Spring Annotations and Component Scanning

Written by Buddhi



2018-05-01



Best practices for defining components and scanning them

Annotate the class by including the exact place where to scan for components with @ComponentScan({“com.app.service”,” com.app.repo”}) rather than providing a path like @ComponentScan(“com”) or @ComponentScan(“com.app”)

Stereotype Annotations

Spring provides predefined annotations that can be used to define specific roles or purposes of that component. @ComponentScan will scan for the annotations.

@Component is the main stereotype annotation in Spring.

Annotations defined below uses @Component implicitly.

@Service
@Repository
@Controller
@RestController
@Configuration

Meta Annotations

Meta annotations are written by developers using the predefined stereotype annotations or annotations which can be used to annotate other annotations.

E.g of custom annotation:


  @Retention(RetentionPolicy.Runtime)
  @Target(ElementType.Type)
  @Service
  @Transctional(timeout=60)
  public @interface MyTransactionalService {
  String value() default "";
  }


  @ComponentScan("com.app.service")

recognises the above custom annotation and scans it.


@MyTransactionalService
  public class TransferServiceImpl implements TransferService{
  ...
  }

How to define Spring Beans?

Explicitly using @Bean annotation on methods inside configuration class. ie., classes annoted with @Configuration annotation
Implicitly using @Component and component scanning @ComponentScan

@PostConstruct and @PreDestroy

@PostConstruct: Method called at startup after all dependencies are injected. Initialization phase. E.g.: connecting to database after bean creation and dependencies are injected.
@PreDestroy: Method called at shutdown pripr to destroying the bean instance. Cleanup phase. E.g.: closing connections after query to db.

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