Would like Caffeine instance builder to support an objectServedFromCacheListener

[yschondorf-cr]

When creating a Caffeine bean the builder is used as in the following example:

@bean
public Caffeine caffeineConfig() {
return Caffeine.newBuilder().maximumSize(1000).expireAfterWrite(4, TimeUnit.HOURS);
}
The builder has a removalListener() to attach a listener for removal events. I wish there was such an easy way to attach a listener for other events such as cache hits and misses. My particular use case needed a objectServedFromCacheListener, but I can see this request easily extended to support objectNotInCacheListener and objectSavedInCacheListener.

[ben-manes]

The primary use-case for this is for statistics, which you can enable on the builder. That can be given its own StatsCounter to record with if you want to push them, rather than poll via cache.stats(). This does not capture key/value fields, only the metrics.

Otherwise you can manually intercept via a ThreadLocal that is set within the CacheLoader if invoked. This way the caller can check and clear the flag to determine if a load occurred. If instead you use cache.get(key, mappingFunction), then the lamda can capture a local variable and set the flag before returning the value. Either approach lets you know if the loading function was used, and react accordingly