@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.