[neonsunset]

This FAQ is a bit outdated in places, and is not something most users should worry about in practice.

JVM Green Threads here serve predominantly back-end scenarios, where most of the items on the list are not of concern. This list also exists to address bad habits that carried over from before the tasks were introduced, many years ago.

In general, the perceived want of green threads is in part caused by misunderstanding of that one bad article about function coloring. And that one bad article about function coloring also does not talk about the way you do async in C#.

Async/await in C# in back-end is a very easy to work with model with explicit understanding where a method returns an operation that promises to complete in the future or not, and composing tasks[0] for easy (massive) concurrency is significantly more idiomatic than doing so with green threads or completable futures that existed in Java before these. And as evidenced by adoption of green threads by large scale Java projects, turns out the failure modes share similarities except green threads end up violating way more expectations and the code author may not have any indication or explicit mechanism to address this, like using AsyncLocal.

Also one change to look for is "Runtime Handled Tasks" project in .NET that will replace Roslyn-generated state machine code with runtime-provided suspension mechanism which will only ever suspend at true suspension points where task's execution actually yields asynchronously. So far numbers show at least 5x decrease in overhead, which is massive and will bring performance of computation heavy async paths in line with sync ones:

https://github.com/dotnet/runtimelab/blob/feature/async2-exp...

Note that you were trivially able to have millions of scheduled tasks even before that as they are very lightweight.

[0]: e.g. sending requests in parallel is just this

    using var http = new HttpClient() {
        BaseAddress = new("https://news.ycombinator.com/news")
    };

    var requests = Enumerable
        .Range(1, 4)
        .Select(n => $"?p={n}")
        .Select(http.GetStringAsync);

    var pages = await Task.WhenAll(requests);

[no_wizard]

I take your point about the aforementioned article[0][1] being a popular reference when discussing async / await (and to a lesser extent, async programming in modern languages more generally) I think its popularity is highlighting the fact that it is a pain point for folks.

Take for instance Go. It is well liked in part, because its so easy to do concurrency with goroutines, and they're easy to reason about, easy to call, easy to write, and for how much heavy weight they're lifting, relatively simple to understand.

The reason Java is getting alot of kudos here for their implementation of green threads is exactly the same reason people talk about Go being an easy language to use for concurrency: It doesn't gate code behind specialized idioms / syntax / features that are only specific to asynchronous work. Rather, it largely utilizes the same idioms / syntax as synchronous code, and therefore is easier to reason about, adopt, and ultimately I think history is starting to show, to use.

Java is taking an approach paved by Go, and ultimately I think its the right choice, because having worked extensively with C# and other languages that use async / await, there are simply less footguns for the average developer to hit when you reduce the surface area of having to understand async / sync boundaries.

[0]: https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...

[1]: HN discussion: https://news.ycombinator.com/item?id=8984648

[neonsunset]

Green Threads increase the footgun count as methods which return tasks are rather explicit about their nature. The domain of async/await is well-studied, and enables crucial patterns that, like in my previous example, Green Threads do nothing to improve the UX of in any way. This also applies to Go approach which expects you to use Channels, which have their own plethora of footguns, even for things trivially solved by firing off a couple of tasks and awaiting their result. In Go, you are also expected to use explicit synchronization primitives for trivial concurrent code that require no cognitive effort in C# whatsoever. C# does have channels that work well, but turns out you rarely need them when you can just write simple task-based code instead.

I'm tired of this, that one article is bad, and incorrect, and promotes straight-up harmful intuition and probably sets the industry in terms of concurrent and asynchronous programming back by 10 years in the same way misinterpreting Donald Knuth's quote did in terms of performance.

[kaba0]

That’s a very simplistic view. Especially that java does/will provide “structured concurrency” as something analogous to structured control flow, vs gotos.

Also, nothing prevents you from building your own, more limited but safer (the two always come together!) abstraction on top, but you couldn’t express Loom on async as the primitive.

[ffsm8]

I don't think that this would be a good showcase for Virtual Threads. The "async" API for Java is CompletableFutures, right? thats been stable for something like 10 years, so no real change since Java 8.

You'd jsut have to define a ThreadPool with n Threads before, where each request would've blocked one pending thread. Now it just keeps going.

So your equivalent Java example should've been something like this, but again: the completeable futures api is pretty old at this point.

    @HttpExchange(value = "https://news.ycombinator.com")
    interface HnClient {
        @GetExchange("news?p={page}")
        CompletableFuture<String> getNews(@PathVariable("page") Integer page);
    }

    @RequiredArgsConstructor
    @Service
    class HnService {
        private final HnClient hnClient;
        List<String> getNews() {
            var requests = IntStream.rangeClosed(1, 4)
                                    .boxed().map(hnClient::getNews).toList();
            return requests.stream().map(CompletableFuture::join).toList();
        }
    }

[vips7L]

Structured concurrency is still being developed: https://openjdk.org/jeps/453

Also, I wouldnt consider that the equivalent Java code. That is all Spring and Lombok magic. Just write the code and just use java.net.HttpClient.

[ffsm8]

> and just use java.net.HttpClient.

No.

[no_wizard]

it might be obvious to others, but why the 'No'?

[vips7L]

The standard http client doesn’t have as great of UX as other community libs. Most of us (including me) don’t like to use it.

That being said, imo you can’t call something equivalent when doing a bunch of spring magic. This disregards that OPs logic isn’t equivalent at all. It waits for each future 1 by 1 instead of doing something like CompletableFuture.allOf or in JS: Promise.all.