---

🌱 Spring Boot – Dynamic Bean Initialization

1️⃣ What is Dynamic Bean Initialization?

Dynamic Bean Initialization means deciding which bean implementation should be created at runtime, based on:

• Configuration values
• Conditions
• Properties
• Environment variables

👉 Instead of fixing the bean at compile time, Spring decides dynamically during startup.

---

2️⃣ Problem: Unsatisfied Dependency

🔴 Scenario Explained

• You have an interface (e.g., Order)
• Multiple implementations exist:
  • OnlineOrder
  • OfflineOrder
• Spring does not know which one to inject

❌ Error Seen

UnsatisfiedDependencyException:
Error creating bean with name 'user'

❓ Why this happens

• Spring finds more than one bean of the same interface
• No clear instruction is provided to choose one

---

3️⃣ Example Structure (From PDF – Page 1)

Interface

public interface Order {
    void createOrder();
}

Implementations

@Component
public class OnlineOrder implements Order {
    public void createOrder() {
        System.out.println("Created Online Order");
    }
}

@Component
public class OfflineOrder implements Order {
    public void createOrder() {
        System.out.println("Created Offline Order");
    }
}

Controller

@Autowired
Order order;

❌ Result: Spring fails because it finds two beans of type Order

---

4️⃣ Solution 1: Using @Qualifier

✅ What is @Qualifier?

@Qualifier tells Spring exactly which bean to inject.

✔ Example

@Autowired
@Qualifier("onlineOrder")
Order order;

✔ Result

• Spring injects OnlineOrder
• Application starts successfully

⭐ Important Points

• Bean name = class name (camelCase by default)
• Use @Qualifier when multiple beans exist

---

5️⃣ Solution 2: Dynamic Bean Selection using @Configuration

📌 Best approach for real projects

Step 1: Interface

public interface Order {
    void createOrder();
}

---

Step 2: Implementations (NO @Component)

public class OnlineOrder implements Order {
    public void createOrder() {
        System.out.println("Created Online Order");
    }
}

public class OfflineOrder implements Order {
    public void createOrder() {
        System.out.println("Created Offline Order");
    }
}

---

Step 3: Configuration Class

@Configuration
public class OrderConfig {

    @Bean
    public Order createOrder(@Value("${isOnlineOrder}") boolean isOnlineOrder) {
        if (isOnlineOrder) {
            return new OnlineOrder();
        } else {
            return new OfflineOrder();
        }
    }
}

---

Step 4: application.properties

isOnlineOrder=false

---

✅ Output (From PDF – Page 2 Logs)

Created Offline Order

Change to:

isOnlineOrder=true

➡ Output:

Created Online Order

---

6️⃣ @Value Annotation (Important Concept)

🔹 What is @Value?

Used to inject values from:

• application.properties
• Environment variables
• Inline literals

✔ Example

@Value("${isOnlineOrder}")
boolean isOnlineOrder;

---

Inline Literal Example (From PDF)

@Bean
public Order createOrderBean(@Value("false") boolean isOnlineOrder) {
    if (isOnlineOrder) {
        return new OnlineOrder();
    } else {
        return new OfflineOrder();
    }
}

---

7️⃣ Extra Points (Not Clearly Mentioned in PDF)

🔸 Why Dynamic Bean Initialization is Powerful

• Environment-based configuration
• No code changes needed
• Easy switching between implementations
• Ideal for microservices

---

🔸 Best Practices

• Prefer @Configuration + @Bean for logic-based creation
• Use @Qualifier only when logic is simple
• Avoid multiple @Component beans of same type without control

---

8️⃣ Interview / Exam Questions & Answers

Q1. What is UnsatisfiedDependencyException?

Answer:
It occurs when Spring cannot resolve which bean to inject due to missing or multiple matching beans.

---

Q2. Difference between @Qualifier and @Primary?

Answer:

• @Qualifier: Explicitly specifies bean name
• @Primary: Sets default bean when multiple beans exist

---

Q3. When should we use dynamic bean initialization?

Answer:
When bean selection depends on:

• Configuration
• Environment
• Runtime conditions

---

Q4. Can @Value inject default values?

Answer:
Yes

@Value("${isOnlineOrder:false}")

---

Q5. Which approach is better for enterprise apps?

Answer:
@Configuration + @Bean approach because it provides better control and flexibility.

---

9️⃣ One-Line Summary (Exam Gold ✨)

Dynamic Bean Initialization allows Spring Boot to decide bean creation at runtime using configuration and conditions, solving multiple implementation conflicts cleanly.

---