@ConfigurationProperties for type-safe config in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when binding properties with @ConfigurationProperties?

Given the following Spring Boot configuration class and application.properties, what will be the value of appConfig.getTimeout() after the application starts?

AppConfig.java:

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

@Component
@ConfigurationProperties(prefix = "app")
public class AppConfig {
    private int timeout;
    public int getTimeout() { return timeout; }
    public void setTimeout(int timeout) { this.timeout = timeout; }
}

application.properties:

app.timeout=5000

Spring Boot

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

@Component
@ConfigurationProperties(prefix = "app")
public class AppConfig {
    private int timeout;
    public int getTimeout() { return timeout; }
    public void setTimeout(int timeout) { this.timeout = timeout; }
}

A null

B 0

C Throws a runtime exception

D 5000

💡 Hint

Check how the prefix in @ConfigurationProperties matches the property keys.

The property 'app.timeout=5000' matches the prefix 'app' and binds to the 'timeout' field. Spring Boot automatically converts the string '5000' to int and sets it.

📝 Syntax

intermediate

Which option correctly defines a nested property class for @ConfigurationProperties?

You want to bind nested properties under prefix 'server' like server.address.host=localhost and server.address.port=8080. Which of the following nested class definitions inside a @ConfigurationProperties class is syntactically correct?

Spring Boot

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

@Component
@ConfigurationProperties(prefix = "server")
public class ServerConfig {
    private Address address = new Address();
    public Address getAddress() { return address; }
    public void setAddress(Address address) { this.address = address; }

    public static class Address {
        private String host;
        private int port;
        // getters and setters
    }
}

A public static class Address { private String host; private int port; public String getHost() { return host; } public void setHost(String host) { this.host = host; } public int getPort() { return port; } public void setPort(int port) { this.port = port; } }

B public class Address { public String host; public int port; }

C static class Address { private String host; private int port; }

D public static class Address { private String host; private int port; }

💡 Hint

Remember that nested classes for binding need proper getters and setters.

Option A correctly defines a public static nested class with private fields and public getters and setters, which is required for Spring Boot to bind nested properties.

🔧 Debug

advanced

Why does this @ConfigurationProperties class fail to bind properties?

Consider this class:

@Component
@ConfigurationProperties(prefix = "mail")
public class MailConfig {
    private String host;
    private int port;
}

Given mail.host=smtp.example.com and mail.port=25 in application.properties, why are the properties not bound?

Spring Boot

@Component
@ConfigurationProperties(prefix = "mail")
public class MailConfig {
    private String host;
    private int port;
}

A Fields are private but missing getters and setters, so Spring Boot cannot bind them.

B The prefix 'mail' does not match the property keys.

C The class is missing @EnableConfigurationProperties annotation.

D The fields should be static for binding to work.

💡 Hint

Check how Spring Boot binds properties to fields.

Spring Boot requires either getters/setters for private fields, public fields, or annotations like @ConstructorBinding. Private fields without getters/setters are ignored by default.

🧠 Conceptual

advanced

What is the effect of using @ConstructorBinding with @ConfigurationProperties?

When you annotate a class with @ConfigurationProperties and @ConstructorBinding, what changes in how Spring Boot binds properties?

A It disables binding and requires manual property injection.

B Properties are bound only if setters are present.

C Spring Boot uses the constructor parameters to bind properties instead of setters.

D Binding happens only for static fields.

💡 Hint

Think about immutable configuration classes.

@ConstructorBinding tells Spring Boot to bind properties by calling the constructor with matching parameters, enabling immutable configuration classes without setters.

❓ state_output

expert

What is the value of 'enabled' after binding with relaxed binding rules?

Given this configuration class:

@Component
@ConfigurationProperties(prefix = "feature")
public class FeatureConfig {
    private boolean enabled;
    public boolean isEnabled() { return enabled; }
    public void setEnabled(boolean enabled) { this.enabled = enabled; }
}

And this property in application.properties:

feature.ENABLED=true

What will be the value of featureConfig.isEnabled() after startup?

Spring Boot

@Component
@ConfigurationProperties(prefix = "feature")
public class FeatureConfig {
    private boolean enabled;
    public boolean isEnabled() { return enabled; }
    public void setEnabled(boolean enabled) { this.enabled = enabled; }
}

A false

B true

C null

D Throws a binding exception

💡 Hint

Spring Boot property binding is case-insensitive and supports relaxed binding.

Spring Boot's relaxed binding allows property keys to be uppercase or lowercase and still bind correctly. So 'feature.ENABLED' binds to 'enabled' field.

Practice

1. What is the main purpose of using @ConfigurationProperties in a Spring Boot application?

easy

A. To create REST endpoints automatically

B. To bind external configuration properties to a Java class for type-safe access

C. To define database entities

D. To handle HTTP requests and responses

Solution

1. Step 1: Understand the role of @ConfigurationProperties

   This annotation is used to map properties from configuration files (like application.properties or application.yml) into Java classes.

2. Step 2: Identify the benefit of type-safe access

   By binding properties to a class, you avoid manual parsing and get compile-time checking of property names and types.

3. Final Answer:

   To bind external configuration properties to a Java class for type-safe access -> Option B

4. Quick Check:

   @ConfigurationProperties = type-safe config binding [OK]

Hint: Remember: @ConfigurationProperties maps config to Java class [OK]

Common Mistakes:

• Confusing @ConfigurationProperties with REST controllers
• Thinking it creates database entities
• Assuming it handles HTTP requests

2. Which of the following is the correct way to enable @ConfigurationProperties support in a Spring Boot application?

easy

A. Add @RestController on the configuration class

B. Annotate the class with @Entity

C. Annotate the class with @Component or use @EnableConfigurationProperties on a configuration class

D. Use @Service annotation on the properties class

Solution

1. Step 1: Identify how Spring Boot detects @ConfigurationProperties classes

   Spring Boot requires the class to be a Spring bean, which can be done by annotating it with @Component or by enabling it via @EnableConfigurationProperties.

