Question

I have run into a situation in which I would like to store an in-memory cache of spatial data which is not immediately needed, and is not loaded from disk, but generated algorithmically. Because the data is accessed spatially, data would be deleted from the cache based on irrelevance factors and the distance from the location of the most recent read operation. The problem is that Java’s garbage collection does not seem to integrate well with this system. I would like to use the spatial knowledge of the data to enable it to be garbage-collected by the JVM. Is there a way to mark these cache objects as garbage-collectible? If the JVM encounters an out-of-memory exception, is there a way to catch that exception and delete the cache objects to free up memory?

Or is this the wrong way to do things?

Answer 1

Is there a way to mark these cache objects as garbage-collectible?

The simplest way is to store

• some data with strong references e.g. in a LinkedHashMap, possible as a LRU cache.
• data which you would like to retain if possible in a SoftReferences cache. These will not be cleaned up immediately but will be cleaned up before an OOME.
• data which can be discarded with little cost in a WeakHashMap. This data is available until the GC is performed.

If the JVM encounters an out-of-memory exception, is there a way to catch that exception and delete the cache objects to free up memory?

You can do this but its not ideal as the error can be thrown anywhere in just about any thread.