Default properties provided by Spring Boot’s SpringApplication.setDefaultProperties() method.

@PropertySource-annotated @Configuration classes.

Config data (such as application.properties files).

A RandomValuePropertySource that has properties only in random.*.

OS environment variables.

Java system properties (System.getProperties()).

JNDI attributes from java:comp/env.

ServletContext init parameters.

ServletConfig init parameters.

Properties from SPRING_APPLICATION_JSON (inline JSON embedded in an environment variable or system property).

Command-line arguments.

The properties attribute on your tests. This is available with the @SpringBootTest annotation and also slice-based testing (which we’ll cover later in Chapter 5, Testing with Spring Boot).

@TestPropertySource annotations on your tests.

DevTools global settings properties (the $HOME/.config/spring-boot directory when Spring Boot DevTools is active).

https://docs.spring.io/spring-boot/docs/2.1.9.RELEASE/reference/html/boot-features-external-config.html