computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...

, the lapsed listener problem is a common source of

memory leak In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that Computer memory, memory which is no longer needed is not released. A memory leak may also happe ...

s for

object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pro ...

programming languages, among the most common ones for

garbage collected Garbage, trash, rubbish, or refuse is waste material that is discarded by humans, usually due to a perceived lack of utility. The term generally does not encompass bodily waste products, purely liquid or gaseous wastes, or toxic waste T ...

languages.Memory Loiterers in Java
Ethan Henry and Ed Lycklama It originates in the

observer pattern In software design and engineering, the observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by ca ...

, where observers (or listeners) register with a subject (or publisher) to receive

events Event may refer to: Gatherings of people * Ceremony, an event of ritual significance, performed on a special occasion * Convention (meeting), a gathering of individuals engaged in some common interest * Event management, the organization of ev ...

. In basic implementation, this requires both explicit registration and explicit deregistration, as in the

dispose pattern In object-oriented programming, the dispose pattern is a design pattern for resource management. In this pattern, a resource is held by an object, and released by calling a conventional method – usually called close, dispose, free, release dep ...

, because the subject holds strong

references Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name'' ...

to the observers, keeping them alive. The leak happens when an observer fails to unsubscribe from the subject when it no longer needs to listen. Consequently, the subject 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 subject 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 reference In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector, unlike a strong reference. An object referenced ''only'' by weak references – meaning "every chain of ref ...

s to the observers, allowing them to be garbage collected as normal without needing to be unregistered.

References

{{Memory management navbox Software bugs Software anomalies