Custom auto-configuration in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Spring Boot custom auto-configuration class is missing @ConditionalOnClass?

Consider a custom auto-configuration class that configures a bean but does not use @ConditionalOnClass. What is the likely behavior when the application starts?

Spring Boot

package com.example.autoconfig;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MyAutoConfiguration {

    @Bean
    public String myBean() {
        return "AutoConfiguredBean";
    }
}

A The bean is never created because @ConditionalOnClass is required.

B The application fails to start due to missing class errors.

C The bean is created only if the dependent class is present on the classpath.

D The bean is always created regardless of whether the dependent class is on the classpath.

💡 Hint

Think about what @ConditionalOnClass does and what happens if you omit it.

Without @ConditionalOnClass, Spring Boot does not check for the presence of any class before creating the bean. So the bean is always created, which can cause errors if the bean depends on missing classes.

📝 Syntax

intermediate

Which annotation is required to mark a class as a Spring Boot auto-configuration?

Select the correct annotation to declare a class as an auto-configuration class in Spring Boot.

A @Configuration

B @AutoConfiguration

C @EnableAutoConfiguration

D @SpringBootApplication

💡 Hint

Auto-configuration classes are regular configuration classes with some extra metadata.

Auto-configuration classes are annotated with @Configuration. The @EnableAutoConfiguration annotation is used on the main application class to enable auto-configuration.

🔧 Debug

advanced

Why does a custom auto-configuration bean not appear in the application context?

You created a custom auto-configuration class and added it to META-INF/spring.factories but the bean is not available at runtime. What is the most likely cause?

A The spring.factories file is missing the fully qualified name of the auto-configuration class under the key org.springframework.boot.autoconfigure.EnableAutoConfiguration.

B The auto-configuration class is missing the @Component annotation.

C The bean method is private instead of public.

D The application.properties file disables all auto-configurations.

💡 Hint

Check the spring.factories file format and keys carefully.

Spring Boot loads auto-configuration classes listed under the key org.springframework.boot.autoconfigure.EnableAutoConfiguration in META-INF/spring.factories. If the class is missing or the key is wrong, the auto-configuration won't be loaded.

🧠 Conceptual

advanced

What is the purpose of @ConditionalOnMissingBean in custom auto-configuration?

Why do Spring Boot custom auto-configurations often use @ConditionalOnMissingBean on bean methods?

A To always override existing beans with the auto-configured bean.

B To create the bean only if no other bean of the same type is already defined in the context.

C To prevent the bean from being created under any circumstances.

D To delay bean creation until explicitly requested.

💡 Hint

Think about how auto-configuration should behave when user-defined beans exist.

@ConditionalOnMissingBean ensures that the auto-configuration provides a default bean only if the user has not defined one already, allowing customization.

❓ state_output

expert

What is the output when a custom auto-configuration bean depends on a missing class with @ConditionalOnClass?

Given this custom auto-configuration class: @Configuration @ConditionalOnClass(name = "com.example.ExternalService") public class ExternalServiceAutoConfig { @Bean public String externalServiceBean() { return "ServiceReady"; } } If com.example.ExternalService is NOT on the classpath, what happens when the application starts?

A The bean externalServiceBean is created with a null value.

B The bean externalServiceBean is created but throws ClassNotFoundException at runtime.

C The bean externalServiceBean is NOT created and no error occurs.

D The application fails to start with a NoClassDefFoundError.

💡 Hint

Consider what @ConditionalOnClass does when the class is missing.

@ConditionalOnClass prevents the auto-configuration class from being processed if the specified class is missing, so the bean is not created and no error occurs.

Practice

1. What is the main purpose of custom auto-configuration in Spring Boot?

easy

A. To automatically create beans based on specific conditions

B. To manually register beans in the application context

C. To disable all default Spring Boot configurations

D. To write custom SQL queries for database access

Solution

1. Step 1: Understand auto-configuration concept

   Custom auto-configuration automatically creates beans when certain conditions are met, reducing manual setup.

2. Step 2: Compare options with purpose

   Only To automatically create beans based on specific conditions describes automatic bean creation based on conditions, which matches the purpose.

3. Final Answer:

   To automatically create beans based on specific conditions -> Option A

4. Quick Check:

   Auto-configuration = automatic bean creation [OK]

Hint: Auto-configuration means beans created automatically [OK]

Common Mistakes:

• Confusing auto-configuration with manual bean registration
• Thinking it disables default configurations
• Mixing it with database query writing

2. Which annotation is commonly used to mark a class as a custom auto-configuration in Spring Boot?

easy

A. @Configuration

B. @Component

C. @EnableAutoConfiguration

D. @SpringBootApplication

Solution

1. Step 1: Identify annotation for configuration classes

   @Configuration marks a class as a source of bean definitions, suitable for auto-configuration classes.

