<!-- Maven -->
<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-actor_2.12</artifactId>
  <version>2.5.21</version>
</dependency>

<!-- Gradle -->
dependencies {
  compile group: 'com.typesafe.akka', name: 'akka-actor_2.12', version: '2.5.21'
}

<!-- sbt -->
libraryDependencies += "com.typesafe.akka" %% "akka-actor" % "2.5.21"

import akka.actor.AbstractActor;
import akka.event.Logging;
import akka.event.LoggingAdapter;

public class MyActor extends AbstractActor {
  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            String.class,
            s -> {
              log.info("Received String message: {}", s);
            })
        .matchAny(o -> log.info("received unknown message"))
        .build();
  }
}

import akka.actor.Props;
Props props1 = Props.create(MyActor.class);
Props props2 =
    Props.create(ActorWithArgs.class, () -> new ActorWithArgs("arg")); // careful, see below
Props props3 = Props.create(ActorWithArgs.class, "arg");

// NOT RECOMMENDED within another actor:
// encourages to close over enclosing class
Props props7 = Props.create(ActorWithArgs.class, () -> new ActorWithArgs("arg"));

static class DemoActor extends AbstractActor {
  /**
   * Create Props for an actor of this type.
   *
   * @param magicNumber The magic number to be passed to this actor’s constructor.
   * @return a Props for creating this actor, which can then be further configured (e.g. calling
   *     `.withDispatcher()` on it)
   */
  static Props props(Integer magicNumber) {
    // You need to specify the actual type of the returned actor
    // since Java 8 lambdas have some runtime type information erased
    return Props.create(DemoActor.class, () -> new DemoActor(magicNumber));
  }

  private final Integer magicNumber;

  public DemoActor(Integer magicNumber) {
    this.magicNumber = magicNumber;
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            Integer.class,
            i -> {
              getSender().tell(i + magicNumber, getSelf());
            })
        .build();
  }
}

static class SomeOtherActor extends AbstractActor {
  // Props(new DemoActor(42)) would not be safe
  ActorRef demoActor = getContext().actorOf(DemoActor.props(42), "demo");
  // ...
}

static class DemoMessagesActor extends AbstractLoggingActor {

  public static class Greeting {
    private final String from;

    public Greeting(String from) {
      this.from = from;
    }

    public String getGreeter() {
      return from;
    }
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            Greeting.class,
            g -> {
              log().info("I was greeted by {}", g.getGreeter());
            })
        .build();
  }
}

import akka.actor.ActorRef;
import akka.actor.ActorSystem;

static class FirstActor extends AbstractActor {
  final ActorRef child = getContext().actorOf(Props.create(MyActor.class), "myChild");

  @Override
  public Receive createReceive() {
    return receiveBuilder().matchAny(x -> getSender().tell(x, getSelf())).build();
  }
}

import akka.actor.Actor;
import akka.actor.IndirectActorProducer;
class DependencyInjector implements IndirectActorProducer {
  final Object applicationContext;
  final String beanName;

  public DependencyInjector(Object applicationContext, String beanName) {
    this.applicationContext = applicationContext;
    this.beanName = beanName;
  }

  @Override
  public Class<? extends Actor> actorClass() {
    return TheActor.class;
  }

  @Override
  public TheActor produce() {
    TheActor result;
    result = new TheActor((String) applicationContext);
    return result;
  }
}

  final ActorRef myActor =
      getContext()
          .actorOf(
              Props.create(DependencyInjector.class, applicationContext, "TheActor"), "TheActor");

final Inbox inbox = Inbox.create(system);
inbox.send(target, "hello");
try {
  assert inbox.receive(Duration.ofSeconds(1)).equals("world");
} catch (java.util.concurrent.TimeoutException e) {
  // timeout
}

final Inbox inbox = Inbox.create(system);
inbox.watch(target);
target.tell(PoisonPill.getInstance(), ActorRef.noSender());
try {
  assert inbox.receive(Duration.ofSeconds(1)) instanceof Terminated;
} catch (java.util.concurrent.TimeoutException e) {
  // timeout
}

