2.5.3.3 Simplicity and robustness

Write finalizers that are simple and robust. They might be called in any context, including within other threads; with careful design, your finalizers will work in most or all possible situations.

A finalizer might be called on the same object more than once. This could occur if the object was registered for finalization more than once, or if your application registered the object for finalization and also called finalize on it directly. To account for this, write finalizers that are idempotent: that is, the effect of multiple calls is the same as the effect of a single call. See Section 2.5.2.6 for more on the effects of multiple registrations.

Remember that the order in which the finalization queue is processed is not defined. Finalizers cannot make assumptions about ordering.

This is particularly important to note when writing finalizers for classes that are typically used to form circular or otherwise interestingly connected graphs of objects. If guarantees about finalization in graphs of objects are important, we suggest registering a root object for finalization and making its finalizer traverse the graph (in some graph-specific well-ordered fashion) and call the finalize method for each object in the graph requiring finalization.

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Common Dylan and Functional Extensions - 31 Mar 00