Configuration properties are a great feature of Spring Boot for reading properties with type safety. This section will explain the concept with the following DemoApplicationProperties class file:

@ConfigurationProperties(prefix = "demo")
public class DemoApplicationProperties {

    private Integer number;

    private String username;

    private String telephoneNumber;

    private List<String> emailAddresses = 
    Arrays.asList("[email protected]");

    private String firstName;

    private String lastName;

    private Duration workingTime;

    // Getters and Setters
}

The application.yml has the following configuration properties:

demo:
  number: 10
  user-Name: shazin
  firstName: Shazin
  lAsTNamE: Sadakath
  telephone_number: "0094777329939"
  workingTime: 8h
  EMAILADDRESSES:
    - [email protected]
    - [email protected]
  addresses:
    - number: "B 22 2/3"
      city: Colombo
      street: "Ramya Place"
      country: "Sri Lanka"
      zipCode: "01000"
    - number: "123"
      city: Kandy
      street: "Dalada Weediya"
      country: "Sri Lanka"
      zipCode: "01332"

For configuration properties, an application.properties file also can be used over an application.yml file. Lately, YML files are becoming famous among developers because they can provide a structure with indentations, the ability to use collections, and so on. Spring Boot supports both at the moment.