New candidate JEP: 462: Structured Concurrency (Second Preview)

Eric Kolotyluk eric at
Fri Oct 27 16:37:14 UTC 2023

Looks good... thanks.

One slight area of confusion for me was,

|<T> List<Future<T>> executeAll(List<Callable<T>> tasks) throws 
InterruptedException { try (var scope = new 
StructuredTaskScope.ShutdownOnFailure()) { List<? extends 
Supplier<Future<T>>> futures = .map(task -> 
asFuture(task)) .map(scope::fork) .toList(); scope.join(); return; } } static <T> 
Callable<Future<T>> asFuture(Callable<T> task) { return () -> { try { 
return CompletableFuture.completedFuture(; } catch 
(Exception ex) { return CompletableFuture.failedFuture(ex); } }; }|

And what happens if |ShutdownOnSuccess| is called instead. Eventually I 
reasoned that the right thing should happen, but there should only ever 
be one element in the list. Does the scope guarantee only one result?

 1. It would be slightly helpful to point this out in a note so that it
    is more obvious.
 2. What is less obvious is that with |ShutdownOnSuccess| what happens
    if one or more of the siblings throw an exception?
      * I would hope that so long at least one task succeeds, this
        should not cause the overall success of the scope to fail.
      * It would be nice to see this explained more clearly.
      *   Maybe |ShutdownOnSuccess| deserves its own example, discussing
        possible edge cases.

Somehow I am remembering Scala 'for comprehensions' with concurrent 
tasks that 'yield' a result... 😉

The current situation seems kinda clunky with using one stream to 
collect futures, and yet another stream to collect results. Maybe one of 
the Java architects hates such boilerplate, and will come up with an 
elegant way to reduce/remove such boilerplate.

Sincerely, Eric

On 2023-10-27 7:39 a.m., Mark Reinhold wrote:
>    Summary: Simplify concurrent programming by introducing an API for
>    structured concurrency. Structured concurrency treats groups of related
>    tasks running in different threads as a single unit of work, thereby
>    streamlining error handling and cancellation, improving reliability,
>    and enhancing observability. This is a preview API.
> - Mark
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