What is Spring Boot's application.properties / application.yml and how does configuration work?

# application.yml (YAML format - preferred for complex config)
server:
  port: 8080
  servlet:
    context-path: /api

spring:
  application:
    name: order-service
  datasource:
    url: jdbc:postgresql://localhost:5432/orders
    username: ${DB_USER}         # reference env variable
    password: ${DB_PASSWORD}
  jpa:
    hibernate:
      ddl-auto: validate
    show-sql: false

# Bind to a typed configuration class:
# application.yml:
app:
  order:
    max-items: 50
    allowed-currencies: [USD, EUR, GBP]

// @ConfigurationProperties class:
@ConfigurationProperties(prefix = "app.order")
@Validated  // enables Bean Validation on the properties
public record OrderProperties(
    @Min(1) @Max(1000) int maxItems,
    @NotEmpty List<String> allowedCurrencies
) {}

// Register:
@SpringBootApplication
@EnableConfigurationProperties(OrderProperties.class)
public class App { ... }

// Inject:
@Service
@RequiredArgsConstructor
public class OrderService {
    private final OrderProperties props;
}