Timeouts in structured concurrency

[Eric Kolotyluk]

Upon reviewing the current documentation, I see that I misstated an 
earlier point. StructuredTaskScope.join() returns a result of type R (as 
produced by the configured Joiner), rather than returning null 
unconditionally on success.

That said, the outcome contract remains: join() either returns a value 
or throws. After waiting for completion or cancellation, the scope 
invokes Joiner.result(); if that method throws, join() throws 
FailedException with the underlying exception as the cause (with 
timeouts and cancellation also surfaced as exceptions).

So while Joiners make join() result-producing and more configurable, the 
failure channel is still exception-based. From that perspective, I can 
still see value in a functional, value-oriented result type—where 
success and failure are both represented explicitly as values—coexisting 
alongside exceptions, rather than routing expected failure exclusively 
through throws.

Joiners improve policy flexibility, but they don’t quite address that 
particular concern.

Respectfully,

Eric Kolotyluk


On 2025-12-18 4:46 PM, Eric Kolotyluk wrote:
> Respectfully, I think we’re talking past each other a bit.
>
> Calling Rust’s error handling “horrible” is a subjective judgment 
> about trade-offs, not an objective flaw. Rust’s Result<T, E> is 
> deliberately value-oriented and explicit; Java’s exception model is 
> deliberately stack-oriented and implicit. Each optimizes for different 
> things, and each has real costs.
>
> My appreciation of Java’s evolution is that it has consistently 
> expanded the set of available tools, rather than insisting on a single 
> paradigm. Generics, lambdas, streams, records, sealed types, Optional, 
> and now Loom itself all reflect that trajectory. They didn’t replace 
> older mechanisms; they complemented them.
>
> There has been sustained criticism in the Java community of both null 
> and over-reliance on exceptions, particularly where failure is 
> expected rather than exceptional. I’m not here to relitigate either 
> debate, nor to argue that exceptions should go away. My point is 
> simply that other options exist, and Java has historically been at its 
> best when APIs acknowledge and support them.
>
> In that light, my concern with StructuredTaskScope.join() is not that 
> it uses exceptions at all, but that it offers only an exception-based 
> outcome model, with null representing success. That feels like a 
> missed opportunity in an otherwise forward-looking API.
>
> I’m advocating for additional, not replacement, abstractions—ones that 
> allow structured concurrency outcomes to be expressed explicitly when 
> appropriate, while leaving exceptions fully available for genuinely 
> exceptional conditions.
>
> Respectfully,
> Eric Kolotyluk
>
>
> On 2025-12-18 1:34 PM, Robert Engels wrote:
>> My two cents… Rust’s error handling is horrible - it is designed to 
>> work in functional contexts, so like Java streams - the error 
>> handling feels “random” (and finding out where the error actually 
>> occurred is extremely difficult since it is a value type).
>>
>> Java’s Exceptions are for ‘exceptional conditions’ and should not be 
>> used for flow control (which I don’t think they are in this case - 
>> they signify unexpected error conditions).
>>
>>
>>> On Dec 18, 2025, at 3:24 PM, Eric Kolotyluk <eric at kolotyluk.net> wrote:
>>>
>>> My $0.02
>>>
>>> Why are we still relying so heavily on exceptions as a control-flow 
>>> mechanism?
>>>
>>> Consider the current StructuredTaskScope design:
>>>
>>> The join() method waits for all subtasks to succeed or any subtask 
>>> to fail.
>>> The join() method returns null if all subtasks complete successfully.
>>> It throws StructuredTaskScope.FailedException if any subtask fails, 
>>> with the exception from the first subtask to fail as the cause.
>>>
>>> This design encodes normal outcomes as null and expected failure 
>>> modes as exceptions. That choice forces callers into the least 
>>> informative and least composable error-handling model Java has.
>>>
>>> Returning null for success is especially problematic. null conveys 
>>> no semantic information, cannot carry context, and pushes 
>>> correctness checks to runtime. It remains one of Java’s most 
>>> damaging design decisions, and Loom should not be perpetuating it.
>>>
>>> Optional<T> exists, but it is only a partial solution and does not 
>>> address error information. In this context, even Optional<Void> 
>>> would be an improvement over null, but it still leaves failure 
>>> modeled exclusively as exceptional control flow.
>>>
