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🌱 Spring Boot – @ConditionalOnProperty

1️⃣ What is @ConditionalOnProperty?

@ConditionalOnProperty is a Spring Boot conditional annotation that controls whether a bean should be created or not, based on the value of a property defined in application.properties or application.yml.

👉 In simple words:
Bean creation depends on configuration values.

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2️⃣ Why do we need Conditional Bean Creation?

In real-world applications:

• Different environments need different configurations
• Some features must be enabled/disabled dynamically
• We may want only one implementation active at a time

Spring Boot solves this using conditional annotations, and one of the most commonly used is @ConditionalOnProperty.

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3️⃣ Problem Without @ConditionalOnProperty

❌ Default Behavior

If multiple beans exist:

• Spring creates all beans
• May lead to:
  • Unnecessary initialization
  • Conflicts
  • Higher memory usage
  • Startup errors

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4️⃣ Use Cases (From Your PDF)

✅ Use Case 1

👉 Create only ONE bean
Either:

• MySQLConnection
• OR NoSQLConnection

Not both at the same time.

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✅ Use Case 2

👉 Same codebase, different components need different DBs

• Component A → MySQL
• Component B → NoSQL

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5️⃣ Solution: @ConditionalOnProperty

Spring Boot provides:

@ConditionalOnProperty(
    name = "db.type",
    havingValue = "mysql",
    matchIfMissing = false
)

This tells Spring:

• Create this bean only if property matches
• Otherwise → skip bean creation

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6️⃣ Example Implementation

🔹 MySQL Bean

@Component
@ConditionalOnProperty(
    name = "database",
    havingValue = "mysql"
)
public class MySQLConnection {

    public MySQLConnection() {
        System.out.println("Initialization of MySQLConnection Bean");
    }
}

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🔹 NoSQL Bean

@Component
@ConditionalOnProperty(
    name = "database",
    havingValue = "nosql"
)
public class NoSQLConnection {

    public NoSQLConnection() {
        System.out.println("Initialization of NoSQLConnection Bean");
    }
}

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🔹 application.properties

database=mysql

✔ Result:

• Only MySQLConnection bean is created
• NoSQLConnection is ignored

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7️⃣ Important Attributes of @ConditionalOnProperty

Attribute	Meaning
name	Property key
havingValue	Required value to enable bean
matchIfMissing	Create bean if property is absent

Example:

@ConditionalOnProperty(
    name = "feature.logging",
    havingValue = "true",
    matchIfMissing = true
)

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8️⃣ Real-World Examples (Extra Points)

✅ Enable/Disable:

• Payment gateways (Razorpay / Paytm)
• Cache (Redis ON/OFF)
• Messaging (Kafka / RabbitMQ)
• Feature flags

✅ Environment based:

• Dev → H2 DB
• Prod → MySQL / Oracle

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9️⃣ Advantages (From PDF + Added)

✔ Feature toggling
✔ Avoid unnecessary beans
✔ Reduce memory usage
✔ Faster application startup
✔ Clean environment-based configuration

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🔟 Disadvantages (From PDF + Added)

❌ Misconfiguration can break app
❌ Hard to debug if properties are wrong
❌ Overuse increases code complexity
❌ Multiple similar beans cause confusion
❌ Difficult to manage in very large projects

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1️⃣1️⃣ Best Practices (Added)

✅ Use clear property names
✅ Document properties properly
✅ Avoid overusing conditional beans
✅ Prefer profiles (@Profile) when suitable
✅ Log active configurations at startup

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🧠 Interview Questions & Answers

Q1️⃣ What is @ConditionalOnProperty?

Answer:
It is a Spring Boot annotation used to create beans conditionally based on application property values.

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Q2️⃣ Why do we need @ConditionalOnProperty?

Answer:
To control bean creation dynamically, avoid unnecessary beans, and support environment-based configurations.

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Q3️⃣ Difference between @Profile and @ConditionalOnProperty?

Answer:

• @Profile → Environment based
• @ConditionalOnProperty → Property value based (more flexible)

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Q4️⃣ What happens if property value does not match?

Answer:
The bean is not created and skipped during application startup.

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Q5️⃣ What is matchIfMissing = true?

Answer:
If the property is missing, the bean will still be created.

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Q6️⃣ Can multiple beans use the same property?

Answer:
Yes, but different havingValue must be used to avoid conflicts.

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Q7️⃣ Is @ConditionalOnProperty evaluated at runtime?

Answer:
No, it is evaluated during application startup.

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Q8️⃣ Common mistake with @ConditionalOnProperty?

Answer:
Wrong property key or value in application.properties.

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📌 Summary (One-Line)

@ConditionalOnProperty helps build flexible, configurable, and environment-aware Spring Boot applications.