Conditional bean creation in Spring Boot

Introduction

Conditional bean creation helps your Spring Boot app create beans only when certain conditions are true. This keeps your app clean and efficient.

You want to create a bean only if a specific property is set in your configuration.

You want to load a bean only when running in a certain environment, like development or production.

You want to create a bean only if another bean is missing or present.

You want to enable a feature only when a certain class is available on the classpath.

Syntax

Spring Boot

@ConditionalOnProperty(name = "property.name", havingValue = "value")
@Bean
public MyBean myBean() {
    return new MyBean();
}

Use @ConditionalOnProperty to create beans based on config properties.

Spring Boot offers other conditions like @ConditionalOnMissingBean and @ConditionalOnClass.

Examples

This bean is created only if the property feature.enabled is set to true.

Spring Boot

@ConditionalOnProperty(name = "feature.enabled", havingValue = "true")
@Bean
public FeatureService featureService() {
    return new FeatureService();
}

This bean is created only if no other bean of the same type exists.

Spring Boot

@ConditionalOnMissingBean
@Bean
public DefaultService defaultService() {
    return new DefaultService();
}

This bean is created only if SomeClass is on the classpath.

Spring Boot

@ConditionalOnClass(name = "com.example.SomeClass")
@Bean
public OptionalService optionalService() {
    return new OptionalService();
}

Sample Program

This Spring Boot app creates the GreetingService bean only if the property app.greeting.enabled is set to true in application.properties. The main method checks if the bean exists and prints the result.

Spring Boot

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        var context = SpringApplication.run(DemoApplication.class, args);
        boolean hasBean = context.containsBean("greetingService");
        System.out.println("Is GreetingService bean created? " + hasBean);
    }

    @Bean
    @ConditionalOnProperty(name = "app.greeting.enabled", havingValue = "true")
    public GreetingService greetingService() {
        return new GreetingService();
    }

    public static class GreetingService {
        public String greet() {
            return "Hello!";
        }
    }
}

Important Notes

Make sure to set the property in application.properties or application.yml to control bean creation.

Conditional annotations help keep your app flexible and avoid loading unnecessary beans.

Summary

Conditional bean creation lets Spring Boot create beans only when needed.

Use annotations like @ConditionalOnProperty to control when beans load.

This helps keep your app clean, fast, and easy to manage.

Practice

1. What is the main purpose of using @ConditionalOnProperty in Spring Boot?

easy

A. To disable all beans in the application

B. To always create a bean regardless of configuration

C. To create a bean only if a specific property is set in the configuration

D. To create beans only during testing

Solution

1. Step 1: Understand the role of @ConditionalOnProperty

   This annotation checks if a property is set in the application configuration.

2. Step 2: Effect on bean creation

   If the property matches the condition, Spring Boot creates the bean; otherwise, it skips it.

3. Final Answer:

   To create a bean only if a specific property is set in the configuration -> Option C

4. Quick Check:

   Conditional bean creation = property check [OK]

Hint: Remember: @ConditionalOnProperty controls bean creation by config [OK]

Common Mistakes:

• Thinking it always creates beans
• Confusing it with disabling all beans
• Assuming it only works in tests

2. Which of the following is the correct way to use @ConditionalOnProperty to create a bean only when feature.enabled is true?

easy

A. @ConditionalOnProperty(property = "feature.enabled", value = "true")

B. @ConditionalOnProperty(name = "feature.enabled", havingValue = "true")

C. @ConditionalOnProperty(key = "feature.enabled", equals = "true")

D. @ConditionalOnProperty(enabled = "feature.enabled", isTrue = true)

Solution

1. Step 1: Recall correct annotation parameters

   The correct parameters are name for the property key and havingValue for the expected value.

2. Step 2: Match with options

   @ConditionalOnProperty(name = "feature.enabled", havingValue = "true") uses name and havingValue correctly, others use invalid parameter names.

