How to cache null values in caffeine cache

Question

I'm using Caffeine cache to store data received from an external system.

LoadingCache<String, String> clientCache = Caffeine.newBuilder().expireAfterWrite(1, TimeUnit.MINUTES).build(id -> {
  System.out.println("Generating new value for " + id);
  if (id.equalsIgnoreCase("1")) {
    return null;
  }
  return new Date().toString();
});

System.out.println(clientCache.get("1"));
System.out.println(clientCache.get("1"));
System.out.println(clientCache.get("2"));
System.out.println(clientCache.get("3"));

This resulted,

Generating new value for 1
null
Generating new value for 1
null
Generating new value for 2
Wed May 20 17:11:01 IST 2020
Generating new value for 3
Wed May 20 17:11:01 IST 2020
Wed May 20 17:11:01 IST 2020

Caffeine is not saving null values in cache. How to store nulls in Caffeine?

Borderline

I don't know Caffeine but I'd guess that it can't distinguish between null meaning "I tried but got null" and "I didn't try yet". Maybe you could use Optional for the values instead, i.e. instead of returning null you return Optional.empty(). Of course you'd then have to deal with the optionals then but that might improve your code or at least would be easy to adapt to the current logic with a orElse(null). – Miksen

@Miksen good observation. Just like Map.computeIfAbsent a null return value indicates that it cannot be stored, so a subsequent call will try to recompute. – Meissner

Answer 1

From this issue report:

The cache does not allow storing null values. If you perform a computation and return null then this indicates the data doesn't exist and the caller receives null.

In addition, it advises:

Optional is a good technique for negative caching.

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