Soft/Weak References: rule of thumbs?

Question

After reading <a target="extern" href="http://java.sun.com/developer/technicalArticles/ALT/RefObj/index.html">Reference Objects and Garbage Collection</a> i browsed my code and replaced my listener containers with versions that store WeakReference objects. The main idea is to avoid memory leaks by storing references to objects that are not needed anymore (the listener container is the only strong reference left).
Another rule is to use SoftReferences for caches. But I haven't figured out a good way of implementing it (for caches that fill itself with one single SQL statement).
Are there any other examples where it's good advice to use Soft/Weak References?

Answer 1

For listeners, you have to make sure that the listener object is hard referenced - which is usually the case. After you release the listener object from the main application/object/thread, because it's finished or you don't need it anymore, the listener holder should not hold on to such references to avoid OutOfMemoryExceptions. Of course, the best way is always to unregister a listener, but unfortunately, we're not living in a perfect world. No listener is forced to unregister, they can simple forget to unregister. For this reason, it's better to tell the listener: if you register yourself, you should know that you are just referenced via WeakReferences. If the listener suddenly doesn't get listener events, this is a coding error which can be traced and fixed. But if a listener forgets to unregister itself, the resulting memory bloat is much harder to trace.
SQL table caches are difficult. On one hand you want to cache the data, on the other, you want to free this cache memory (temporarily) for more important memory usage. It's always a trade off between 2 advantages. If performance is not the big issue, but you get OutOfMemoryExceptions, then this is critical for the application to continue working. Better a little slower than not at all ... ;-)