2. Step 2: Exclude other annotations

   @Component is generic, @EnableAutoConfiguration is used on main app class, @SpringBootApplication is a composite annotation.

3. Final Answer:

   @Configuration -> Option A

4. Quick Check:

   Auto-config class needs @Configuration [OK]

Hint: Auto-config classes use @Configuration annotation [OK]

Common Mistakes:

• Using @Component instead of @Configuration
• Confusing @EnableAutoConfiguration with auto-config class annotation
• Using @SpringBootApplication on config classes

3. Given this custom auto-configuration snippet, what bean will be created if com.example.MyService is on the classpath and property my.feature.enabled=true?

@Configuration
@ConditionalOnClass(name = "com.example.MyService")
@ConditionalOnProperty(prefix = "my.feature", name = "enabled", havingValue = "true")
public class MyFeatureAutoConfiguration {
    @Bean
    public String featureBean() {
        return "Feature Enabled";
    }
}

medium

A. No bean will be created

B. A bean named featureBean with value "Feature Enabled" will be created

C. A bean named myService will be created

D. An exception will be thrown at startup

Solution

1. Step 1: Check conditions for bean creation

   Bean is created only if class com.example.MyService exists and property my.feature.enabled=true.

2. Step 2: Given conditions are met

   Since both conditions are true, the featureBean method creates a bean with value "Feature Enabled".

3. Final Answer:

   A bean named featureBean with value "Feature Enabled" will be created -> Option B

4. Quick Check:

   Conditions met = bean created [OK]

Hint: Bean created only if class and property conditions are true [OK]

Common Mistakes:

• Ignoring @ConditionalOnProperty and assuming bean always created
• Confusing bean name with class name
• Expecting exceptions when conditions are met

4. Identify the error in this custom auto-configuration class:

@Configuration
@ConditionalOnClass(name = "com.example.Service")
public class ServiceAutoConfiguration {
    @Bean
    public Service service() {
        return new Service();
    }

    @Bean
    public Service service() {
        return new Service();
    }
}

medium

A. Missing @ConditionalOnProperty annotation

B. No error, code is valid

C. Service class not imported

D. Duplicate bean method names causing a compilation error

Solution

1. Step 1: Check bean method definitions

   Two methods named service() exist, both annotated with @Bean, causing duplicate bean names.

2. Step 2: Understand Spring bean naming rules

   Bean names must be unique; duplicate method names cause compilation or context startup errors.

3. Final Answer:

   Duplicate bean method names causing a compilation error -> Option D

4. Quick Check:

   Duplicate @Bean methods = error [OK]

Hint: Bean method names must be unique in config classes [OK]

Common Mistakes:

• Ignoring duplicate method names
• Thinking missing @ConditionalOnProperty is an error here
• Assuming missing import causes this error

5. You want to create a custom auto-configuration that only creates a bean if no other bean of the same type exists and a property app.feature.enabled is true. Which combination of annotations should you use on your configuration class and bean method? A) @Configuration on class, @ConditionalOnMissingBean and @ConditionalOnProperty on bean method B) @Component on class, @ConditionalOnBean on bean method C) @Configuration on class, @ConditionalOnClass on bean method D) @EnableAutoConfiguration on class, no conditions on bean method

hard

A. @Configuration on class, @ConditionalOnClass on bean method

B. @Component on class, @ConditionalOnBean on bean method

C. @Configuration on class, @ConditionalOnMissingBean and @ConditionalOnProperty on bean method

D. @EnableAutoConfiguration on class, no conditions on bean method

Solution

1. Step 1: Identify class-level annotation for config

   @Configuration is correct to mark a class as configuration for auto-configuration.

2. Step 2: Choose bean method conditions

   @ConditionalOnMissingBean ensures no existing bean of the type exists; @ConditionalOnProperty checks the property is true.

3. Step 3: Exclude incorrect options

   @Component is generic, @ConditionalOnBean requires existing bean, @ConditionalOnClass checks class presence, not bean existence, @EnableAutoConfiguration is for main app class.

4. Final Answer:

   @Configuration on class, @ConditionalOnMissingBean and @ConditionalOnProperty on bean method -> Option C

5. Quick Check:

   Missing bean + property conditions = @Configuration on class, @ConditionalOnMissingBean and @ConditionalOnProperty on bean method [OK]

Hint: Use @ConditionalOnMissingBean and @ConditionalOnProperty together [OK]

Common Mistakes:

• Using @ConditionalOnBean instead of @ConditionalOnMissingBean
• Putting conditions on class instead of bean method
• Confusing @EnableAutoConfiguration with config class annotation