public void preStart() {}

public void preRestart(Throwable reason, Optional<Object> message) {
  for (ActorRef each : getContext().getChildren()) {
    getContext().unwatch(each);
    getContext().stop(each);
  }
  postStop();
}

public void postRestart(Throwable reason) {
  preStart();
}

public void postStop() {}

import akka.actor.Terminated;
static class WatchActor extends AbstractActor {
  private final ActorRef child = getContext().actorOf(Props.empty(), "target");
  private ActorRef lastSender = system.deadLetters();

  public WatchActor() {
    getContext().watch(child); // <-- this is the only call needed for registration
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals(
            "kill",
            s -> {
              getContext().stop(child);
              lastSender = getSender();
            })
        .match(
            Terminated.class,
            t -> t.actor().equals(child),
            t -> {
              lastSender.tell("finished", getSelf());
            })
        .build();
  }
}

@Override
public void preStart() {
  target = getContext().actorOf(Props.create(MyActor.class, "target"));
}

// will look up this absolute path
getContext().actorSelection("/user/serviceA/actor");
// will look up sibling beneath same supervisor
getContext().actorSelection("../joe");

// will look all children to serviceB with names starting with worker
getContext().actorSelection("/user/serviceB/worker*");
// will look up all siblings beneath same supervisor
getContext().actorSelection("../*");

import akka.actor.ActorIdentity;
import akka.actor.ActorSelection;
import akka.actor.Identify;
static class Follower extends AbstractActor {
  final Integer identifyId = 1;

  public Follower() {
    ActorSelection selection = getContext().actorSelection("/user/another");
    selection.tell(new Identify(identifyId), getSelf());
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            ActorIdentity.class,
            id -> id.getActorRef().isPresent(),
            id -> {
              ActorRef ref = id.getActorRef().get();
              getContext().watch(ref);
              getContext().become(active(ref));
            })
        .match(
            ActorIdentity.class,
            id -> !id.getActorRef().isPresent(),
            id -> {
              getContext().stop(getSelf());
            })
        .build();
  }

  final AbstractActor.Receive active(final ActorRef another) {
    return receiveBuilder()
        .match(
            Terminated.class, t -> t.actor().equals(another), t -> getContext().stop(getSelf()))
        .build();
  }
}

getContext().actorSelection("akka.tcp://app@otherhost:1234/user/serviceB");

public class ImmutableMessage {
  private final int sequenceNumber;
  private final List<String> values;

  public ImmutableMessage(int sequenceNumber, List<String> values) {
    this.sequenceNumber = sequenceNumber;
    this.values = Collections.unmodifiableList(new ArrayList<String>(values));
  }

  public int getSequenceNumber() {
    return sequenceNumber;
  }

  public List<String> getValues() {
    return values;
  }
}

// don’t forget to think about who is the sender (2nd argument)
target.tell(message, getSelf());

import static akka.pattern.Patterns.ask;
import static akka.pattern.Patterns.pipe;

import java.util.concurrent.CompletableFuture;
final Duration t = Duration.ofSeconds(5);

// using 1000ms timeout
CompletableFuture<Object> future1 =
    ask(actorA, "request", Duration.ofMillis(1000)).toCompletableFuture();

// using timeout from above
CompletableFuture<Object> future2 = ask(actorB, "another request", t).toCompletableFuture();

CompletableFuture<Result> transformed =
    CompletableFuture.allOf(future1, future2)
        .thenApply(
            v -> {
              String x = (String) future1.join();
              String s = (String) future2.join();
              return new Result(x, s);
            });

pipe(transformed, system.dispatcher()).to(actorC);

try {
  String result = operation();
  getSender().tell(result, getSelf());
} catch (Exception e) {
  getSender().tell(new akka.actor.Status.Failure(e), getSelf());
  throw e;
}

target.forward(result, getContext());

