application.properties vs application.yml. Property hierarchy and override order.

application.properties and application.yml are two formats for the same thing — externalized configuration — and Boot binds both into the same Environment. The real interview content isn't the syntax; it's the override order that decides which value wins when a key is defined in several places.

The two formats

# application.properties
spring.datasource.url=jdbc:postgresql://localhost/shop
spring.jpa.hibernate.ddl-auto=validate
logging.level.com.acme=DEBUG

# application.yml — hierarchical, less repetition
spring:
  datasource:
    url: jdbc:postgresql://localhost/shop
  jpa:
    hibernate:
      ddl-auto: validate
logging:
  level:
    com.acme: DEBUG

YAML is nicer for nested structure and lists; .properties is flat and avoids YAML's indentation/type gotchas (on/off/yes parse as booleans). If both files exist, .properties is loaded after .yml, so a key in .properties overrides the same key in .yml. Don't rely on that — pick one format.

The property hierarchy (override order)

Boot layers many property sources; later sources override earlier ones. Highest-priority first, roughly:

1. Command-line arguments (--server.port=9090)
2. SPRING_APPLICATION_JSON (inline JSON)
3. OS environment variables (SERVER_PORT=9090 — relaxed binding)
4. Java system properties (-Dserver.port=9090)
5. Profile-specific files outside the jar (application-prod.yml next to the jar)
6. Profile-specific files inside the jar
7. Plain application.yml/.properties outside the jar
8. Plain files inside the jar
9. @PropertySource on @Configuration
10. Default properties (SpringApplication.setDefaultProperties)

Two rules cover most cases: command line / env vars beat files, and profile-specific beats plain, external beats packaged.

Relaxed binding

A single property is reachable under several spellings, which is why env vars work:

spring.datasource.url   ==   SPRING_DATASOURCE_URL   ==   spring.datasource_url

Kebab-case (my-prop), camelCase, and UPPER_UNDERSCORE all bind to the same target.

Profile groups and multi-document files

A single YAML file can hold several profile-specific documents separated by ---:

spring:
  config:
    activate:
      on-profile: prod
server:
  port: 8443