Configuration & application.yml

application.properties vs application.yml

Both configure the app; YAML is more readable for nested config (and is the more common modern choice).

# application.yml
server:
  port: 8080

spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/fintrack
    username: ${DB_USER}
    password: ${DB_PASSWORD}
  jpa:
    hibernate:
      ddl-auto: validate
    show-sql: true

app:
  jwt:
    secret: ${JWT_SECRET}
    expiration-ms: 3600000

# equivalent application.properties
server.port=8080
spring.datasource.url=jdbc:postgresql://localhost:5432/fintrack

Externalizing Secrets with Environment Variables

Never hardcode secrets. Use ${ENV_VAR} placeholders and supply them via:

• .env file + Docker Compose env_file
• CI/CD secret store (GitHub Actions secrets)
• Cloud provider secret manager (Oracle Cloud Vault, AWS Secrets Manager)

Type-Safe Configuration with @ConfigurationProperties

Instead of scattering @Value("${app.jwt.secret}") everywhere, bind a whole config block to a class:

@Configuration
@ConfigurationProperties(prefix = "app.jwt")
public class JwtProperties {
    private String secret;
    private long expirationMs;
    // getters/setters
}

Then inject JwtProperties wherever needed — it's testable, IDE-autocompletes, and validates types at startup.

@Value for One-Off Values

@Value("${server.port}")
private int port;

Fine for a single simple value; prefer @ConfigurationProperties for related groups of values.

Profile-Specific Configuration (preview)

application.yml           # common config
application-dev.yml       # dev overrides
application-prod.yml      # prod overrides

Activate with spring.profiles.active=dev (env var, JVM arg, or in the base YAML). Full details in the Profiles topic later.

Configuration Precedence (high to low, common ones)

1. Command-line arguments (--server.port=9090)
2. SPRING_APPLICATION_JSON env var
3. OS environment variables
4. application-{profile}.yml
5. application.yml
6. @PropertySource annotated values
7. Default values in code