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 usually enough; @Autowired is unnecessary in that case.