javactor

Java actor API, with an implementation for Akka.

This began as a library to make using Java Akka easier. Through its evolution, though, it became clear that clients of the API didn't need to have a dependency on Akka at all; their needs could be satisfied with an actor API generic to any actor system provider. This is with the rather considerable caveat that many of the features Javactor exposes are provided by and modeled after Akka. In theory, though, any actor system providing the same features could supplant Akka in an application written to Javactor, without the need to rewrite any of the actors.

The principal value of Javactor at this time, though, is to make Java Akka easier. Here's how:

• Actors are POJOs, and don't need to inherit from UntypedActor
• Handle messages simply by implementing a method annotated with @Handle
• Make supervisor decisions simply by implementing a method annotated with @OnException
• Annotations for all the standard actor life cycle events, e.g., @PostStart
• Builder DSL for sending messages, e.g., ctx.msg(new MyMessage()).to(someOtherActor).sender(replyTo).fireAndForget()
• Safety for message sends that should receive a response, e.g., ctx.msg(xxx).to(someActor).request(ReplyMsg.class). Javactor will ensure that the sender has a corresponding handle method for the reply message type. It will also ensure the sender handles a timeout message, which is sent if the reply does not arrive within a default or specified timeout.
• Builder DSL for scheduling messages, e.g., ctx.schedule(new MyMessage()).to(ctx.self()).delay(1, TimeUnit.SECONDS).period(2, TimeUnit.MINUTES).go()

Performance

Javactor uses reflection to invoke handler methods, which entails a performance penalty. Also, there is some memory overhead on each actor; I have tried to keep this to a minimum. The degree of these effects is not yet known, as I have yet to measure them. This will not be a concern, of course, for applications using the actor pattern for purposes of concurrency correctness and resilience, rather than performance.

Creating javactors

Create javactor class:

public class MyActor
{
  JavactorContext ctx;//Will be set by the Javactor implementation before each interaction with the Javactor
  
  @Handle
  public void handle(SomeMessage msg) {
    // Handle message here
  }
}

Note that it is not a subclass of anything.

Create Akka actor based on this Javactor:

ActorSystem system = ActorSystem.create();
system.actorOf(Props.create(new MyActorCreator(new MyActor())), "myactor");

Where MyActorCreator is:

	@RequiredArgsConstructor//using project lombok for brevity
	private static final class MyActorCreator implements
		Creator<JavactorUntypedActor>
	{
		private final Object myActor;
		@Override
		public JavactorUntypedActor create() throws Exception
		{
			return AkkaJavactorBuilder.builder(myActor).build();
		}
	}

This might seem ownerous, but thereafter, all Javactors can create child actors like so:

Object javactor = ctx.actorBuilder(SomeOtherJavactor.class, "otheractor").build();

Note that the result of this method is an Object. A more specific type is unnecessary, as when using Javactors, methods are never called on actor references. (This design decision has not yet been finalized. I understand that an API should provide meaningful types, and may introduce a Javactor actor reference type.)

Need an actor to have some specific initialization state? Just specify a JavactorPreparer:

child = ctx.actorBuilder(ChildActor.class, "child")
	.preparer(new JavactorPreparer<ChildActor>()
	{
		@Override
		public void prepare(ChildActor javactor)
		{
			javactor.setSomeState(someState);
		}
	})
	.build();

The Javactor Context

Every javactor uses a JavactorContext to interact with the Javactor system. This is obtained by creating a field in your javactor of the corresponding type. This field is set before any interaction with the javactor, e.g., handle methods, exception handler methods, actor lifecycle handler methods, etc.

Be carful not to close over the ctx instance in callback methods, etc., as its value may change by the time the callback is invoked. This is the same gotcha as with regular Akka programming and closing over methods like UntypedActor.sender(). I considered passing the JavactorContext to javactors via a method parameter, which would obviate this problem, but have hesitated doing so in the interest of ease of use. This is still an open design issue.

Sending messages

Javactor provides a sigle builder DSL for both sending messages to specific actors, and posting to the event stream.

ctx.msg(new MyMsg()).to(someActor).fireAndForget();

The msg(..) call creates the builer. Note the name of the method makes it clear that we don't care if we get a response.

Now, simply omit the specification of the target actor, and the message will be posted to the event stream:

ctx.msg(new MyMsg()).fireAndForget();//off to the event stream

Requiring responses

An actor system guarantees neither that a message will reach its target, nor that the reply will reach the sender. So, an actor that sends a message that requires a reply should always do two things: handle the reply message, and trigger a timeout if the reply does not arrive. Javactor enforces these requirements. To trigger this, send a message like so:

ctx.msg(new MyMsg()).to(destActor).request(ReplyMsg.class, "Requesting reply");

If this is executed by an actor that is missing either the handling of ReplyMsg or TimeoutMsg, an error will result. The second argument to request(..) is set as a property of the timeout message. This is convenient for an error message indicating what the actor was waiting for.

Note that this request mechanism can just as easily be used for messages posted to the event stream:

ctx.msg(new MyMsg()).request(ReplyMsg.class, "Requesting reply");//no to() means post to event stream

Listening to the event stream

Javactors can handle methods on the event stream simply by invoking a config method on the actor builder and implementing the handle method for the desired message type.

Object javactor = ctx.actorBuilder(MyJavactor.class, "myactor").subscribeToEventBus().build();

Then in the javactor:

@Handle
public void handle(SomeEventStreamMessage msg) {
	// handle code here	
}

The Javactor implementation will automatically subscribe this actor to the event stream, and call the handle method when the message of the handled type is posted.