Reinstate RedisCacheConfiguration.getTtl()

[jxblum]

See #2628

[christophstrobl]

The key parameter of TtlFunction#getTimeToLive does not seem to be nullable. Also why cast to FixedDurationTtlFunction?

[jxblum]

Why a null key?

Because this is what the old, mistakingly removed, now deprecated (once again) getTtl() method did with the introduction of dynamic TTL cache configuration using the new RedisCacheWriter.TtlFunction interface.

See original Issue tickets and Pull Requests referenced in Issue #2628.

[jxblum]

Why cast to FixedDurationTtlFunction?

Because it is still possible to configure a "fixed" Duration TTL cache entry expiration policy using the overloaded entryTtl(:Duration) (source) builder method in RedisCacheConfiguration.

NOTE: entryTtl(:Duration) (fixed) is an alternative to entryTtl(:TtlFunction) (dynamic; here), hence the overload. Internally, a "fixed" Duration is still represented with a TtlFunction.

From this line, it will then create a (non-dynamically computed) FixedDurationTtlFunction instance (here).

[christophstrobl]

FixedDurationTtlFunction#getTimeToLive would return the duration nevertheless, so really I do not see the need for a cast here.

[jxblum]

Outside of a potential JVM runtime optimization here, it is true, there probably is no a need for the instance of check and implicit cast to a FixedDurationTtlFunction and direct call to the duration() method.

Feel free to change it in the polishing commit if you like. Otherwise, let me know, and I can take care of it.

[jxblum]

Merged in commits fe88d31 & d25de89.