Object finalization and cleanup: How to design classes for proper object cleanup

“…By now you may be getting the feeling that you don’t have much use for finalizers. While it is likely that most of the classes you design won’t include a finalizer, there are some reasons to use finalizers.

One reasonable, though rare, application for a finalizer is to free memory allocated by native methods. If an object invokes a native method that allocates memory (perhaps a C function that calls malloc()), that object’s finalizer could invoke a native method that frees that memory (calls free()). In this situation, you would be using the finalizer to free up memory allocated on behalf of an object — memory that will not be automatically reclaimed by the garbage collector.

Another, more common, use of finalizers is to provide a fallback mechanism for releasing non-memory finite resources such as file handles or sockets. As mentioned previously, you shouldn’t rely on finalizers for releasing finite non-memory resources. Instead, you should provide a method that will release the resource. But you may also wish to include a finalizer that checks to make sure the resource has already been released, and if it hasn’t, that goes ahead and releases it. Such a finalizer guards against (and hopefully will not encourage) sloppy use of your class. If a client programmer forgets to invoke the method you provided to release the resource, the finalizer will release the resource if the object is ever garbage collected.”