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🌱 Spring Boot – Bean and Its Lifecycle

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1️⃣ What is a Bean?

A Bean is a Java object that is created, managed, and destroyed by the Spring IoC (Inversion of Control) Container.

🔑 Key Points

• Beans are managed by the IoC Container
• The container handles:
  • Instantiation
  • Dependency Injection
  • Lifecycle callbacks
  • Destruction

🧠 Internal Working

• Spring scans the classpath
• Identifies bean definitions
• Creates bean instances
• Stores them in the ApplicationContext

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✅ Quick Summary

• Bean = managed Java object
• Lifecycle controlled by Spring
• Core of Spring Framework

❓ Interview / Exam Questions (with Answers)

1. What is a Spring Bean?
   → A Java object managed by the Spring IoC container.
2. Who manages the bean lifecycle?
   → Spring IoC Container.
3. Are all objects beans?
   → No, only objects created by Spring.

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2️⃣ How to Create a Bean?

Spring provides two main approaches.

🔹 Approach 1: @Component Annotation

@Component
public class User {
    private String username;
    private String email;
}

🔸 Notes

• Follows Convention over Configuration
• Spring auto-detects via Component Scanning
• Related stereotypes:
  • @Controller
  • @Service
  • @Repository

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🔹 Approach 2: @Bean Annotation

Used when constructor parameters or custom logic are required.

@Configuration
public class AppConfig {

    @Bean
    public User userBean() {
        return new User("defaultUser", "default@email.com");
    }
}

⚠️ Why @Bean is Needed?

If a class has a parameterized constructor, Spring does not know what values to pass automatically.

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✅ Quick Summary

• @Component → auto creation
• @Bean → manual configuration
• Choose based on constructor needs

❓ Interview / Exam Questions

1. Difference between @Component and @Bean?
   → @Component is auto-scanned, @Bean is explicitly defined.
2. When should @Bean be preferred?
   → When constructor parameters are required.
3. Can we create multiple beans of same type?
   → Yes, via multiple @Bean methods.

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3️⃣ How Spring Boot Finds Beans?

🔹 Method 1: Component Scanning

@SpringBootApplication
@ComponentScan(basePackages = "com.example")
public class Application { }

• Scans base package & sub-packages
• Finds classes annotated with stereotypes

🔹 Method 2: Explicit Bean Definition

• Beans defined using @Bean inside @Configuration

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✅ Quick Summary

• Beans found via scanning or configuration
• Base package is critical
• Misplacement causes bean not found errors

❓ Interview / Exam Questions

1. What happens if a bean is outside base package?
   → It will not be detected.
2. What does @ComponentScan do?
   → Scans packages for beans.
3. Can @Bean work without component scan?
   → Yes.

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4️⃣ When Are Beans Created? (Eager vs Lazy)

🔹 Eager Initialization (Default)

• Beans created at application startup
• Applies to Singleton scope

🔹 Lazy Initialization

• Beans created only when requested
• Enabled using @Lazy

@Lazy
@Component
public class OrderService {
    public OrderService() {
        System.out.println("OrderService Initialized");
    }
}

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✅ Quick Summary

• Singleton → eager by default
• Lazy improves startup time
• Use carefully in production

❓ Interview / Exam Questions

1. Default bean initialization type?
   → Eager.
2. When is @Lazy used?
   → To delay bean creation.
3. Does @Lazy work with prototype?
   → Prototype beans are always lazy.

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5️⃣ Lifecycle of a Spring Bean

🔄 Complete Lifecycle Steps

1. Application Start
2. IoC Container Created
3. Bean Instantiated
4. Dependencies Injected
5. @PostConstruct
6. Bean Ready for Use
7. @PreDestroy
8. Bean
9. 

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🔹 Lifecycle Callback Example

@Component
public class User {

    @PostConstruct
    public void init() {
        System.out.println("Bean Initialized");
    }

    @PreDestroy
    public void destroy() {
        System.out.println("Bean Destroyed");
    }
}

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🧠 Internal Working

• @PostConstruct → called after DI
• @PreDestroy → called before container shutdown
• Container shutdown occurs via:
  • context.close()
  • JVM shutdown hook

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⚙️ Performance & Best Practices

• Avoid heavy logic in constructors
• Use @PostConstruct for initialization
• Release resources in @PreDestroy
• Prefer constructor injection

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✅ Quick Summary

• Lifecycle is fully managed by Spring
• Hooks available for customization
• Critical for resource management

❓ Interview / Exam Questions

1. When is @PostConstruct executed?
   → After dependency injection.
2. Is @PreDestroy called for prototype beans?
   → No.
3. Who controls bean destruction?
   → IoC Container.

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🎯 FINAL CONCLUSION

• Beans are the backbone of Spring
• Lifecycle understanding is frequently asked
• Correct usage prevents memory leaks and startup issues

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