spring-boot-dependency-injection

SKILL.md

Constructor-first dependency injection patterns for Spring Boot with optional collaborator handling and bean selection strategies.

Prioritizes constructor injection to keep dependencies explicit, immutable, and testable without Spring context.

Handles optional dependencies through guarded setters, ObjectProvider, and deterministic no-op defaults.

Resolves bean ambiguity using @Primary, @Qualifier, profiles, and conditional annotations (@ConditionalOnProperty, @ConditionalOnMissingBean).

Includes validation workflows covering unit tests with mocks, slice tests, and Spring context verification before integration testing.

Requires Java 17+ and Spring Boot 3.5.x; provides reference materials and progressive examples from basic constructor injection to multi-module configurations.

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