Java Design Patterns

Define an interface with a getType method, annotate each implementation with @Component, then inject List<Interface> into a factory class. Spring auto-collects all implementations into a Map keyed by type, so new implementations require no factory changes.

Factory creates different objects based on a condition, while Strategy selects different behaviors for the same task at runtime. Both can use Spring's list injection to auto-collect implementations and avoid if/else chains.

Use Decorator for business-level wrapping like adding tax or discounts to a calculation, where explicit control matters. Use Spring AOP for cross-cutting concerns such as logging, security, and auditing that span many components.

No. Spring beans are singletons by default within the ApplicationContext, so manual getInstance implementations are unnecessary. Be careful with mutable fields in singleton beans, since shared state can cause race conditions.

Use @EventListener for side effects that can run immediately within the same transaction. Use @TransactionalEventListener with AFTER_COMMIT when the handler depends on data being successfully persisted, such as assigning roles after user creation.

Avoid patterns when a simpler construct suffices: skip Factory for a single implementation, skip Strategy for one or two fixed cases, and skip Builder for POJOs with one or two fields. Patterns should solve real problems, not follow best practice blindly.