources that have to serialise errors
into JSON. I've added support for an @WithExceptionMapper(Class<?
extends ExceptionMapper>) annotation that you can place on the resource
method or class to override the error mapper for this call.

WDYT?

I'm definitly game for pushing PRs for those things, BTW.

How far along are you guys with implementing JAX-RS 2.1 in RE?

Cheers.
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