yml(yaml): Yet Another Markup Language --> JSON的子集；
17种姿势作配置管理(优先级递减)：
1.spring-boot-devtools.properties;
2.@TestPropertySource
3.properties
4.Command line arguments
5.Properties form SPRING_APPLICATION_JSON
6.ServletConfig
7.ServletContext
8.JNDI from java:comp/env
9.java System propertis
10.OS environment varialbles
11.a RandomValueProeprtySource
12.Profile-specific application properties outside of your packaged jar
13.Profile-specific application properties inside of your packaged jar
14.application properties outside of your packaged jar
15.application properties packaged inside your jar
16.@PropertySoruce annotation on your @Configuration classes
17.Default properteis (specified by setting SpringApplication.setDefaultProperteis).
