strategy-registry-pattern

Define a Strategy interface with a canHandle discriminator method, implement concrete strategies as @Component beans, and inject List<Strategy> into a Registry that filters by canHandle. Spring auto-discovers all implementations, so new strategies require no changes to existing code.

Use a Resolver component that extracts a discriminator from the request, such as a URI pattern, caller type, or query parameter. The Registry then routes that discriminator to the matching strategy via its canHandle method, returning null when nothing matches.

Use a Filter extending OncePerRequestFilter for request-scoped concerns like metrics or logging that apply to every request. Use a Handler invoked explicitly in business logic for synchronous operations like permission checks or entity-to-DTO mapping.

Avoid it when only one implementation exists, when a simple two or three case if/else suffices, or when compile-time selection via Spring @Conditional beans is enough. It also fits poorly when strategies share tightly coupled state.

Parsers return non-blank sentinels like UNKNOWN or DEFAULT so downstream tag building and metrics recording never break on extraction failures. Exceptions are caught, logged with log.error, and converted to the sentinel value.