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.