Provide programmatic alternative to `@ConcurrencyLimit`

[odrotbohm]

It would be nice if the features exposed via @ConcurrencyLimit could be used through a template-style API taking a callback.

[parkgeonhu]

Hi, @jhoeller, @odrotbohm.
I'm interested in this issue and would like to help. Please let me know if I can take it

[jhoeller]

After some consideration, I went with a different approach: adding concurrency throttle capabilities to our SyncTaskExecutor, repurposing its role in a Virtual Threads setup. This goes nicely next to the existing concurrency throttle support in SimpleAsyncTaskExecutor and is quite convenient to use, just like a typical template class:

SyncTaskExecutor taskExecutor = new SyncTaskExecutor();
taskExecutor.setConcurrencyLimit(2);
taskExecutor.execute(() -> {...});

There is also a new SyncTaskExecutor-specific overloaded variant that takes a TaskCallback with return value and flexible exception support (which only makes sense for synchronous invocations since on AsyncTaskExecutor, all such return value and exception handling goes through Future handles).

Last but not least, this arrangement leaves room for further concurrency-related features to be added to SyncTaskExecutor, rather than introducing new template classes for each such feature.