>>> I also want to be clear that I am not confusing try-with-resources 
>>> with exceptions. StructuredTaskScope being AutoCloseable is the 
>>> right design choice for lifetime management and cancellation, and 
>>> try blocks are the correct mechanism for that. However, scope 
>>> lifetime and outcome reporting are separable concerns. The use of 
>>> try does not require that task outcomes be surfaced exclusively via 
>>> thrown exceptions.
>>>
>>> As a recent Rust convert, the contrast is stark. Rust’s Result<T, E> 
>>> treats failure as a first-class, explicit outcome, enforced by the 
>>> type system. Java doesn’t need to abandon exceptions—but it does 
>>> need to support alternate paradigms where failure is expected, 
>>> structured, and composable.
>>>
>>> APIs like join() should envision a future beyond “success = null, 
>>> failure = throw”. Even a simple structured outcome type—success or 
>>> failure—would be a step forward. Exceptions could remain available 
>>> for truly exceptional conditions, not routine concurrency outcomes.
>>>
>>> Loom is a rare opportunity to modernize not just how Java runs 
>>> concurrent code, but how Java models correctness and failure. 
>>> Re-entrenching null and exception-only outcomes misses that opportunity.
>>>
>>> I’ll stop bloviating now.
>>>
>>> Sincerely,
>>> Eric Kolotyluk
>>>
>>>
>>> On 2025-12-18 1:00 PM, David Alayachew wrote:
>>>
>>>> For 1, the javadoc absolutely does help you. Please read for open.
>>>>
>>>> https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/StructuredTaskScope.html#open()
>>>>
>>>> As for verbose, can you go into more detail? This is a traditional 
>>>> builder pattern addition, so it is literally 1 static method call.
>>>>
>>>> That said, if you dislike a 0 parameter call being forced into 
>>>> being a 2 paramefer call when you need to add timeout, then sure, I 
>>>> think adding an overload for that static method that takes in the 
>>>> configFunction is reasonable. I'd support that.
>>>>
>>>>
>>>> On Thu, Dec 18, 2025, 3:46 PM Holo The Sage Wolf 
>>>> <holo3146 at gmail.com> wrote:
>>>>
>>>>     Hello Loom devs,
>>>>     Few years ago I experimented in a personal PoC project with
>>>>     StructuredConcurrency in Java 19 and I had to stop working on
>>>>     it for personal reasons.
>>>>
>>>>     Recently I came back to the project and updated it to Java 25
>>>>     and had to change my code to the new way the API is built and
>>>>     while doing that I noticed a couple of stuff I want to point out:
>>>>
>>>>     1. The default Joiner method can't receive timeout
>>>>     Obviously that is wrong, but the API and JavaDoc don't actually
>>>>     help you. Say you start with:
>>>>      ```java
>>>>     try (var scope = StructuredTaskScope.open()) {
>>>>         ...
>>>>     }
>>>>     ```
>>>>     And I want to evolve the code to add timeout, I look at
>>>>     the StructuredTaskScope static methods, and won't see any way
>>>>     to do that. After reading a bit
>>>>     what StructuredTaskScope.open(Joiner, configFunction) does, I
>>>>     will realise that I can set the timeout using the configFunction.
>>>>     But then I will encounter the problem that I need to provide a
>>>>     Joiner, currently the only way to actually get the "no args
>>>>     method"-joiner is to look at the source code of the method, see
>>>>     which Joiner it uses and copy that into my method to get:
>>>>      ```java
>>>>     try (var scope =
>>>>     StructuredTaskScope.open(Joiner.awaitAllSuccessfulOrThrow(),
>>>>     (conf) -> ...)) {
>>>>         ...
>>>>     }
>>>>     ```
>>>>     Not only is this a lot of work to do something very simple,
>>>>     there is a high chance that people who start learning
>>>>     concurrency will want to use timeout before they even know what
>>>>     the Joiner object is.
>>>>
>>>>     2. Changing only the timeout is "verbose".
>>>>     I can only talk from my experience, so I may have the wrong
>>>>     impression, but I feel like setting timeout is orders of
>>>>     magnitude more common than changing the default ThreadFactory
>>>>     (especially when using virtual threads) or setting a name.
>>>>     I feel like adding a couple of overloads of the open method
>>>>     that takes only an extra parameter of duration will be convenient:
>>>>     > StructuredTaskScope.open()
>>>>     > StructuredTaskScope.open(Duration timeout)
>>>>     > StructuredTaskScope.open(Joiner joiner)
>>>>     > StructuredTaskScope.open(Joiner joiner, Duration timeout)
>>>>     > StructuredTaskScope.open(Joiner
>>>>     joiner, Function<Configuration, Configuration> configFunction)
>>>>
>>
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