@Override
public Receive createReceive() {
  return receiveBuilder().match(String.class, s -> System.out.println(s.toLowerCase())).build();
}

import akka.actor.AbstractActor;
import akka.event.Logging;
import akka.event.LoggingAdapter;

public class MyActor extends AbstractActor {
  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            String.class,
            s -> {
              log.info("Received String message: {}", s);
            })
        .matchAny(o -> log.info("received unknown message"))
        .build();
  }
}

import akka.actor.AbstractActor;
import akka.event.Logging;
import akka.event.LoggingAdapter;
import akka.japi.pf.ReceiveBuilder;

public class GraduallyBuiltActor extends AbstractActor {
  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);

  @Override
  public Receive createReceive() {
    ReceiveBuilder builder = ReceiveBuilder.create();

    builder.match(
        String.class,
        s -> {
          log.info("Received String message: {}", s);
        });

    // do some other stuff in between

    builder.matchAny(o -> log.info("received unknown message"));

    return builder.build();
  }
}

static class WellStructuredActor extends AbstractActor {

  public static class Msg1 {}

  public static class Msg2 {}

  public static class Msg3 {}

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(Msg1.class, this::receiveMsg1)
        .match(Msg2.class, this::receiveMsg2)
        .match(Msg3.class, this::receiveMsg3)
        .build();
  }

  private void receiveMsg1(Msg1 msg) {
    // actual work
  }

  private void receiveMsg2(Msg2 msg) {
    // actual work
  }

  private void receiveMsg3(Msg3 msg) {
    // actual work
  }
}

static class OptimizedActor extends UntypedAbstractActor {

  public static class Msg1 {}

  public static class Msg2 {}

  public static class Msg3 {}

  @Override
  public void onReceive(Object msg) throws Exception {
    if (msg instanceof Msg1) receiveMsg1((Msg1) msg);
    else if (msg instanceof Msg2) receiveMsg2((Msg2) msg);
    else if (msg instanceof Msg3) receiveMsg3((Msg3) msg);
    else unhandled(msg);
  }

  private void receiveMsg1(Msg1 msg) {
    // actual work
  }

  private void receiveMsg2(Msg2 msg) {
    // actual work
  }

  private void receiveMsg3(Msg3 msg) {
    // actual work
  }
}

getSender().tell(s, getSelf());

static class ReceiveTimeoutActor extends AbstractActor {
  public ReceiveTimeoutActor() {
    // To set an initial delay
    getContext().setReceiveTimeout(Duration.ofSeconds(10));
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals(
            "Hello",
            s -> {
              // To set in a response to a message
              getContext().setReceiveTimeout(Duration.ofSeconds(1));
            })
        .match(
            ReceiveTimeout.class,
            r -> {
              // To turn it off
              getContext().cancelReceiveTimeout();
            })
        .build();
  }
}

import java.time.Duration;
import akka.actor.AbstractActorWithTimers;

static class MyActor extends AbstractActorWithTimers {

  private static Object TICK_KEY = "TickKey";

  private static final class FirstTick {}

  private static final class Tick {}

  public MyActor() {
    getTimers().startSingleTimer(TICK_KEY, new FirstTick(), Duration.ofMillis(500));
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            FirstTick.class,
            message -> {
              // do something useful here
              getTimers().startPeriodicTimer(TICK_KEY, new Tick(), Duration.ofSeconds(1));
            })
        .match(
            Tick.class,
            message -> {
              // do something useful here
            })
        .build();
  }
}

import akka.actor.ActorRef;
import akka.actor.AbstractActor;

public class MyStoppingActor extends AbstractActor {

  ActorRef child = null;

  // ... creation of child ...

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("interrupt-child", m -> getContext().stop(child))
        .matchEquals("done", m -> getContext().stop(getSelf()))
        .build();
  }
}

@Override
public void postStop() {
  final String message = "stopped";
  // don’t forget to think about who is the sender (2nd argument)
  target.tell(message, getSelf());
  final Object result = "";
  target.forward(result, getContext());
  target = null;
}

victim.tell(akka.actor.PoisonPill.getInstance(), ActorRef.noSender());

victim.tell(akka.actor.Kill.getInstance(), ActorRef.noSender());

