Custom auto-configuration in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mark a class as a Spring Boot auto-configuration.

Spring Boot

@[1]
public class MyAutoConfiguration {
}

Drag options to blanks, or click blank then click option'

AEnableAutoConfiguration

BComponent

CConfiguration

DSpringBootApplication

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Component instead of @Configuration

Using @EnableAutoConfiguration on the class directly

The @Configuration annotation marks the class as a source of bean definitions for Spring.

2fill in blank

medium

Complete the code to conditionally enable the auto-configuration only if a bean of type DataSource is missing.

Spring Boot

@ConditionalOnMissingBean([1].class)
public class MyAutoConfiguration {
}

Drag options to blanks, or click blank then click option'

ADataSource

BRestTemplate

CJdbcTemplate

DEntityManager

Attempts:

3 left

💡 Hint

Common Mistakes

Using JdbcTemplate instead of DataSource

Using EntityManager which is JPA specific

The @ConditionalOnMissingBean(DataSource.class) annotation ensures the configuration applies only if no DataSource bean exists.

3fill in blank

hard

Fix the error in the code to properly register the auto-configuration class in Spring Boot.

Spring Boot

META-INF/spring.factories:
org.springframework.boot.autoconfigure.EnableAutoConfiguration=[1]

Drag options to blanks, or click blank then click option'

Acom.example.MyAutoConfiguration.class

Bcom.example.MyAutoConfiguration.java

Corg.springframework.boot.autoconfigure.EnableAutoConfiguration

Dcom.example.MyAutoConfiguration

Attempts:

3 left

💡 Hint

Common Mistakes

Adding '.class' suffix

Using the annotation name instead of class name

The spring.factories file must list the fully qualified class name without '.class' or file extensions.

4fill in blank

hard

Fill both blanks to create a bean method that returns a RestTemplate only if no other RestTemplate bean exists.

Spring Boot

@Bean
@ConditionalOnMissingBean([1].class)
public [2] restTemplate() {
    return new RestTemplate();
}

Drag options to blanks, or click blank then click option'

ARestTemplate

BDataSource

DJdbcTemplate

Attempts:

3 left

💡 Hint

Common Mistakes

Using DataSource in condition but returning RestTemplate

Mismatching return type and condition class

The method returns a RestTemplate bean and is conditional on missing RestTemplate beans.

5fill in blank

hard

Fill all three blanks to create a conditional auto-configuration that loads only if property 'app.feature.enabled' is true and a bean of type FeatureService is missing.

Spring Boot

@ConditionalOnProperty(name = "app.feature.enabled", havingValue = "[1]")
@ConditionalOnMissingBean([2].class)
@Configuration
public class FeatureAutoConfiguration {
    @Bean
    public [3] featureService() {
        return new FeatureService();
    }
}

Drag options to blanks, or click blank then click option'

Atrue

BFeatureService

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'false' as property value

Mismatching bean type in condition and method return

The property must be 'true' to enable the config, and the bean type must be FeatureService for both condition and return type.

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