Spring provides several annotations to declare components as managed beans with in the container.

@Controller: Designates a class as a Spring MVC controller, typically used in conjunction with @RequestMapping.

@Controller
@RequestMapping("user")
public class UserController {

    @GetMapping("get/{id}")
    public String getUser(@PathVariable String id) {
        return "";
    }
}

Accessing /user/get/1 invokes getUser() with id set to "1".

@Service: Indicates that a class contains business logic, commonly paired with @Transactional.

@Service
@Transactional
public class UserServiceImpl implements UserService {
    public void processOrder(...) {
        // ...
    }
}

@Repository: Marks a class as a Data Access Object (DAO) responsible for database interactions.

@Repository
public class UserDao implements CrudDao<User, String> {
    // ...
}

@RestController: Combines @Controller and @ResponseBody, used for RESTful services that return JSON responses.

@Component: A generic annotation for classes managed by Spring, used when other stereotypes don't fit.

@Configuraton: Declares a class as a source of bean definitions, often used with @Bean.

@Configuration
public class DatabaseConfig {

    @Bean(name = "dataSource")
    public DataSource dataSource(Environment env) {
        HikariDataSource ds = new HikariDataSource();
        ds.setDriverClassName(env.getProperty("spring.datasource.driver-class-name"));
        ds.setJdbcUrl(env.getProperty("spring.datasource.url"));
        ds.setUsername(env.getProperty("spring.datasource.username"));
        ds.setPassword(env.getProperty("spring.datasource.password"));
        return ds;
    }
}

The DatabaseConfig class configures a HikariDataSource bean.

@Bean: Applied to methods to indicate that they return a bean managed by the Spring container.

Spring handles bean instantiation and provides lifecycle callbacks:

@PostConstruct: Invokes the annotated method after bean initialization.

@Component
public class DemoComponent {
    
    @PostConstruct
    public void initialize() { ... }
}

@PreDestroy: Calls the annotated method before container destruction.

@Service
public class DemoService {
    
    @PreDestroy
    public void cleanup() { ... }
}

Beans can be referenced by name or type:

• Name-based: Use @Qualifier to specify and reference a bean by its name.

@Service
@Qualifier("primaryService")
public class PrimaryService { ... }

@Service
public class ConsumerService {
    
    @Qualifier("primaryService")
    private PrimaryService service;
}

• Type-based: Use @Autowired to inject by type.

@Service
public class PrimaryService { ... }

@Service
public class ConsumerService {
    
    @Autowired
    private PrimaryService service;
}

@Autowired injects dependencies into fields, constructor parameters, or method arguments. For differences between @Autowired, @Resource, and @Inject, consult Spring documentation.