@ConfigurationProperties for type-safe config in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable binding of properties to the class.

Spring Boot

@[1]("app.settings")
public class AppSettings {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

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

AComponent

BConfigurationProperties

CService

DRepository

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Component instead of @ConfigurationProperties

Forgetting to specify the prefix in the annotation

The @ConfigurationProperties annotation binds external properties to this class using the specified prefix.

2fill in blank

medium

Complete the code to register the configuration properties class as a Spring bean.

Spring Boot

@Bean
public [1] appSettings() {
    return new AppSettings();
}

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

AAppSettings

BComponent

CConfigurationProperties

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the annotation name instead of the class

Returning a Spring stereotype annotation instead of the class

The method returns an instance of the AppSettings class to register it as a bean.

3fill in blank

hard

Fix the error in the class to properly bind nested properties.

Spring Boot

public class DatabaseProperties {
    private String url;
    private Credentials [1];

    public static class Credentials {
        private String username;
        private String password;

        public String getUsername() {
            return username;
        }

        public void setUsername(String username) {
            this.username = username;
        }

        public String getPassword() {
            return password;
        }

        public void setPassword(String password) {
            this.password = password;
        }
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public Credentials getCredentials() {
        return credentials;
    }

    public void setCredentials(Credentials credentials) {
        this.credentials = credentials;
    }
}

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

Acredentials

Bcredential

CCredentials

DcredentialInfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase for the field name

Using a different name than the getter/setter methods

The field name must match the getter/setter and property name to bind nested properties correctly.

4fill in blank

hard

Fill both blanks to complete the annotation and enable validation.

Spring Boot

@[1]("server.config")
@[2]
public class ServerConfig {
    @NotNull
    private Integer port;

    public Integer getPort() {
        return port;
    }

    public void setPort(Integer port) {
        this.port = port;
    }
}

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

AConfigurationProperties

BValidated

CComponent

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting @Validated annotation

Using @Component instead of @ConfigurationProperties

Use @ConfigurationProperties to bind properties and @Validated to enable validation annotations like @NotNull.

5fill in blank

hard

Fill all three blanks to create a type-safe configuration with nested properties and validation.

Spring Boot

@[1]("mail.settings")
@[2]
public class MailSettings {
    private Smtp smtp;

    public static class Smtp {
        @NotEmpty
        private String host;

        @Min([3])
        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;
        }
    }

    public Smtp getSmtp() {
        return smtp;
    }

    public void setSmtp(Smtp smtp) {
        this.smtp = smtp;
    }
}

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

AConfigurationProperties

BValidated

C1

DComponent

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting @Validated annotation

Setting @Min to 0 or negative values

Using @Component instead of @ConfigurationProperties

Use @ConfigurationProperties to bind, @Validated to enable validation, and @Min(1) to ensure port is at least 1.

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