[cdi-dev] [JBoss JIRA] (CDI-563) Event.fireAsync() - clarify the usage of the returned CompletionStage

[Martin Kouba (JIRA)]

Martin Kouba created CDI-563:
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             Summary: Event.fireAsync() - clarify the usage of the returned CompletionStage
                 Key: CDI-563
                 URL: https://issues.jboss.org/browse/CDI-563
             Project: CDI Specification Issues
          Issue Type: Clarification
          Components: Events
    Affects Versions: 2.0-EDR1
            Reporter: Martin Kouba


So far {{CompletionStage}} is only mentioned in "10.5.1. Handling multiple exceptions thrown during an asynchronous event" and the {{Event.fireAsync()}} javadoc is too general. However, the {{CompletionStage}} itself does not define an unambiguous contract for its methods. E.g. what thread is used to execute a given callback? Or what's the _"stage's default asynchronous execution facility"_? I believe this is left on implementors. In Weld 3.0 Alpha we're using {{CompletableFuture}} under the hood, and its more concrete in this area, e.g.:
{quote}
* Actions supplied for dependent completions of _non-async_ methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.
{quote}
So as a result, if an async delivery is finished before a sync dependent action is registered, the callback is executed in the caller thread:
{code:java}
event.fireAsync(new Message()).thenAccept((m) -> System.out.println("This might be executed in a caller thread or in a different thread!"));
{code}
And this might be confusing. Especially from the context propagation point of view. I think the spec should clarify the contract of a returned {{CompletionStage}}.



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