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

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing a custom property in Spring Boot?

Given a Spring Boot application with a custom configuration property app.title=MyApp defined in application.properties and a @ConfigurationProperties class binding it, what will be the output of System.out.println(appProperties.getTitle())?

Spring Boot

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

@Component
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private String title;
    public String getTitle() { return title; }
    public void setTitle(String title) { this.title = title; }
}

// application.properties
// app.title=MyApp

// In some service or main method:
// System.out.println(appProperties.getTitle());

A app.title

B null

C MyApp

D Compilation error due to missing @EnableConfigurationProperties

💡 Hint

Remember that Spring Boot automatically binds properties with matching prefix to the fields in the class annotated with @ConfigurationProperties.

The property 'app.title' in application.properties is bound to the 'title' field in the AppProperties class because of the prefix 'app'. Spring Boot automatically injects the value 'MyApp' into the field, so calling getTitle() returns 'MyApp'.

📝 Syntax

intermediate

Which code snippet correctly enables custom configuration properties in Spring Boot?

You want to enable a class DatabaseSettings annotated with @ConfigurationProperties(prefix = "db") to be loaded as a bean. Which of the following code snippets correctly enables this?

A

@SpringBootApplication
@EnableAutoConfiguration
public class App {}

B

@SpringBootApplication
@ComponentScan(basePackages = "db")
public class App {}

C

@SpringBootApplication
@ConfigurationPropertiesScan
public class App {}

D

@SpringBootApplication
@EnableConfigurationProperties(DatabaseSettings.class)
public class App {}

💡 Hint

Use the annotation that explicitly enables configuration properties classes.

The @EnableConfigurationProperties annotation with the class as parameter tells Spring Boot to create a bean for that class and bind properties with the specified prefix. Other options either scan components or enable auto-configuration but do not specifically enable configuration properties binding.

🔧 Debug

advanced

Why does the custom configuration property not bind correctly?

Given the class below, why does the property server.port=8081 not bind to the port field?

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

@Component
@ConfigurationProperties(prefix = "server")
public class ServerConfig {
    private int port;
    public int getPort() { return port; }
    // Missing setter method
}

A Missing setter method for 'port' prevents Spring from setting the value.

B The prefix 'server' is incorrect; it should be 'server.port'.

C The class must be annotated with @Configuration instead of @Component.

D The field 'port' must be public for binding to work.

💡 Hint

Spring Boot uses setters to inject property values.

Spring Boot uses JavaBean conventions to bind properties. Without a setter method for 'port', Spring cannot set the value, so the field remains at its default (0). The prefix 'server' is correct, and @Component is valid. The field does not need to be public if there is a setter.

❓ state_output

advanced

What is the value of the list property after binding?

Given the following properties and class, what will be the value of appProperties.getServers()?

application.yml:
app:
  servers:
    - host1.example.com
    - host2.example.com

Java class:
@Component
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private List servers;
    public List getServers() { return servers; }
    public void setServers(List servers) { this.servers = servers; }
}

A Compilation error due to missing @EnableConfigurationProperties

B ["host1.example.com", "host2.example.com"]

C ["app.servers"]

D null

💡 Hint

YAML lists map to Java List fields with matching names.

The YAML list under 'app.servers' binds to the List servers field in AppProperties. The list contains two strings as defined, so getServers() returns a list with those two hostnames.

🧠 Conceptual

expert

Which statement about @ConfigurationProperties and validation is correct?

Consider a class annotated with @ConfigurationProperties(prefix = "mail") and fields annotated with @NotNull from javax.validation.constraints. Which statement is true about validation of these properties?

A Validation annotations are ignored unless @Validated is added to the class.

B Validation happens automatically without any extra annotation.

C Validation requires implementing Validator interface manually.

D Validation only works if properties are loaded from YAML, not properties files.

💡 Hint

Spring Boot requires an extra annotation to trigger validation on configuration properties.

To enable validation of @ConfigurationProperties fields using javax.validation annotations, the class must also be annotated with @Validated. Without it, validation annotations are ignored. Validation does not depend on the source format and does not require manual Validator implementation.