LRU byte Cache java

I need to implement a cache in java with a maximum size, would like to do it using the real size of the cache in the memory and not the number of elements in the cache. This cache will basically have String as key and String as value. I have already implemented the cache using the LinkedHashMap structure of java but the question is how开发者_C百科 to know the actual size of the cache so that i can adapt the policy to drop an object when the size is too big.

Wanted to compute it using the getObjectSize() of the instrumentation package but it seems not working as desired.

When I do getObjectSize( a string ) whatever the size of the string is, it returns the same size : 32. I guess it's just using the reference size of the string or something like that and not the content. So don't know how to solve this problem efficiently.

Do you have any ideas ?

Thanks a lot!

You might want to consider using Ehcache with memory based cache sizing.

If your keys and values are both strings, then the calculation is easy: object overhead + 2 bytes per character in the strings. On a 32-bit Sun JVM, 32 bytes for overhead sounds correct.

There are a couple of caveats: first, the Map that you use to hold the cache adds its own overhead. This will depend on the size of the hash table and the number of entries in the map. Personally, I'd just ignore all overheads and base the calculation on the string lengths.

Second, unless you track strings by identity, you may over-count because the same string may be stored with multiple keys. Since tracking strings by identity would add yet more overhead, this is probably not worth doing.

And finally: while memory-limited caches seem like a good idea, they rarely are. If you know your application well enough, you should know the average string length, and can control the cache based on number of entries. And if you don't know your application that well, a simple LRU expiration policy is likely to get you into trouble: a large entry can cause many small entries to be expired. And if that happens, unless the cost to rebuild is proportional to the size, you've just made your cache less effective.