How does Spring resolve a dependency when multiple beans implement the same interface, and what are @Primary, @Qualifier, and parameter-name matching doing?

Injection is by type first; if more than one candidate matches, Spring narrows by @Primary (a single default winner), then by @Qualifier name, and finally falls back to matching the injection point's field or parameter name against the bean name.

If it still can't disambiguate it throws NoUniqueBeanDefinitionException at startup. In Spring Boot 3 you can also define custom qualifier annotations or inject a List<T>/Map<String,T> to receive all implementations, which is handy for strategy patterns.