// expecting the actor to indeed terminate:
expectTerminated(Duration.ofSeconds(3), victim);

import static akka.pattern.Patterns.gracefulStop;
import akka.pattern.AskTimeoutException;
import java.util.concurrent.CompletionStage;

try {
  CompletionStage<Boolean> stopped =
      gracefulStop(actorRef, Duration.ofSeconds(5), Manager.SHUTDOWN);
  stopped.toCompletableFuture().get(6, TimeUnit.SECONDS);
  // the actor has been stopped
} catch (AskTimeoutException e) {
  // the actor wasn't stopped within 5 seconds
}

# CoordinatedShutdown is enabled by default and will run the tasks that
# are added to these phases by individual Akka modules and user logic.
#
# The phases are ordered as a DAG by defining the dependencies between the phases
# to make sure shutdown tasks are run in the right order.
#
# In general user tasks belong in the first few phases, but there may be use
# cases where you would want to hook in new phases or register tasks later in
# the DAG.
#
# Each phase is defined as a named config section with the
# following optional properties:
# - timeout=15s: Override the default-phase-timeout for this phase.
# - recover=off: If the phase fails the shutdown is aborted
#                and depending phases will not be executed.
# - enabled=off: Skip all tasks registered in this phase. DO NOT use
#                this to disable phases unless you are absolutely sure what the
#                consequences are. Many of the built in tasks depend on other tasks
#                having been executed in earlier phases and may break if those are disabled.
# depends-on=[]: Run the phase after the given phases
phases {

  # The first pre-defined phase that applications can add tasks to.
  # Note that more phases can be added in the application's
  # configuration by overriding this phase with an additional
  # depends-on.
  before-service-unbind {
  }

  # Stop accepting new incoming connections.
  # This is where you can register tasks that makes a server stop accepting new connections. Already
  # established connections should be allowed to continue and complete if possible.
  service-unbind {
    depends-on = [before-service-unbind]
  }

  # Wait for requests that are in progress to be completed.
  # This is where you register tasks that will wait for already established connections to complete, potentially
  # also first telling them that it is time to close down.
  service-requests-done {
    depends-on = [service-unbind]
  }

  # Final shutdown of service endpoints.
  # This is where you would add tasks that forcefully kill connections that are still around.
  service-stop {
    depends-on = [service-requests-done]
  }

  # Phase for custom application tasks that are to be run
  # after service shutdown and before cluster shutdown.
  before-cluster-shutdown {
    depends-on = [service-stop]
  }

  # Graceful shutdown of the Cluster Sharding regions.
  # This phase is not meant for users to add tasks to.
  cluster-sharding-shutdown-region {
    timeout = 10 s
    depends-on = [before-cluster-shutdown]
  }

  # Emit the leave command for the node that is shutting down.
  # This phase is not meant for users to add tasks to.
  cluster-leave {
    depends-on = [cluster-sharding-shutdown-region]
  }

  # Shutdown cluster singletons
  # This is done as late as possible to allow the shard region shutdown triggered in
  # the "cluster-sharding-shutdown-region" phase to complete before the shard coordinator is shut down.
  # This phase is not meant for users to add tasks to.
  cluster-exiting {
    timeout = 10 s
    depends-on = [cluster-leave]
  }

  # Wait until exiting has been completed
  # This phase is not meant for users to add tasks to.
  cluster-exiting-done {
    depends-on = [cluster-exiting]
  }

  # Shutdown the cluster extension
  # This phase is not meant for users to add tasks to.
  cluster-shutdown {
    depends-on = [cluster-exiting-done]
  }

  # Phase for custom application tasks that are to be run
  # after cluster shutdown and before ActorSystem termination.
  before-actor-system-terminate {
    depends-on = [cluster-shutdown]
  }

  # Last phase. See terminate-actor-system and exit-jvm above.
  # Don't add phases that depends on this phase because the
  # dispatcher and scheduler of the ActorSystem have been shutdown.
  # This phase is not meant for users to add tasks to.
  actor-system-terminate {
    timeout = 10 s
    depends-on = [before-actor-system-terminate]
  }
}

CoordinatedShutdown.get(system)
    .addTask(
        CoordinatedShutdown.PhaseBeforeServiceUnbind(),
        "someTaskName",
        () -> {
          return akka.pattern.Patterns.ask(someActor, "stop", Duration.ofSeconds(5))
              .thenApply(reply -> Done.getInstance());
        });

