What does the @Qualifier annotation do in Spring, and when do you need it?

@Qualifier resolves ambiguity when multiple beans of the same type exist and Spring can't determine which one to autowire by type alone — you pass @Qualifier("beanName") alongside @Autowired to specify exactly which implementation to inject. Without it, Spring throws a NoUniqueBeanDefinitionException the moment more than one matching bean is found for a given injection point.