var f = async_api_returns_future();
...
var res = f.join();
but join() won't block OS/JVM thread, but make it to perform other tasks in the queue.
Or you can design API which will receive executorService as params, and run callback there, e.g.:
async_call(Callable callback, ExecutorService threadPool);
BTW, as for fork-join's `join`, not only is it designed for pure computation only and cannot help with IO, but every `join` increases the depth of the stack, so it is fundamentally limited in how much it can help. FJ is designed for pure computation workloads, so in practice that's not a huge limitation, but virtual threads are designed for IO workloads.
I apologise for not going into more depth here, but as you can imagine, with a user base numbering in the many millions, we can only afford to put in depth explanations in texts and videos that gain a large audience, but once you've familiarised yourself with the material I'll gladly answer specific questions (and perhaps your questions will help us improve our material, too).
my concern is that you somehow can find time to write long comments with lots of handwavings (our framework is designed for that, and their framework is not designed for that), but refuse to provide specific code pointers and example in support of your opinion. For example, in this specific case, can you give example how green threads can be used with current Java IO library, or Java JDBC library?
If there's something unclear in that material, please ask. Also, there is no our framework vs. their framework here. I'm only discussing the JDK's own various thread pools vs. virtual threads. They were all designed and implemented by the JDK team.
BTW, I'm not trying to support any opinion. Pretty much all popular Java server frameworks are adopting virtual threads, and Java's virtual threads are poised to become the most popular lightweight user mode threads. We've already convinced everyone that needed convincing. I'm merely offering pointers in case you're interested to learn how to use virtual threads and understand how they offer high throughput and good observability at the same time (whereas before you could have one or the other). Of course, if you're satisfied with the throughput and observability you can get with our old mechanisms, you don't have to use virtual threads. We've not taken anything away.
