@Primary & @Qualifier Annotations - Advanced Guide

1️⃣ Introduction

@Primary and @Qualifier annotations are essential tools in Spring for managing multiple bean implementations and resolving dependency injection ambiguity. This article explores their usage and best practices.

Key features include:

  • @Primary - Marks a bean as the default choice
  • @Qualifier - Specifies which bean to inject
  • Dependency resolution
  • Bean selection control

2️⃣ Key Concepts & Terminology

  • @Primary: Indicates that a bean should be given preference when multiple candidates exist.
  • @Qualifier: Specifies which bean to inject when multiple candidates exist.
  • Dependency Resolution: The process of selecting the appropriate bean for injection.
  • Bean Ambiguity: Situation when Spring finds multiple beans of the same type.

3️⃣ Hands-on Implementation 🛠

🔹 Step 1: Using @Primary

@Configuration
public class PaymentConfig {
    @Bean
    @Primary
    public PaymentService creditCardPaymentService() {
        return new CreditCardPaymentService();
    }
    
    @Bean
    public PaymentService paypalPaymentService() {
        return new PayPalPaymentService();
    }
}

🔹 Step 2: Using @Qualifier

@Service
public class OrderService {
    private final PaymentService paymentService;
    
    @Autowired
    public OrderService(@Qualifier("paypalPaymentService") PaymentService paymentService) {
        this.paymentService = paymentService;
    }
    
    public void processOrder(Order order) {
        paymentService.processPayment(order.getAmount());
    }
}

🔹 Step 3: Combining Both

@Configuration
public class NotificationConfig {
    @Bean
    @Primary
    public NotificationService emailNotificationService() {
        return new EmailNotificationService();
    }
    
    @Bean
    @Qualifier("sms")
    public NotificationService smsNotificationService() {
        return new SMSNotificationService();
    }
}

4️⃣ Common Issues & Debugging 🐞

Common Issues and Solutions

Issue Solution
Multiple beans found Use @Primary or @Qualifier to specify the bean
Qualifier not working Check bean name matches qualifier value
Primary bean conflicts Ensure only one @Primary bean per type

5️⃣ Q&A / Frequently Asked Questions

Use @Primary when you want to specify a default bean implementation, and use @Qualifier when you need explicit control over which bean to inject in specific cases.

Yes, you can use both annotations together. @Qualifier takes precedence over @Primary when both are present.

6️⃣ Best Practices & Pro Tips 🚀

  • Use @Primary for default implementations
  • Use @Qualifier for explicit bean selection
  • Keep qualifier names meaningful
  • Document bean selection strategy
  • Test bean resolution in different scenarios

7️⃣ Read Next 📖

8️⃣ Conclusion

@Primary and @Qualifier annotations are powerful tools for managing bean selection in Spring applications. Understanding when and how to use them is crucial for building maintainable and flexible applications.

Remember to use these annotations judiciously and document your bean selection strategy for better code maintainability.

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