Generally software applications, we need to be able to run it under different environments or under different setups. For example, say our application sends emails. For sending emails we need to configure SMTP servers with correct server URL, user, password etc. While developing we will generally have different SMTP servers credentials then the Production servers. Let's assume we have two different kinds of environments, dev- Development and prod-Production. So we would like to run the same code in dev and prod environments but with different configurations for email. Similarly we might have similar requirements for database, logs, security, etc, where we want to have different services/configurations plugged while in developing vs in productions. Spring Profiles helps to segregate our application configurations and make them available only in certain environments. 

application-{_profile_}.properties

mail.smtp.host=mail.dev.com
mail.smtp.port=7777  
mail.smtp.user=test_dev 

mail.smtp.host=mail.prod.com
mail.smtp.port=8888  
mail.smtp.user=prod_user

#Other configurations
spring.profiles.active=dev
logging.level.root=info

@Component
@PropertySource("classpath:application-${spring.profiles.active}.properties")
public class MailSenderProperties {

  @Value("${mail.smtp.host}")
  private String smtpHost;

  @Value("${mail.smtp.port}")
  private Integer smtpPort;

  public String getSmtpHost() {
    return smtpHost;
  }

  public void setSmtpHost(String smtpHost) {
    this.smtpHost = smtpHost;
  }

  public Integer getSmtpPort() {
    return smtpPort;
  }

  public void setSmtpPort(Integer smtpPort) {
    this.smtpPort = smtpPort;
  }
}