spring-boot-dependency-injection

Constructor injection in Spring Boot requires adding a constructor accepting dependencies like UserRepository as parameters, promoting immutability and deterministic bean configuration. This approach makes mandatory collaborators explicit and easier to unit test.

Optional dependencies in Spring Boot are managed using setter injection or ObjectProvider. This allows flexible collaborator wiring without forcing mandatory bean resolution during application context initialization.

Field injection creates hidden dependencies and complicates unit testing. Migrating to constructor injection enforces explicit mandatory dependencies, ensuring your Spring Boot service is immutable, maintainable, and deterministic.

Resolve ambiguous beans during dependency injection using @Qualifier to target specific collaborators or @Primary to set a default bean. Conditional beans also manage complex wiring scenarios effectively.

Circular dependencies occur when beans require each other for initialization. Prevent this anti-pattern by refactoring to constructor injection for deterministic configuration, or use optional collaborators via setter injection to break the cycle.