import context.dispatcher nullpointerexception

[Jeff]

I have noticed an issue where if a future maps/flatmaps after actor shutdown, a NullPointerException is thrown. I've narrowed it down to the import context.dispatcher, which I technically understand since that is closing over actor state. If I change that to implicit val ec = context.dispatcher, everything works fine. 

I'd like to understand what is the best practice in this case, since the documentation for context.dispatcher indicates that it is threadsafe and looking at the actor trait, context is a val. Most documentation seems to indicate that import context.dispatcher is preferred. 

Thanks

Jeff

[Viktor Klang]

Context is bound to the lifecycle of the actor.

--
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ: http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
---
You received this message because you are subscribed to the Google Groups "Akka User List" group.
To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+unsubscribe@googlegroups.com.
To post to this group, send email to akka...@googlegroups.com.
Visit this group at https://groups.google.com/group/akka-user.
For more options, visit https://groups.google.com/d/optout.

--

Cheers,

√

[Jeff]

I suspected as much. So what would you suggest as for handling use cases where you could have chained flatmaps on futures that at the end will pipeTo a message back to the actor. Should we set the ExecutionContext to a val? 

On Tuesday, March 6, 2018 at 12:09:22 PM UTC-8, √ wrote:

Context is bound to the lifecycle of the actor.

On Tue, Mar 6, 2018 at 8:37 PM, Jeff <jknigh...@gmail.com> wrote:

I have noticed an issue where if a future maps/flatmaps after actor shutdown, a NullPointerException is thrown. I've narrowed it down to the import context.dispatcher, which I technically understand since that is closing over actor state. If I change that to implicit val ec = context.dispatcher, everything works fine. 

I'd like to understand what is the best practice in this case, since the documentation for context.dispatcher indicates that it is threadsafe and looking at the actor trait, context is a val. Most documentation seems to indicate that import context.dispatcher is preferred. 

Thanks

Jeff

--
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ: http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
---
You received this message because you are subscribed to the Google Groups "Akka User List" group.

To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+...@googlegroups.com.

To post to this group, send email to akka...@googlegroups.com.
Visit this group at https://groups.google.com/group/akka-user.
For more options, visit https://groups.google.com/d/optout.

--

Cheers,

√

[Patrik Nordwall]

It's because when the actor is stopped some of the fields are cleared (yeah, even though they are vals) to "help" the GC in case something (e.g. an local ActorRef) is still referencing the actor instance.

implicit val ec = context.dispatcher

would solve it here.

Where in the documentation is the import recommended? We should probably update that. Would you be able creating a PR fixing it? Thanks.

/Patrik

To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+unsubscribe@googlegroups.com.

To post to this group, send email to akka...@googlegroups.com.
Visit this group at https://groups.google.com/group/akka-user.
For more options, visit https://groups.google.com/d/optout.

--

Patrik Nordwall
Akka Tech Lead
Lightbend -  Reactive apps on the JVM
Twitter: @patriknw

[Jeff]

An example of documentation for using import context.dispatcher is here https://doc.akka.io/docs/akka/2.5/futures.html#within-actors

I can create some PR to update the documentation

Patrik Nordwall
Akka Tech Lead

[Patrik Nordwall]

Thanks!

To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+unsubscribe@googlegroups.com.

To post to this group, send email to akka...@googlegroups.com.
Visit this group at https://groups.google.com/group/akka-user.
For more options, visit https://groups.google.com/d/optout.

[Heiko Seeberger]

`import context.dispatcher` is what „everybody“ is doing, not? I’m using it all over the place, because I have learned that one can import from stable identifiers (e.g. vals) in Scala. Hence I don’t think changing the docs has the necessary effect.

How can a final val be „nulled"? Unsafe? Reflection?

Cheers

Heiko

--

Heiko Seeberger

Home: heikoseeberger.de

Twitter: @hseeberger

Public key: keybase.io/hseeberger

[Konrad “ktoso” Malawski]

Yes it is nulled using unsafe.

-- 

Cheers,

Konrad 'ktoso' Malawski

Akka @ Lightbend

[Jeff]

I agree with Heiko on this one - import context.dispatcher is used everywhere. It is mentioned in the docs not to close over 'context' and access it outside of message handlers, however I think more education is needed here. 

To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+...@googlegroups.com.
To post to this group, send email to akka...@googlegroups.com.
Visit this group at https://groups.google.com/group/akka-user.
For more options, visit https://groups.google.com/d/optout.

[Patrik Nordwall]

For the record, I created issue https://github.com/akka/akka/issues/25040

To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+unsubscribe@googlegroups.com.

To post to this group, send email to akka...@googlegroups.com.
Visit this group at https://groups.google.com/group/akka-user.
For more options, visit https://groups.google.com/d/optout.