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

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}}.