2. Step 2: Recognize incorrect annotations

   @Entity is for JPA entities, @RestController is for web controllers, and @Service is for service layer beans, none of which enable configuration properties binding.

3. Final Answer:

   Annotate the class with @Component or use @EnableConfigurationProperties on a configuration class -> Option C

4. Quick Check:

   Enable with @Component or @EnableConfigurationProperties [OK]

Hint: Use @Component or @EnableConfigurationProperties to activate binding [OK]

Common Mistakes:

• Using @Entity or @RestController instead of @Component
• Forgetting to register the properties class as a bean
• Assuming @Service enables configuration binding

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

application.yml:
app:
  timeout: 5000

Java class:
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private int timeout;
    public int getTimeout() { return timeout; }
    public void setTimeout(int timeout) { this.timeout = timeout; }
}

medium

A. 5000

B. 0

C. null

D. Throws an exception

Solution

1. Step 1: Match property prefix and field

   The prefix 'app' matches the class prefix, and 'timeout' matches the field name, so the value 5000 binds correctly.

2. Step 2: Understand default value and binding

   Since the property is set in the YAML, the field will be set to 5000, not the default 0 or null.

3. Final Answer:

   5000 -> Option A

4. Quick Check:

   Property value binds correctly = 5000 [OK]

Hint: Match prefix and field names exactly for binding [OK]

Common Mistakes:

• Assuming default 0 because of missing @Value
• Confusing null with primitive int default
• Expecting exception without validation

4. Consider this configuration class:

@ConfigurationProperties(prefix = "server")
public class ServerConfig {
    private String host;
    private int port;

    public String getHost() { return host; }
    public void setHost(String host) { this.host = host; }

    public int getPort() { return port; }
    public void setPort(int port) { this.port = port; }
}

If the application.yml contains:

server:
  host: localhost
  port: notANumber

What will happen when Spring Boot starts?

medium

A. Port is set to 0 by default, application starts normally

B. Application starts, port is set to string 'notANumber'

C. Port is ignored, host is set, application starts with warning

D. Application fails to start with a binding exception due to invalid port value

Solution

1. Step 1: Analyze property type mismatch

   The 'port' field is an int, but the YAML provides a non-numeric string 'notANumber', causing a type mismatch.

2. Step 2: Understand Spring Boot binding behavior

   Spring Boot throws a binding exception on startup if it cannot convert the property to the required type.

3. Final Answer:

   Application fails to start with a binding exception due to invalid port value -> Option D

4. Quick Check:

   Invalid type causes startup failure [OK]

Hint: Type mismatch in config causes startup failure [OK]

Common Mistakes:

• Assuming default values are used silently
• Expecting warnings instead of errors
• Thinking string can be assigned to int field

5. You want to create a nested configuration properties class to map this YAML:

app:
  security:
    enabled: true
    roles:
      - USER
      - ADMIN

Which of the following Java class structures correctly uses @ConfigurationProperties to map this nested config?

hard

A. public class AppProperties {
private Security security;
public static class Security {
private boolean enabled;
private List<String> roles;
// getters and setters
}
// getters and setters
}

@ConfigurationProperties(prefix = "app")
@Component
public class AppProperties {}

B. @ConfigurationProperties(prefix = "app.security")
@Component
public class Security {
private boolean enabled;
private List<String> roles;
// getters and setters
}

C. @ConfigurationProperties(prefix = "app")
@Component
public class AppProperties {
private boolean enabled;
private List<String> roles;
// getters and setters
}

D. @ConfigurationProperties(prefix = "security")
@Component
public class Security {
private boolean enabled;
private List<String> roles;
// getters and setters
}

Solution

1. Step 1: Understand nested property mapping

   To map nested properties, create a nested class inside the main properties class and define fields matching the nested keys.

2. Step 2: Match prefix and class structure

   The prefix 'app' matches the root, and the nested Security class maps the 'security' subtree with 'enabled' and 'roles'. The main class must be annotated with @ConfigurationProperties and registered as a bean.

3. Step 3: Evaluate options

   public class AppProperties { private Security security; public static class Security { private boolean enabled; private List<String> roles; // getters and setters } // getters and setters } @ConfigurationProperties(prefix = "app") @Component public class AppProperties {} correctly nests Security inside AppProperties and uses prefix 'app'. @ConfigurationProperties(prefix = "app.security") @Component public class Security { private boolean enabled; private List<String> roles; // getters and setters } uses prefix 'app.security' but does not nest. @ConfigurationProperties(prefix = "app") @Component public class AppProperties { private boolean enabled; private List<String> roles; // getters and setters } flattens properties incorrectly. @ConfigurationProperties(prefix = "security") @Component public class Security { private boolean enabled; private List<String> roles; // getters and setters } uses wrong prefix.

4. Final Answer:

   Option A with nested Security class inside AppProperties and prefix 'app' -> Option A

5. Quick Check:

   Nested class + prefix 'app' = correct mapping [OK]

Hint: Use nested classes for nested config with root prefix [OK]

Common Mistakes:

• Using wrong prefix for nested properties
• Flattening nested properties into one class
• Not annotating main class with @ConfigurationProperties