Add Java API for Retry with examples

[synox]

It's hard to figure out how to use Retry with Java. An example should be provided.

so for I got this - please tell me how it can be improved.

  @Wither
    @Value
    class Meta {
        String payload;
        int attempt;
    }

    @Test
    public void retry2() throws InterruptedException {

        Flow<Tuple2<String, Meta>, Tuple2<Try<String>, Meta>, NotUsed> flow = Flow.<Tuple2<String, Meta>>create()
                .map(t -> {
                    System.out.println(t);
                    if (t._1.equals("banana")) {
                        return Tuple2.apply(Failure.apply(new IllegalArgumentException("oops")), t._2);
                    } else {
                        return Tuple2.apply(Success.apply(t._1), t._2);
                    }
                });

        Source.from(Arrays.asList("apple", "banana"))
                .map(s -> Tuple2.apply(s, new Meta(s, 1)))
                .via(Retry.apply(flow, (JFunction1<Meta, Option<Tuple2<String, Meta>>>) state -> {
                    if (state.attempt < 4) {
                        return Some.apply(Tuple2.apply(state.payload, state.withAttempt(state.attempt + 1)));
                    } else {
                        return Option.empty();
                    }
                }))
                .runForeach(t -> log.info("out: {}, state={}", t._1, t._2), mat);

        Thread.sleep(5000);

    }

[johanandren]

I think there should be a Java API rather. And then possibly a sample.

Adding one could be rather straight forward if you would be up for it, mostly converting between Java and Scala function types, using akka.japi.Pair instead of Tuple2, and java.util.Optional instead of Option. You should be able to find examples of the conversions in the Akka streams Java DSL code.

Are you up for doing a PR with that perhaps?

[synox]

I agree on Java API, but currently no time to do it.

btw. there is also RestartFlow akka/akka#19950, but it does not retry the Events.

[johanandren]

Let's keep this one open so that someone can pick it up in the future.