Every real application needs different configuration for different environments — a local database for dev, a connection pool for staging, a secret manager for prod. This article covers everything Spring Boot gives you to handle this cleanly. application.properties vs application.yml Spring Boot reads configuration from src/main/resources/application.properties (or .yml) by default. Both formats express the same thing: application.properties: server.port=8080 spring.datasource.url=jdbc:postgresql://localhost:5432/orders spring.datasource.username=app spring.datasource.password=secret spring.jpa.show-sql=false application.yml: server: port: 8080 spring: datasource: url: jdbc:postgresql://localhost:5432/orders username: app password: secret jpa: show-sql: false YAML is generally preferred for nested properties — it’s less repetitive.