SpringBoot裡的官方文檔叫做Externalized Configuration：
優先級依次如下：
(1) Devtools global settings properties in the $HOME/.config/spring-boot folder when devtools is active.
(2) @TestPropertySource annotations on your tests.
(3) properties attribute on your tests. Available on @SpringBootTest and the test annotations for testing a particular slice of your application.
(4) Command line arguments.
(5) Properties from SPRING_APPLICATION_JSON (inline JSON embedded in an environment variable or system property).
(6) ServletConfig init parameters.
(7) ServletContext init parameters.
(8) JNDI attributes from java:comp/env.
(9) Java System properties (System.getProperties()).
(10) OS environment variables.
(11) A RandomValuePropertySource that has properties only in random.*.
(12) Profile-specific application properties outside of your packaged jar (application-{profile}.properties and YAML variants).
(13) Profile-specific application properties packaged inside your jar (application-{profile}.properties and YAML variants).
(14) Application properties outside of your packaged jar (application.properties and YAML variants).
(15) Application properties packaged inside your jar (application.properties and YAML variants).
(16) @PropertySource annotations on your @Configuration classes. Please note that such property sources are not added to the Environment until the application context is being refreshed. This is too late to configure certain properties such as logging.* and spring.main.* which are read before refresh begins.(17) Default properties (specified by setting SpringApplication.setDefaultProperties).
做個實驗，在run as configuration裡，program argument設定為server.port=8001,
Environment環境變量設定為server.port=8002.
在SpringBoot項目内部的Application.properties檔案設定成8000：
最後運作時，生效的端口是環境變量設定進去的8002：
在shell裡使用set指令設定環境變量，也能按照期望的方式工作：