spring-boot-dependency-injection_skill

Overview

This skill provides a practical dependency injection guide for Spring Boot projects, promoting constructor-first design, explicit optional collaborator handling, and deterministic bean configuration. It focuses on keeping services framework-agnostic, testable, and easy to reason about during development and refactors.

How this skill works

The skill inspects dependency wiring patterns and prescribes concrete implementation steps: identify mandatory vs optional collaborators, prefer constructor injection, supply guarded setters or ObjectProvider for optional beans, and declare deterministic @Bean factories. It also covers resolving bean ambiguity with @Primary, @Qualifier, profiles, and conditional annotations, and validates wiring with focused unit and slice tests.

When to use it

• Creating new @Service, @Component, or @Repository classes to ensure explicit wiring
• Replacing legacy field injection and modernizing modules for better testability
• Configuring optional or pluggable collaborators behind feature flags or multi-tenant adapters
• Auditing bean definitions before adding integration tests or upgrading Spring Boot
• Resolving bean ambiguity when multiple implementations exist

Best practices

• Prefer constructor injection for mandatory dependencies and mark injected fields final
• Handle optional collaborators via guarded setters (@Autowired(required = false)), ObjectProvider, or no-op defaults
• Use @Primary, @Qualifier, profiles, and @ConditionalOn... annotations to make bean selection intentional
• Write unit tests that instantiate classes directly with mocks before running Spring slice or integration tests
• Avoid field injection, service locators, and excessive @Lazy; extract services if constructors grow beyond a few collaborators

Example use cases

• Implement UserService with @RequiredArgsConstructor and inject repositories and notification services via constructor
• Add a CacheService optional collaborator using a guarded setter and a CacheService.noOp() fallback
• Register NotificationService conditionally using @ConditionalOnProperty and provide a @ConditionalOnMissingBean noop implementation
• Migrate a legacy module by replacing @Autowired fields with constructor parameters and adding focused unit tests to validate wiring
• Resolve ambiguous implementations using @Qualifier and document qualifier names in shared constants