Implementing caches that may be cleared if heap space shrinks away

Hi all,

I have a client that queries a server via RMI and uses some caches for
the results to speed up things. Unfortunatily, these caches can grow
till heap space is completely filled up and I get an exception.

So now my cache classes don't use Map<S,T> anymore but
SoftReference<Map<S,T>>. But with that approach I always have to check
if the soft reference has already been cleared before accessing the
referenced Map, i.e. in each of my cache classes I have something like
this:

--8<---------------cut here---------------start------------->8---
    private void initializeCache() {
        words = new SoftReference<HashMap<String, WordInfo>>(
                new HashMap<String, WordInfo>());
    }

    WordInfo getWordInfo(String word) throws RemoteException, JGWNLException {
        if (words.get() == null) {
            initializeCache();
        } else if (words.get().containsKey(word)) {
            return words.get().get(word);
        }
        WordInfo wi = jgwnl.lookupWord(word);
        for (SynsetInfo si : wi.getSynsetInfos()) {
            synsetCache.updateSynsetInfo(si.getUid(), si);
        }
        words.get().put(word, wi);
        return wi;
    }
--8<---------------cut here---------------end--------------->8---

And there may be the possibility that after the check the soft ref is
cleared before I access it...

One poor-man's alternative would be to poll Runtime.freeMemory() in
another Thread and clear the caches when it falls beyond a certain
limit.

The other alternative I've thought about is deriving the cache classes
directly from SoftReference and overriding the clear() method. But its
docs state that the garbage collector won't call clear() but delete the
referent directly. So that's no solution, too.

Basically, what I want is an interface Clearable which defines a clear()
method that's called automatically if heap space shrinks away, but I
couldn't find anything like that.

So what's the right way to implement a cache in Java?

Thanks for any pointers,
Tassilo
--
A morning without coffee is like something without something else.