Spring Boot 有一个非常强的能力：外部化配置。
最常见的是 application.yml：
spring:
application:
name: city-hall
datasource:
url: jdbc:postgresql://localhost:5432/city_hall
username: city
password: city
jpa:
hibernate:
ddl-auto: update
open-in-view: false
management:
endpoints:
web:
exposure:
include: health,info,metrics,prometheus
city:
approval:
max-daily-approvals: 200
timeout: 5s配置覆盖心智可以先记成：
默认值
< application.yml
< application-{profile}.yml
< 环境变量
< 命令行参数