How does logging into the console works in the Java Spring framework

I have a very simple Spring application, but I can't not get a System.out.println statement to print into the console.

This is the main app file where I am printing an env variable set in a .yml file

import path.config.Config;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MainApplication {

	@Autowired
	private Config config;

	public static void main(String[] args) {
		SpringApplication app = new SpringApplication(MainApplication.class);
		app.run();
	}

	public void run(String... args) throws Exception {
		System.out.println("env: " + config.getEnv());
	}

}

The configuration file looks like this:

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Configuration;

@Configuration
@EnableConfigurationProperties
@ConfigurationProperties
public class Config {
    private String env;

    public void setEnv(String env) {
        this.env = env;
    }

    public String getEnv() {
        return this.env;
    }
}

Finally the properties yml file

spring: 
    profiles.active: dev
    h2:
        console:
            enabled: true
---
spring:
    profiles: dev
env: dev
---
spring:
    profiles: test
env: test
---
spring:
    profiles: prod
env: prod

The Spring app builds a runs fine, however, I can't see the env variable to show in the terminal. I have seen examples of people using Controllers with a Request endpoint just to debug the environment variables in the browser. Is that the only option?

Couple of changes to your code, you don't need to use new keyword for starting spring application, you can directly use static run method

 public static void main(String[] args) {
    SpringApplication.run(MainApplication.class);
  
}

Second thing the run method in MainApplication will only execute if that class implements CommandLineRunner

@SpringBootApplication
public class MainApplication implements CommandLineRunner {

    @Autowired
    private Config config;

    public static void main(String[] args) {
          SpringApplication.run(MainApplication.class);
    }

    public void run(String... args) throws Exception {
          System.out.println("env: " + config.getEnv());
     }

  }