Migration Guide 2.0.x to 2.4.x - Version 2.4.16

Migration Guide 2.0.x to 2.4.x

General notes

Java DSL now uses Java 8 types: CompletionStage and Optional

In order to provide a top-notch Java API we switched from Scala’s Future and Akka’s akka.japi.Option interim solutions to the JDK’s own types for deferred computation and optional results. This has been done throughout Streams & HTTP, most notably changing most materialized types, but also the signature of the mapAsync combinator and the asynchronous route result combinators in the HTTP DSL.

The akka.pattern package has been updated with a new set of implementations within the PatternCS class that provide the ability to interact between Actors and Futures (or streams) for CompletionStage.

Should you have the need to use Scala Futures with these new Java APIs please use the scala-java8-compat library that comes as a dependency of Akka. For more information see the documentation.

akka.Done and akka.NotUsed replacing Unit and BoxedUnit

To provide more clear signatures and have a unified API for both Java and Scala two new types have been introduced:

akka.NotUsed is meant to be used instead of Unit in Scala and BoxedUnit in Java to signify that the type parameter is required but not actually used. This is commonly the case with Source, Flow and Sink that do not materialize into any value.

akka.Done is added for the use case where it is boxed inside another object to signify completion but there is no actual value attached to the completion. It is used to replace occurrences of Future<BoxedUnit> with Future<Done> in Java and Future[Unit] with Future[Done] in Scala.

All previous usage of Unit and BoxedUnit for these two cases in the Akka Streams APIs has been updated.

This means that Java code like this:

Source<String, BoxedUnit> source = Source.from(Arrays.asList("1", "2", "3"));
Sink<String, Future<BoxedUnit>> sink = Sink.ignore();

needs to be changed into:

Source<String, NotUsed> source = Source.from(Arrays.asList("1", "2", "3"));
Sink<String, Future<Done>> sink = Sink.ignore();

These changes apply to all the places where streams are used, which means that signatures in the persistent query APIs also are affected.

Changed Operators

expand() is now based on an Iterator

Previously the expand combinator required two functions as input: the first one lifted incoming values into an extrapolation state and the second one extracted values from that, possibly evolving that state. This has been simplified into a single function that turns the incoming element into an Iterator.

The most prominent use-case previously was to just repeat the previously received value:

// This no longer works!
Flow.of(Integer.class).expand(i -> i)(i -> new Pair<>(i, i));

In Akka 2.4.x this is simplified to:

Flow.of(Integer.class).expand(in -> Stream.iterate(in, i -> i).iterator());

If state needs to be be kept during the expansion process then this state will need to be managed by the Iterator. The example of counting the number of expansions might previously have looked like:

// This no longer works!
Flow.of(Integer.class).expand(i -> new Pair<>(i, 0))(
  pair -> new Pair<>(new Pair<>(pair.first(), pair.second()),
                     new Pair<>(pair.first(), pair.second() + 1)));

In Akka 2.4.x this is formulated like so:

Flow.of(Integer.class).expand(in ->
    Stream.iterate(new Pair<>(in, 0),
                   p -> new Pair<>(in, p.second() + 1)).iterator());

conflate has been renamed to conflateWithSeed()

The new conflate operator is a special case of the original behavior (renamed to conflateWithSeed) that does not change the type of the stream. The usage of the new operator is as simple as:

Flow.of(Integer.class).conflate((a, b) -> a + b) // Add numbers while downstream is not ready

Which is the same as using conflateWithSeed with an identity function:

Flow.of(Integer.class).conflateWithSeed(x -> x, (a, b) -> a + b) // Add numbers while downstream is not ready

viaAsync and viaAsyncMat has been replaced with async()

async() is available from Sink, Source, Flow and the sub flows. It provides a shortcut for setting the attribute Attributes.asyncBoundary on a flow. The existing methods Flow.viaAsync and Flow.viaAsyncMat has been removed to make marking out asynchronous boundaries more consistent:

// This no longer works

In Akka 2.4.x this will instead look lile this:

Flow<Integer, Integer, NotUsed> flow = Flow.of(Integer.class).map(n -> n + 1);
Source.range(1, 10).via(flow.async());

Changed Sources / Sinks

Sink.asPublisher is now configured using an enum

In order to not use a meaningless boolean parameter we have changed the signature to:

import static akka.stream.javadsl.AsPublisher.*;
Sink.asPublisher(WITH_FANOUT);    // instead of Sink.asPublisher(true)
Sink.asPublisher(WITHOUT_FANOUT); // instead of Sink.asPublisher(false)

IO Sources / Sinks materialize IOResult

Materialized values of the following sources and sinks:

  • FileIO.fromPath
  • FileIO.toPath
  • StreamConverters.fromInputStream
  • StreamConverters.fromOutputStream

have been changed from Long to akka.stream.io.IOResult. This allows to signal more complicated completion scenarios. For example, on failure it is now possible to return the exception and the number of bytes written until that exception occured.

PushStage, PushPullStage and DetachedStage have been deprecated in favor of GraphStage

The PushStage PushPullStage and DetachedStage classes have been deprecated and should be replaced by GraphStage (Custom processing with GraphStage) which is now a single powerful API for custom stream processing.

Update procedure

Please consult the GraphStage documentation (Custom processing with GraphStage) and the previous migration guide on migrating from AsyncStage to GraphStage.

Changes in Akka HTTP

Routing settings parameter name

RoutingSettings were previously the only setting available on RequestContext, and were accessible via settings. We now made it possible to configure the parsers settings as well, so RoutingSettings is now routingSettings and ParserSettings is now accessible via parserSettings.

Client / server behaviour on cancelled entity

Previously if request or response were cancelled or consumed only partially (e.g. by using take combinator) the remaining data was silently drained to prevent stalling the connection, since there could still be more requests / responses incoming. Now the default behaviour is to close the connection in order to prevent using excessive resource usage in case of huge entities.

The old behaviour can be achieved by explicitly draining the entity:


SslTls has been renamed to TLS and moved

The DSL to access a TLS (or SSL) BidiFlow have now split between the javadsl and scaladsl packages and have been renamed to TLS. Common option types (closing modes, authentication modes, etc.) have been moved to the top level stream package, and the common message types are accessible in the class akka.stream.TLSProtocol

Websocket now consistently named WebSocket

Previously we had a mix of methods and classes called websocket or Websocket, which was in contradiction with how the word is spelled in the spec and some other places of Akka HTTP.

Methods and classes using the word WebSocket now consistently use it as WebSocket, so updating is as simple as find-and-replacing the lower-case s to an upper-case S wherever the word WebSocket appeared.

Java DSL for Http binding and connections changed

In order to minimise the number of needed overloads for each method defined on the Http extension a new mini-DSL has been introduced for connecting to hosts given a hostname, port and optional ConnectionContext.

The availability of the connection context (if it's set to HttpsConnectionContext) makes the server be bound as an HTTPS server, and for outgoing connections those settings are used instead of the default ones if provided.


http.cachedHostConnectionPool(toHost("akka.io"), materializer());
http.cachedHostConnectionPool("akka.io", 80, httpsConnectionContext, materializer()); // does not work anymore

Replace with:

http.cachedHostConnectionPool(toHostHttps("akka.io", 8081), materializer());
http.cachedHostConnectionPool(toHostHttps("akka.io", 8081).withCustomHttpsContext(httpsContext), materializer());

Framing moved to akka.stream.[javadsl/scaladsl]

The Framing object which can be used to chunk up ByteString streams into framing dependent chunks (such as lines) has moved to akka.stream.scaladsl.Framing, and has gotten a Java DSL equivalent type in akka.stream.javadsl.Framing.