Multibinding

Multibinding is a pattern in dependency injection and service registration whereby multiple independent bindings contribute to a single collection-valued dependency. Rather than a single implementation bound to a type, a collection such as a set or a map is built up from several bindings.

In practice, multibinding allows modules or plugins to register their own contributions without depending on a central registry. Frameworks typically provide a binding primitive to accumulate contributions: Guice offers Multibinder and MapBinder to assemble Set<T> or Map<K,V>; Dagger 2 uses @IntoSet and @IntoMap annotations in modules; many Spring-based applications rely on autowiring a collection such as List<T> or Map<String, T> of beans.

Contributors can register their implementation by binding to the target collection, and at runtime the DI container injects the aggregated collection. For example, a plugin system might inject Set<Command> or Map<String, Command> containing all registered commands. This enables extensibility and modular composition.

Common considerations include ordering guarantees, potential duplicates, and lifecycle management. Some frameworks preserve declaration order while others use binding order or semantics that do not guarantee order. Care is needed to avoid conflicts when multiple modules contribute the same key in a map.

Advantages of multibinding include decoupling of modules, simpler extension points, and predictable aggregation of services. Downsides include added complexity in debugging and potential performance costs due to collecting many bindings.