RedisCache should expose ValueWrapper for cached null values returned from retrieve(key)

[jhoeller]

Following up on spring-projects/spring-framework#31637, the core caching infrastructure is able to handle the current RedisCache.retrieve(key) implementation now, making Spring Framework 6.1.1 compatible with the latest Spring Data Redis.

However, there is one remaining concern: RedisCache has support for nullable cache values but currently does not differentiate between a late-determined cache miss - indicated by null as a CompletableFuture value - and null as a cached value, effectively treating a cached null value as a cache miss. You may address this through the retrieve(key) implementation returning a CompletableFuture with a ValueWrapper that holds the nullable value. Since you derive from AbstractValueAdaptingCache anyway, you could simply call toValueWrapper(value) before passing it into the CompletableFuture.

Note that technically just actual null values have to be wrapped in a ValueWrapper while regular values may be exposed directly as a CompletableFuture value. In CaffeineCacheManager, we consistently expose ValueWrapper in case of allowNullValues=true, while we rather expose the regular values in case of allowNullValues=false (where we can expose the Caffeine-provided CompletableFuture directly without post-processing then). The common cache infrastructure can deal with any variant of the cache implementation here, so RedisCache may choose whatever makes sense for it - as long as actual null values are wrapped as a minimum.

[mp911de]

Thanks a lot for bringing this up. It is indeed a gap and aligning with ValueWrapper makes sense.