[Caffeine auto-config] Allow CacheLoader with specific generic types

[shakuzen]

I came across this while trying out the new Caffeine caching auto-configuration. After reading the Spring Boot documentation on Caffeine caching, I was surprised that my provided CacheLoader<String, List<String>> bean was not being used by the auto-configuration. If this is the intended behavior, then this pull request can be closed, but I think the documentation could be a bit more clear.

The UT added here fails without the main code changes. Similar to the UT, I wanted to use the Caffeine auto-configuration but provide a CacheLoader to be used, like the following in a @Configuration class:

@Bean
public CacheLoader<String, List<String>> cacheLoader(SlowService service) {
    return service::getNames;
}

[philwebb]

Seems like we should support different generics. I wonder if CacheLoader<?, ?> would work rather than dropping the generics all together?

[ben-manes]

Can we apply any fixes here to Guava's as well? I would expect similar problems.

[philwebb]

@ben-manes Thanks for the prompt, I've opened #6778 to tackle that.

[snicoll]

For the record, yes this is the intended behaviour. The setter in the related CacheManager is public void setCacheLoader(CacheLoader<Object, Object> cacheLoader) to clearly indicate that we won't support typed caches at that level.

CaffeineCache is using internally a Cache<Object, Object> (see the return type of getNativeCache). I think it's confusing to have a CacheLoader<String, List<String>> in the context and expect Spring Boot to pick that up and associate it with the cache manager. The reason for that is that this CacheLoader is used as a loader for all caches that are managed by that instance. For the reason I explained above, caches in spring land are untyped so the only way for this to make sense is if you would created the cache infrastructure yourself, and not via auto-config.

I am -1 to apply this change without a corresponding change in Spring Framework. The fact you dropped the generic type is a smell.

[snicoll]

As we've discussed on gitter I've improved the documentation to make it clear that only CacheLoader<Object, Object> is taken care by the auto-configuration.