spring-boot-dependency-injection

# Spring Boot Dependency Injection ## Overview Provides constructor-first dependency injection patterns for Spring Boot: - mandatory collaborators via constructor injection - optional collaborators via `ObjectProvider` or no-op fallbacks - bean selection via `@Primary` and `@Qualifier` - validation via minimal context tests before full integration ## When to Use Use this skill when: - creating a new `@Service`, `@Component`, `@Repository`, or `@Configuration` class - replacing field injection in legacy Spring code - resolving multiple beans of the same type with qualifiers or primary beans - handling optional features, adapters, or integrations without null-driven wiring - reviewing circular dependencies or brittle context startup failures - preparing code for direct constructor-based unit testing ## Instructions ### 1. Separate mandatory and optional collaborators For each class, identify: - mandatory collaborators required for correct behavior - optional collaborators that enable integrations, caching, notifications, or feature-flagged behavior Mandatory collaborators belong in the constructor. Optional ones need an explicit strategy such as `ObjectProvider`, conditional beans, or a no-op implementation. ### 2. Default to constructor injection For application services and adapters: - inject mandatory dependencies through the constructor - keep injected fields `final` - instantiate the class directly in unit tests without starting Spring A single constructor is u...