What is Autowiring by Type vs by Name, and what happens when Spring can't resolve a dependency? टाइप बनाम नाम से ऑटोवायरिंग क्या है, और जब स्प्रिंग डिपेंडेंसी रिज़ॉल्व नहीं कर पाता तो क्या होता है?

By default, @Autowired resolves dependencies by type — Spring looks for a bean matching the field or parameter's declared type. If exactly one candidate exists, it's injected directly; if none exist, Spring throws NoSuchBeanDefinitionException at startup (unless the dependency is marked required = false or wrapped in Optional).

If multiple beans of the same type exist, Spring falls back to matching by field/parameter name against bean names before giving up with a NoUniqueBeanDefinitionException — this is effectively "autowiring by name" as a tiebreaker, though it's less explicit and reliable than using @Qualifier or @Primary to disambiguate directly. Failing fast at startup rather than at runtime is one of Spring's key reliability guarantees for dependency wiring.

@Autowired(required = false)
private AnalyticsService analyticsService; // optional dependency, no exception if missing