3. Final Answer:

   @ConditionalOnProperty(name = "feature.enabled", havingValue = "true") -> Option B

4. Quick Check:

   Correct params = name + havingValue [OK]

Hint: Use 'name' and 'havingValue' parameters for @ConditionalOnProperty [OK]

Common Mistakes:

• Using wrong parameter names like 'property' or 'key'
• Confusing 'havingValue' with 'value'
• Using boolean true without quotes

3. Given this Spring Boot bean definition:

@Bean
@ConditionalOnProperty(name = "app.feature", havingValue = "enabled")
public Service myService() {
    return new Service();
}

What happens if the property app.feature=disabled is set in application.properties?

medium

A. The bean myService is created and available

B. The bean is created but disabled

C. Application fails to start with error

D. The bean myService is NOT created

Solution

1. Step 1: Understand the condition

   The bean is created only if app.feature equals "enabled".

2. Step 2: Check the property value

   The property is set to "disabled", which does not match "enabled".

3. Final Answer:

   The bean myService is NOT created -> Option D

4. Quick Check:

   Property mismatch = no bean creation [OK]

Hint: Bean creates only if property matches havingValue exactly [OK]

Common Mistakes:

• Assuming bean creates regardless of property value
• Thinking app fails to start on mismatch
• Believing bean is created but inactive

4. Consider this bean definition:

@Bean
@ConditionalOnProperty(name = "service.enabled", havingValue = "true")
public Service service() {
    return new Service();
}

Why does the bean never get created even when service.enabled=true is set in application.properties?

medium

A. The property key name is incorrect or misspelled

B. The property is a string but havingValue expects a boolean

C. The bean method is missing @Component annotation

D. The bean method must be static to work with @ConditionalOnProperty

Solution

1. Step 1: Verify property key spelling

   If the property key in application.properties is misspelled or different, condition fails.

2. Step 2: Check other options

   HavingValue compares strings, so boolean type mismatch is not an issue; @Component is not needed on @Bean methods; static is not required.

3. Final Answer:

   The property key name is incorrect or misspelled -> Option A

4. Quick Check:

   Property key must match exactly [OK]

Hint: Check property key spelling carefully for conditional beans [OK]

Common Mistakes:

• Assuming boolean type mismatch causes failure
• Thinking @Component is needed on @Bean methods
• Believing bean methods must be static

5. You want to create two beans of the same type but only one should be active based on a property mode which can be "simple" or "advanced".
Which is the best way to define these beans using @ConditionalOnProperty?

hard

A. Use @ConditionalOnProperty(name = "mode", havingValue = "simple") on the simple bean and @ConditionalOnProperty(name = "mode", havingValue = "advanced") on the advanced bean

B. Use @ConditionalOnProperty(name = "mode") on both beans without havingValue

C. Create both beans without conditions and select one at runtime

D. Use @ConditionalOnProperty(name = "mode", matchIfMissing = true) on both beans

Solution

1. Step 1: Understand the requirement

   Only one bean should be active depending on the property value "simple" or "advanced".

2. Step 2: Apply conditional annotations

   Using @ConditionalOnProperty with havingValue set to "simple" or "advanced" on each bean ensures only one bean loads.

3. Step 3: Evaluate other options

   Use @ConditionalOnProperty(name = "mode") on both beans without havingValue lacks value check, so both beans may load; Create both beans without conditions and select one at runtime loads both always; Use @ConditionalOnProperty(name = "mode", matchIfMissing = true) on both beans activates both if property missing.

4. Final Answer:

   Use @ConditionalOnProperty(name = "mode", havingValue = "simple") on the simple bean and @ConditionalOnProperty(name = "mode", havingValue = "advanced") on the advanced bean -> Option A

5. Quick Check:

   Use havingValue to select one bean by property [OK]

Hint: Use havingValue to pick one bean per property value [OK]

Common Mistakes:

• Not specifying havingValue causing both beans to load
• Trying to select bean at runtime instead of conditionally
• Using matchIfMissing on both beans causing conflicts