How auto-configuration works in Spring Boot - Practice Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What triggers Spring Boot's auto-configuration?

In Spring Boot, what causes the auto-configuration classes to be applied during application startup?

A Adding @ComponentScan on any configuration class

B Presence of @EnableAutoConfiguration or @SpringBootApplication annotation on the main class

C Manually registering beans in the application context

D Using @ConfigurationProperties on a POJO

💡 Hint

Think about which annotation bundles auto-configuration in Spring Boot.

Spring Boot applies auto-configuration when the main application class is annotated with @EnableAutoConfiguration or @SpringBootApplication (which includes it). This triggers Spring Boot to scan and apply auto-configuration classes automatically.

❓ component_behavior

intermediate

What happens if a required class is missing for auto-configuration?

Consider an auto-configuration class that requires a specific library class to be present on the classpath. What happens if that class is missing when Spring Boot starts?

A The auto-configuration class is skipped and not applied

B Spring Boot throws a ClassNotFoundException and fails to start

C The auto-configuration applies but logs a warning

D The application runs but with default settings ignoring the missing class

💡 Hint

Auto-configuration classes often use conditional checks on class presence.

Spring Boot auto-configuration classes use conditional annotations like @ConditionalOnClass to check if required classes are present. If the class is missing, the auto-configuration is skipped silently.

📝 Syntax

advanced

Identify the correct conditional annotation for auto-configuration

Which annotation should be used on an auto-configuration class to apply it only if a specific bean is missing in the context?

A @ConditionalOnProperty

B @ConditionalOnBean

C @ConditionalOnClass

D @ConditionalOnMissingBean

💡 Hint

Think about the condition that checks absence of a bean.

@ConditionalOnMissingBean is used to apply auto-configuration only if a certain bean is not already defined in the Spring context.

🔧 Debug

advanced

Why is my custom configuration ignored by Spring Boot auto-configuration?

You created a custom configuration class annotated with @Configuration and @ConditionalOnMissingBean for a DataSource bean. However, your configuration is ignored and the default DataSource from auto-configuration is used. What is the likely cause?

A Your configuration class is not in a package scanned by Spring Boot

B Spring Boot always prefers auto-configuration over user configurations

C You must use @Primary on your DataSource bean to override auto-configuration

D You forgot to add @EnableAutoConfiguration on your class

💡 Hint

Check if Spring Boot can find your configuration class.

Spring Boot scans packages starting from the main application class package. If your custom configuration is outside this scan path, it will be ignored.

❓ lifecycle

expert

Order of auto-configuration and user configuration application

During Spring Boot startup, in what order are auto-configuration classes and user-defined @Configuration classes applied?

A Both are applied simultaneously without guaranteed order

B Auto-configuration classes override user-defined configurations always

C User-defined @Configuration classes are applied after auto-configuration classes

D Auto-configuration classes are applied after user-defined @Configuration classes

💡 Hint

Think about how Spring Boot allows users to override defaults.

Spring Boot applies auto-configuration classes first, then user-defined @Configuration classes are applied later. This allows user configurations to override auto-configured beans.