Lapsed listener problem

The lapsed listener problem is a common source of memory leaks for object-oriented languages, among the most common ones for garbage collected languages.^[1]^[2]

It originates in the observer pattern, where observers (or listeners) register with a subject (or publisher) to receive events. In basic implementation, this requires both explicit registration and explicit deregistration, as in the dispose pattern, because the publisher holds strong references to the observers, keeping them alive. The leak happens when a listener fails to unsubscribe from the publisher when it no longer needs to listen. Consequently, the publisher still holds a reference to the observer which prevents it from being garbage collected — including all other objects it is referring to — for as long as the publisher is alive, which could be until the end of the application.

This causes not only a memory leak, but also a performance degradation with an "uninterested" observer receiving and acting on unwanted events. This can be prevented by the subject holding weak references to the observers, allowing them to be garbage collected as normal without needing to be unregistered.

References

↑ What is the lapsed listener problem?, Andy McMullan
↑ Memory Loiterers in Java, Ethan Henry and Ed Lycklama

Memory management

Memory management as a function of an operating system

Manual memory management	Static memory allocation C dynamic memory allocation new and delete (C++)

Virtual memory	Demand paging Page table Paging Virtual memory compression

Hardware	Memory management unit Translation lookaside buffer

Garbage collection	Boehm garbage collector Finalizer Garbage Mark-compact algorithm Reference counting Strong reference Weak reference

Memory segmentation	Protected mode Real mode Virtual 8086 mode x86 memory segmentation

Memory safety	Buffer overflow Buffer over-read Dangling pointer Stack overflow

Issues	Fragmentation Memory leak Unreachable memory

Other	Automatic variable International Symposium on Memory Management Region-based memory management

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