I feel like this is already indirectly possible with cache systems... yes there is an initial load from DB or whatever, but with Ehcache for example I think(?) all those objects just sit in the JVM, and therefore should be in RAM. If you wrote some app startup batch process to stick every object possible into the cache proactively, I think you'd have essentially what you're asking for.
http://ehcache.org/documentation/2.6/configuration/cache-siz...