Q:

Java and virtual memory: Controlling/influencing which objects are kept in main memory?

I'm writing a cache server in java that will cache image data (jpgs, pngs, tiff etc) in memory for fast access over http. The images are rendered by another service, which is an expensive operation, so I want to cache them on my cache server. 
There are several reasons why I'm writing it from scratch, so the answer I'm looking for is not [some clever software product]
Question: How can I keep certain a set of data objects in main memory, and ensure that data is actually in main memory when I need it, and not pushed to disk by a virtual memory manager? That is, how can i do this in Java?
Further information: Objects could be referenced with any interval, e.g. days or say years apart to be a bit extreme :-)
EDIT: I have found this SO post which asks "can you keep objects in contiguous memory?" - This is not the question I'm asking, although it could help, if objects were referenced all the time, I presume. And btw, the answer to that question was "no", except obviously for value-types in arrays.

A:

I strongly doubt you can do this in Java alone. You'll probably have to use something like mlock through JNI, as well as the requisite JNI incantations to pin the cached objects graphs in memory so the GC doesn't move them. And [insert miracle here] to compact the pinned memory into contiguous pages because that's what mlock operates on.

