[Proposal] Create new standalone method level `@Scheduled`

[hantsy]

In Concurrency 3.1, the new @Schedule annotation that nested in @Asynchronous has some issues in the real world applications.

1. The attributes hour/minute/second etc., are just another duplicated form of the cron.
2. It does not cover the simple fixedRate/interval case.

I hope there is a new annotation that refer to the definition from Spring @Scheduled, cover both simple fixed rate settings and cron expression, also consider EL (via Jakarta Config properties/env) support.

public @interface Scheduled { // a modified versoin of spring @Scheduled
	String cron() default ""; //also should support EL
	ZoneId zone() default UTC;
	long fixedRate() default -1;
	String fixedRateExpression() default ""; // accept EL
	long fixedDelay() default -1;
	String fixedDelayExpression() default ""; // accept EL
	long initialDelay() default -1;
	String initialDelayExpression() default ""; // accept EL
	TimeUnit timeUnit() default TimeUnit.MILLISECONDS;
	String executor() default ""; //scheduledExecutorService name or change it to Qualifier to match the executor to run this scheudule
}

Of course, in theory, it is possible to add the missing attributes to the existing @Schedule.

However, if we add the missing fixedRate related attributes to @Schedule and allow it to use on methods, then we will face another issue: we have to add a executor attribute to @Schedule to set up the executor service used to run the scheduling tasks, which make the @Asychronous looks weird, because @Asynchronous itself includes an executor.

@Schedule(executor=...)
public void aMethod()

@Asynchronous(executor=..., runAt={@Schedule(executor=...)}...)
public void bMethod()

Originally it is discussed here, #624 (comment)

[njr-11]

Added to 3.2 milestone to represent that a new annotation to support fixed rate/fixed delay is being considered.

[hantsy]

According to the original ScheduledExecutorService apidoc(ManagedScheduledExecutorService is derived from this), the rate related settings include 3 mode.

• One shot - only set initialDelay, see: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledExecutorService.html#schedule-java.lang.Runnable-long-java.util.concurrent.TimeUnit-
• Repeatedly running in a fix period after initialed (maybe last execution is not completed in this case) set an initialDelay and fixedRate, see: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledExecutorService.html#scheduleAtFixedRate-java.lang.Runnable-long-long-java.util.concurrent.TimeUnit-
• Repeatedly running in an interval after last execution is done, set initialDelay and fixedDelay https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledExecutorService.html#scheduleWithFixedDelay-java.lang.Runnable-long-long-java.util.concurrent.TimeUnit-

[hantsy]

And for the cron field, also consider supporting the none-standard expression like:

@yearly	
@annually	
@monthly	
@weekly	
@daily	
@hourly	
@reboot	

see: https://crontab.guru/

It is conventional for developers to set up a simple schedule rule with a full cron expression. For example:

// Asia/shanghai 0:00:00 to run the job per day
@Scheduled(cron="@daily", zone=ZoneOffset.of("+8"))  
void doSomething(){}

This schedule expression is also supported in Github actions etc.