Configuration and Profiles Questions

application.properties vs application.yml?

Both set the same properties. .properties is flat key=value; .yml is hierarchical and less repetitive, and one YAML file can hold several profile documents separated by ---. If both exist, .properties wins for the same key. YAML is indentation-sensitive, so typos fail silently.

What are profiles and how do you activate one?

Profiles group configuration and beans per environment (dev, test, prod). Boot loads application.yml then application-{profile}.yml, which overrides it. Beans can be limited with @Profile("prod").

Example

Ways to activate:

java -jar shopease.jar --spring.profiles.active=prod
SPRING_PROFILES_ACTIVE=prod java -jar shopease.jar

Example output

The following 1 profile is active: "prod"

@Value vs @ConfigurationProperties?

@Value	@ConfigurationProperties
one property at a time: `@Value("${app.tax-rate}")`	binds a whole prefix to a typed class/record
supports SpEL	no SpEL, but relaxed binding (`tax-rate` = `TAX_RATE`)
no validation	can be `@Validated`; IDE auto-complete with metadata

@ConfigurationProperties(prefix = "shopease.payment")
@Validated
public record PaymentProperties(@NotBlank String apiKey, Duration timeout, int maxRetries) { }

// enable with @ConfigurationPropertiesScan on the main class

What is the order of precedence for configuration sources?

Higher overrides lower (simplified):

Command-line arguments (--server.port=9000)
SPRING_APPLICATION_JSON, Java system properties (-D)
OS environment variables (SERVER_PORT=9000)
Profile-specific files outside the JAR, then inside the JAR
application.yml/.properties outside the JAR, then inside the JAR
@PropertySource and default properties

Tip: This is why the same JAR runs in dev and prod: only env vars / arguments change.

How do you keep secrets like DB passwords out of the code?

Never commit them. Reference environment variables with placeholders, and inject the values from the platform: Docker/Kubernetes secrets, AWS Secrets Manager / GCP Secret Manager, HashiCorp Vault (Spring Cloud Vault), or a Config Server with encryption.

spring:
  datasource:
    url: ${DB_URL}
    username: ${DB_USER}
    password: ${DB_PASSWORD}

How do you load a bean only when a property is set?

@Bean
@ConditionalOnProperty(name = "shopease.sms.enabled", havingValue = "true")
public SmsSender twilioSmsSender() { return new TwilioSmsSender(); }

Useful for feature flags and switching implementations per environment without code changes.