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The Observer Pattern Revisited

Software design patterns describe elegant solutions to commonly encountered software design problems. The observer pattern is a behavioural object-oriented design pattern first codified by a group of IBM researchers (Gamma, 1995). Observers become listeners by registering their interest with observables that notify all registered listeners when an update is required. Java implementations of the observer pattern may create undesirable run-time behaviour characterised by observers performing multiple updates, cyclic dependencies between objects, and dangling references that inhibit garbage collection. This paper examines the pitfalls and design tradeoffs encountered when implementing the observer pattern, and considers the deficiencies inherent in the native Java support for the pattern. A class that manages the lifecycle of observer and observable instances is introduced. This class recursively updates registered observers while detecting cycles and ensuring that memory leaks do not occur.