Custom Rank Indicator Annotations

[buko]

A very common requirement is to be able to group services of the same contract into "buckets" that will determine how they are used. To facilitate this hk2 provides the @Rank annotation that allows an explicit integer-based ordering of services. The use of integers allows for arbitrary ordering but it can also be error-prone and isn't really type-safe. It would be nice-to-have the ability to specify custom annotations that could indicate a numerical rank.

This might work like the following:

@RankIndicator
@Rank(30_000)
@Documented
@Retention(RUNTIME)
public @interface HighPriority {}

@RankIndicator
@Rank(20_000)
@Documented
@Retention(RUNTIME)
public @interface NormalPriority {}


@RankIndicator
@Rank(10_000)
@Documented
@Retention(RUNTIME)
public @interface LowPriority {}

This would define certain annotations as being Rank indicators. Services annotated with these Rank indicators would be given the Rank specified on the Rank indicator annotation eg:

@HighPriority
final class ConsoleNotifier implements Notifier { ... }

The service ConsoleLogger would have the rank of 30_000 and would only be used for sending high-priority notifications.