`application.properties` is a place to set values for a Spring Boot application. It sits in `src/main/resources` and may be empty when a project is created. Add a property when you want to change a setting without writing bean-configuration code. section 1.2.2 section 6.1
Change the server port
Put `server.port=9090` in `src/main/resources/application.properties` to make the application listen on port 9090 instead of the default 8080. The equivalent YAML file, `application.yml`, expresses the same setting as `server:` followed by an indented `port: 9090`. section 6.1.1
The file is one source of configuration values, not the only one. Spring’s environment also gathers JVM system properties, operating-system environment variables, and command-line arguments, then makes the values available to beans. For example, `--server.port=9090` sets the port at launch, while `SERVER_PORT=9090` sets it through an environment variable. section 6.1.1
Properties can tune automatically configured components without declaring replacement beans:
- Set a database URL and credentials with `spring.datasource` properties; Spring Boot uses them when configuring a `DataSource`. section 6.1.2
- Set logger levels with `logging.level` properties or a log-file path with `logging.file.name`, without creating a `logback.xml` file for those changes. section 6.1.4
- Use `${spring.application.name}` inside another property value when that value should incorporate the application name. section 6.1.5
Your own beans can accept values too. Annotate a Spring bean with `@ConfigurationProperties(prefix="taco.orders")`, give it a `pageSize` property and setter, and set `taco.orders.pageSize` in configuration. An order-list controller can then use that value instead of hardcoding a page size of 20. section 6.2
For settings that differ by environment, keep common properties in `application.properties` and put profile-specific ones in a file such as `application-prod.properties`. A property for the active profile can replace a common value. YAML files support the same naming approach. section 6.3.1
If an IDE flags a custom property as unknown, the property can still work. Optional metadata in `src/main/resources/META-INF/additional-spring-configuration-metadata.json` supplies descriptions and IDE help. section 6.2.2
