@ConfigurationProperties for type-safe config in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @ConfigurationProperties annotation in Spring Boot?

It binds external configuration properties (like those in application.properties or application.yml) to a Java class, enabling type-safe access to configuration values.

intermediate

How do you enable @ConfigurationProperties support in a Spring Boot application?

You add <code>@EnableConfigurationProperties</code> on a configuration class or use <code>@Component</code> on the properties class itself to register it as a Spring bean.

intermediate

What is the benefit of using @ConfigurationProperties over @Value annotations?

@ConfigurationProperties supports grouping related properties into a single class, supports complex types, and provides type safety and easier testing compared to scattered @Value injections.

beginner

How do you define a prefix for properties in a @ConfigurationProperties class?

You specify the prefix in the annotation like <code>@ConfigurationProperties(prefix = "app.settings")</code>. This means properties starting with <code>app.settings.</code> will be mapped to the class fields.

advanced

What must you do to ensure that a <code>@ConfigurationProperties</code> class is immutable?

Use constructor binding by adding <code>@ConstructorBinding</code> and define final fields with a constructor. This makes the configuration class immutable and thread-safe.

Which annotation is used to bind external properties to a Java class in Spring Boot?

A @Value

B @ConfigurationProperties

C @Autowired

D @ComponentScan

@ConfigurationProperties binds external config properties to a class.

How do you specify the prefix for properties in a @ConfigurationProperties class?

A Using <code>@ConfigurationProperties(prefix = "...")</code>

B Using <code>@Value(prefix = "...")</code>

C Using <code>@Component(prefix = "...")</code>

D Using <code>@EnableConfigurationProperties(prefix = "...")</code>

The prefix is set in @ConfigurationProperties(prefix = "...").

What annotation enables constructor binding for immutable configuration classes?

A @FinalBinding

B @ImmutableBinding

C @ConstructorBinding

D @ConfigBinding

@ConstructorBinding enables immutable config classes with constructor injection.

Which of these is NOT a benefit of using @ConfigurationProperties?

A Grouping related properties

B Type safety

C Supports complex nested properties

D Automatically reloads properties at runtime

@ConfigurationProperties does not automatically reload properties at runtime.

To register a @ConfigurationProperties class as a Spring bean, you can:

A Both A and B

B Add <code>@EnableConfigurationProperties</code> on a config class

C Neither A nor B

D Add <code>@Component</code> on the class

Both adding @Component or using @EnableConfigurationProperties registers the class.

Explain how @ConfigurationProperties helps in managing application configuration in Spring Boot.

Think about how you can organize and access config values easily.

Describe the steps to create an immutable configuration class using @ConfigurationProperties.

Focus on annotations and class design for immutability.

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