CompletionStage<Done> done =
    CoordinatedShutdown.get(system).runAll(CoordinatedShutdown.unknownReason());

akka.coordinated-shutdown.exit-jvm = on

akka.coordinated-shutdown.run-by-jvm-shutdown-hook=off

CoordinatedShutdown.get(system)
    .addJvmShutdownHook(() -> System.out.println("custom JVM shutdown hook..."));

# Don't terminate ActorSystem via CoordinatedShutdown in tests
akka.coordinated-shutdown.terminate-actor-system = off
akka.coordinated-shutdown.run-by-jvm-shutdown-hook = off
akka.cluster.run-coordinated-shutdown-when-down = off

static class HotSwapActor extends AbstractActor {
  private AbstractActor.Receive angry;
  private AbstractActor.Receive happy;

  public HotSwapActor() {
    angry =
        receiveBuilder()
            .matchEquals(
                "foo",
                s -> {
                  getSender().tell("I am already angry?", getSelf());
                })
            .matchEquals(
                "bar",
                s -> {
                  getContext().become(happy);
                })
            .build();

    happy =
        receiveBuilder()
            .matchEquals(
                "bar",
                s -> {
                  getSender().tell("I am already happy :-)", getSelf());
                })
            .matchEquals(
                "foo",
                s -> {
                  getContext().become(angry);
                })
            .build();
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("foo", s -> getContext().become(angry))
        .matchEquals("bar", s -> getContext().become(happy))
        .build();
  }
}

static class Swapper extends AbstractLoggingActor {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals(
            Swap,
            s -> {
              log().info("Hi");
              getContext()
                  .become(
                      receiveBuilder()
                          .matchEquals(
                              Swap,
                              x -> {
                                log().info("Ho");
                                getContext()
                                    .unbecome(); // resets the latest 'become' (just for fun)
                              })
                          .build(),
                      false); // push on top instead of replace
            })
        .build();
  }
}

static class SwapperApp {
  public static void main(String[] args) {
    ActorSystem system = ActorSystem.create("SwapperSystem");
    ActorRef swapper = system.actorOf(Props.create(Swapper.class), "swapper");
    swapper.tell(Swap, ActorRef.noSender()); // logs Hi
    swapper.tell(Swap, ActorRef.noSender()); // logs Ho
    swapper.tell(Swap, ActorRef.noSender()); // logs Hi
    swapper.tell(Swap, ActorRef.noSender()); // logs Ho
    swapper.tell(Swap, ActorRef.noSender()); // logs Hi
    swapper.tell(Swap, ActorRef.noSender()); // logs Ho
    system.terminate();
  }
}

static class ActorWithProtocol extends AbstractActorWithStash {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals(
            "open",
            s -> {
              getContext()
                  .become(
                      receiveBuilder()
                          .matchEquals(
                              "write",
                              ws -> {
                                /* do writing */
                              })
                          .matchEquals(
                              "close",
                              cs -> {
                                unstashAll();
                                getContext().unbecome();
                              })
                          .matchAny(msg -> stash())
                          .build(),
                      false);
            })
        .matchAny(msg -> stash())
        .build();
  }
}

@Override
public void preStart() {
  // Initialize children here
}

// Overriding postRestart to disable the call to preStart()
// after restarts
@Override
public void postRestart(Throwable reason) {}

// The default implementation of preRestart() stops all the children
// of the actor. To opt-out from stopping the children, we
// have to override preRestart()
@Override
public void preRestart(Throwable reason, Optional<Object> message) throws Exception {
  // Keep the call to postStop(), but no stopping of children
  postStop();
}

@Override
public Receive createReceive() {
  return receiveBuilder()
      .matchEquals(
          "init",
          m1 -> {
            initializeMe = "Up and running";
            getContext()
                .become(
                    receiveBuilder()
                        .matchEquals(
                            "U OK?",
                            m2 -> {
                              getSender().tell(initializeMe, getSelf());
                            })
                        .build());
          })
      .build();
}