Conditional bean creation in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when using @ConditionalOnProperty with missing property?

Consider a Spring Boot application with a bean annotated as @ConditionalOnProperty(name = "feature.enabled", havingValue = "true"). If the property feature.enabled is not set in application.properties, what happens to the bean?

Spring Boot

@Configuration
public class FeatureConfig {
    @Bean
    @ConditionalOnProperty(name = "feature.enabled", havingValue = "true")
    public String featureBean() {
        return "Feature is enabled";
    }
}

A The bean is created regardless of the property value.

B The bean is created with default value because the property is missing.

C The bean is not created because the property is missing and does not match the required value.

D The application fails to start due to missing property.

💡 Hint

Think about how @ConditionalOnProperty behaves when the property is absent.

The @ConditionalOnProperty annotation creates the bean only if the specified property exists and matches the given value. If the property is missing, the condition fails and the bean is not created.

📝 Syntax

intermediate

Identify the correct syntax for conditional bean creation using @ConditionalOnClass

Which of the following bean definitions correctly uses @ConditionalOnClass to create a bean only if DataSource.class is present on the classpath?

A

@Bean
@ConditionalOnClass(className = "javax.sql.DataSource")
public MyBean myBean() { return new MyBean(); }

B

@Bean
@ConditionalOnClass(name = "javax.sql.DataSource")
public MyBean myBean() { return new MyBean(); }

C

@Bean
@ConditionalOnClass(classes = DataSource.class)
public MyBean myBean() { return new MyBean(); }

D

@Bean
@ConditionalOnClass(DataSource.class)
public MyBean myBean() { return new MyBean(); }

💡 Hint

Check the official attribute name for @ConditionalOnClass.

The correct attribute for @ConditionalOnClass is a varargs of Class types, so using @ConditionalOnClass(DataSource.class) is correct. The other options use incorrect attribute names or syntax.

🔧 Debug

advanced

Why does this conditional bean not get created despite correct property?

Given the following code snippet, the bean specialService is not created even though app.special.enabled=true is set in application.properties. What is the most likely cause?

Spring Boot

@Configuration
public class SpecialConfig {
    @Bean
    @ConditionalOnProperty(name = "app.special.enabled", havingValue = "true", matchIfMissing = false)
    public SpecialService specialService() {
        return new SpecialService();
    }
}

A The property name in the annotation does not exactly match the property key in the properties file.

B The matchIfMissing attribute should be true to create the bean.

C The bean method is missing the @Component annotation.

D The SpecialService class is not annotated with @Service.

💡 Hint

Check for exact spelling and case sensitivity in property names.

The @ConditionalOnProperty annotation requires the property name to exactly match the key in the properties file. If there is a typo or case mismatch, the condition fails and the bean is not created.

❓ state_output

advanced

What is the state of the application context with multiple conditional beans?

In a Spring Boot app, two beans are defined with these conditions: Bean A: @ConditionalOnProperty(name = "feature.a.enabled", havingValue = "true") Bean B: @ConditionalOnProperty(name = "feature.b.enabled", havingValue = "true") If feature.a.enabled=true and feature.b.enabled=false in application.properties, which beans exist in the application context?

A Only Bean B is created; Bean A is not created.

B Only Bean A is created; Bean B is not created.

C Neither Bean A nor Bean B is created.

D Both Bean A and Bean B are created.

💡 Hint

Consider how each property value affects its bean creation independently.

Each bean's creation depends on its own property. Since feature.a.enabled is true, Bean A is created. Since feature.b.enabled is false, Bean B is not created.

🧠 Conceptual

expert

Which condition causes a bean to be created only if another bean is missing?

You want to create a bean only if a bean of type DataSource is NOT already defined in the Spring context. Which annotation achieves this?

A @ConditionalOnMissingBean(DataSource.class)

B @ConditionalOnBean(DataSource.class)

C @ConditionalOnProperty(name = "datasource.enabled", havingValue = "false")

D @ConditionalOnClass(name = "javax.sql.DataSource")

💡 Hint

Think about the annotation that checks absence of a bean.

The @ConditionalOnMissingBean annotation creates the bean only if no bean of the specified type exists in the context. The other annotations check for presence or properties, not absence.

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