Spring Boot Beans and Components

A bean is any object that Spring creates and manages. Every major class in your app — controllers, services, repositories — becomes a bean. Spring keeps track of all beans in its container and injects them wherever needed.

The City Grid Analogy

Think of Spring's container as a city grid. Every building (bean) is registered on a map. When one building needs electricity from another, the city infrastructure (Spring) connects them. You do not run the wires — Spring does.

Stereotype Annotations

Spring uses stereotype annotations to identify which classes should become beans. Each annotation carries meaning about what the class does.

Annotation        Layer              Purpose
───────────────   ─────────────      ──────────────────────────────────
@Component        Any layer          Generic bean — no specific role
@Controller       Web layer          Handles HTTP requests, returns views
@RestController   Web layer          @Controller + @ResponseBody combined
@Service          Business layer     Contains business logic
@Repository       Data layer         Handles database operations

Hierarchy of Stereotype Annotations

         @Component
         (parent of all)
        /     |      \
@Controller  @Service  @Repository
     │
@RestController
(= @Controller + @ResponseBody)

All stereotypes extend @Component. The difference is semantic — @Service and @Repository tell the reader (and Spring) what role the class plays.

Defining Beans with @Component

@Component
public class EmailSender {

    public void send(String to, String message) {
        System.out.println("Sending email to " + to);
    }
}

Spring scans your package, finds this class, creates one instance, and registers it as a bean. Any other class can receive this bean via injection.

Defining Beans with @Bean in @Configuration

Use @Bean inside a @Configuration class when you need to configure a third-party class that you cannot annotate directly:

@Configuration
public class AppConfig {

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();    ← Spring manages this object
    }

    @Bean
    public ObjectMapper objectMapper() {
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        return mapper;
    }
}

RestTemplate is a third-party class — you cannot add @Component to it. The @Bean method lets you create it and hand it to Spring.

Bean Scopes

By default, Spring creates one instance of each bean and reuses it throughout the app. This is the Singleton scope.

Scope           Instances Created     When to Use
─────────────   ───────────────────   ──────────────────────────────
Singleton       One per container     Default — services, repos
Prototype       New one each time     Stateful helper objects
Request         One per HTTP request  Web apps only
Session         One per user session  Web apps with login

@Component
@Scope("prototype")
public class ReportGenerator {
    // A new instance is created every time this is injected
}

How Spring Finds Your Beans

@SpringBootApplication
    │
    └── @ComponentScan
            │
            ▼
  Scans: com.estudy247.demo   (your base package)
            │
            ▼
  Finds every class with:
  @Component / @Service / @Repository / @Controller
            │
            ▼
  Creates one bean per class
  Stores all beans in ApplicationContext

Only classes inside the base package (and its sub-packages) are scanned. Place all your classes under the main class's package to ensure they are found.

Naming Beans

By default, the bean name equals the class name in camelCase. EmailSender becomes the bean named emailSender. You can set a custom name:

@Component("mailer")
public class EmailSender { ... }

@Primary and @Qualifier

When two beans share the same type, Spring does not know which one to inject. Use @Primary to mark the default, or @Qualifier to specify exactly which one you want:

@Service
@Primary
public class GmailSender implements EmailService { ... }

@Service
public class SendGridSender implements EmailService { ... }

// In the consumer:
@Autowired
@Qualifier("sendGridSender")
private EmailService emailService;    ← Gets SendGridSender specifically

Summary

• A bean is an object created and managed by the Spring container
• Use @Component, @Service, @Repository, @Controller to register beans
• Use @Bean in a @Configuration class for third-party objects
• The default scope is Singleton — one instance shared across the app
• @Primary and @Qualifier resolve conflicts when multiple beans share a type