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application.yml vs application.properties in Spring Boot

How Spring reads externalized configuration, when to use YAML or properties, and a typical starter application.yml for this series.

Author: Sushil Kumar

Spring Bootapplication.yml

application.yml vs application.properties in Spring Boot

Introduction

Spring Boot externalizes configuration. You can use application.properties (flat key=value) or application.yml (hierarchical, readable). Both work. This series uses YAML for nested blocks (datasource, JPA, logging) without repeating long prefixes.

Real-world explanation

  • Properties — One line per key. spring.datasource.url=jdbc:... is fine for a few lines; it gets long when you add 30 keys.
  • YAML — Indentation defines tree structure. Easy to read and comment. Watch indentation (use spaces, not tabs, per YAML spec).
  • Order of loading — application.properties and application.yml in src/main/resources both load. Profile-specific files application-dev.yml override base keys when dev is active. Same rules apply to .properties.

Step-by-step: one base file

src/main/resources/application.yml (skeleton, filled more in next articles)

spring:
  application:
    name: demo-service
  jpa:
    hibernate:
      ddl-auto: validate
    show-sql: false
    open-in-view: false
  datasource:
    url: ${SPRING_DATASOURCE_URL:jdbc:postgresql://localhost:5432/demo}
    username: ${SPRING_DATASOURCE_USERNAME:demo}
    password: ${SPRING_DATASOURCE_PASSWORD:demo}
server:
  port: 8080

Same in properties (for comparison)

spring.application.name=demo-service
spring.jpa.hibernate.ddl-auto=validate
server.port=8080

Why we prefer YAML here

  • Datasource and JPA sit under spring, easier to scan in code review
  • You can add spring.profiles: in YAML you often do profile-specific application-prod.yml (article 21)

Common mistakes

  • Tab characters in YAML: parsers fail with obscure errors. Use 2 spaces as team standard.
  • Two keys for the same thing in two files; last loaded wins, which is confusing in debugging.
  • Putting secrets in the repo. Use environment variables: ${VAR:default} as above.

Best practices

  • ddl-auto: validate or none in production; update is only a dev convenience, know the trade-off.
  • Disable spring.jpa.open-in-view in APIs—prevents lazy loading surprises in controllers.

Final summary

Use .yml or .properties; pick one style per project. This series standardizes on application.yml, profile files like application-dev.yml, and placeholders for secrets. Next: configuration discipline beyond syntax.