What does `@Cacheable` actually do the first time vs subsequent times a method is called with the same argument?

The first call executes the method body normally and Spring stores its return value in the configured cache (e.g. Redis) keyed by the method's arguments. Subsequent calls with the same argument skip the method body entirely and return the cached value directly — which is why side-effecting methods should never be annotated `@Cacheable`, since the side effect wouldn't happen on cache hits.