Essential Spring Annotations Overview

Direct Answer

Important Spring annotations are easiest to learn by job: bean definitions (@Configuration, @Bean), component roles (@Component, @Service, @Repository, @Controller), injection (@Autowired, @Qualifier, @Value), web mappings (@RestController, @GetMapping), transactions (@Transactional), and lifecycle callbacks (@PostConstruct, @PreDestroy). Each annotation is processed only when the relevant Spring infrastructure manages the class.

Code Example

@Configuration
public class AppConfig {
    @Bean  // Configuration: explicit bean definition
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder();
    }
}

@Service  // Stereotype: business logic component
public class OrderService {
    private final OrderRepository repository;
    private final int maxRetries;

    @Autowired  // Injection: constructor injection (preferred)
    public OrderService(OrderRepository repository, 
                        @Value("${order.max-retries:3}") int maxRetries) {
        this.repository = repository;
        this.maxRetries = maxRetries;
    }

    @PostConstruct  // Lifecycle: runs after dependency injection
    public void init() {
        System.out.println("OrderService initialized with " + maxRetries + " retries");
    }
}

@RestController  // Web: combines @Controller + @ResponseBody
public class OrderController {
    @GetMapping("/orders/{id}")  // Web: maps HTTP GET to method
    public Order getOrder(@PathVariable Long id) {
        return orderService.findById(id);
    }
}

Common Mistakes & Pitfalls

• Using @Component for everything: Freshers often use @Component for all classes instead of specific stereotypes like @Service or @Repository. This works but loses semantic meaning — @Repository adds persistence exception translation, @Service marks business logic, and @Controller handles web requests. Use the right stereotype for clarity and framework benefits.
• Forgetting @ComponentScan scope: Annotations only work if Spring scans the package. If your @Service is in com.example.external but your main class is in com.example.app, Spring won't find it. Always ensure components are in the same package or subpackages as your @SpringBootApplication class.
• Mixing field and constructor injection: Using @Autowired on fields is tempting but makes testing harder. Constructor injection (implicit @Autowired on the only constructor) is preferred — it makes dependencies explicit and enables immutability with final fields.