IBM User Guide for Java V7 on Windows > IBM SDK for Java > The collector and the allocator > How to coexist with the Garbage Collector > Finalizers

Nature of finalizers

The JVM specification does not describe finalizers, except to state that they are final in nature. It does not state when, how, or whether a finalizer is run. Final, in terms of a finalizer, means that the object is known not to be in use any more.

The object is definitely not in use only when it is not reachable. Only the Garbage Collector (GC) can determine that an object is not reachable. Therefore, when the GC runs, it determines which are the unreachable objects that have a finalizer method attached. Normally, such objects are collected, and the GC can satisfy the memory allocation fault. Finalized garbage must have its finalizer run before it can be collected, so no finalized garbage can be collected in the cycle that finds it. Therefore, finalizers make a garbage collection cycle longer (the cycle has to detect and process the objects) and less productive. Finalizers use more of the processor and resources in addition to regular garbage collection. Because garbage collection is a stop-the-world operation, it is sensible to reduce the processor and resource usage as much as possible.

The GC cannot run finalizers itself when it finds them, because a finalizer might run an operation that takes a long time. The GC cannot risk locking out the application while this operation is running. Therefore, finalizers must be collected into a separate thread for processing. This task adds more processor usage into the garbage collection cycle.

Parent: Finalizers

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