Reading .env and properties.yml files based on profile in Spring Boot?

I am using .env and properties.yml files in my Spring Boot apps and need to be clarified for using them properly. After that, I will also use the other profiles of these files e.g. .env-dev and properties-dev.yml.

Could you please explain these issues?

1. As far as I know, these files are automatically read based on the Run/Debug profile of Intellij or maven. If the active profile is dev, only .env-dev and properties-dev.yml files are read, if the profile is prod, only .env-prod and properties-prod.yml files are read. If the profile "dev,prod", then both of these files are read. Is that true?

2. What if there are only .env and properties.yml files in the project. Then, are these files always read when profile is selected or not?

3. Can I read environment variables from .env file when running/debugging app via Maven by giving file name to the following command?

mvn spring-boot:run -Dspring-boot.run.jvmArguments="-Xdebug" 
-Dspring-boot.run.profiles=dev -Dspring-boot.run.arguments="DB_NAME=employee_db 
DB_USERNAME=postgres DB_PASSWORD=******"

Don't panic! It is/must be working...

Start simple:

1. quickstart-maven, dependencies: web (only)

2. Just a demo (no/empty application.properties, according to foo.bar, foo.baz we can refer to any common/custom "property" ;):

   package com.example.demo;
   
   import org.springframework.beans.factory.InitializingBean;
   import org.springframework.beans.factory.annotation.Value;
   import org.springframework.boot.SpringApplication;
   import org.springframework.boot.autoconfigure.SpringBootApplication;
   import org.springframework.context.annotation.Bean;
   
   @SpringBootApplication
   public class DemoConfigApplication {
   
     public static void main(String[] args) {
       SpringApplication.run(DemoConfigApplication.class, args);
     }
   
     @Bean
     InitializingBean test(
         @Value("${foo.bar:undefined}") String fooBar,
         @Value("${foo.baz:undefined}") String fooBaz
     ) {
       return () -> {
         System.err.format("foo.bar: %s%n", fooBar);
         System.err.format("foo.baz: %s%n", fooBaz);
       };
     }
   
   }
   

3. Build

   ./mvnw clean test
   

   prints:

   ...
   foo.bar:	undefined
   foo.baz:	undefined
   ...
   

To set these via (springboot-)maven-plugin, we can:

((Git)Ba)sh

1. Via "Command Line Arguments"
   (with blanks, with my win+gitbash:

   GNU bash, version 5.2.12(1)-release (x86_64-pc-msys)
   

   )

   Syntax:

   ./mvnw spring-boot:run -Dspring-boot.run.arguments="--foo.bar='Foo Bar' --foo.baz='Foo Baz'"
   

   (prints):

   ...
   2023-03-15T13:45:52.849+01:00  INFO 4196 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1106 ms
   foo.bar: Foo Bar
   foo.baz: Foo Baz
   ...
   

2. Via "System Properties" (same git bash):

   Syntax:

   ./mvnw spring-boot:run -Dspring-boot.run.jvmArguments="-Dfoo.bar='Foo Bar' -Dfoo.baz='Foo Baz'"
   

To make (e.g.) 1. work in

cmd.exe

(Version 10.0.19045.2673), same quoting as gitbash:

.\mvnw.cmd spring-boot:run -Dspring-boot.run.arguments="--foo.bar='Foo Bar' --foo.baz='Foo Baz'"

PowerShell

(v7.3.3)
More complicated, due to/with ref:

.\mvnw.cmd spring-boot:run '-Dspring-boot.run.arguments="--foo.bar=""Foo Bar"" --foo.baz=""Foo Baz"""'

...