title: Get value from application.properties without Main application
body: <p>I' m working in a Spring Boot project, consisted in several applications booted with embedded Tomcat and a unique application used only as library for running applications. In that library application I need to retrieve a value from application.properties file for a variable. Being it a library application, it has not a Main class, so the variable will be always null.
  My try was these:</p>
  
  <pre><code> public class AuthAdapter implements IAuthAdapter {
  
      @Value("${signin.key}")
      private String key;
  
      @Override
      public String getSigningKey() {
          return key;
      }
  }
  </code></pre>
  
  <p>When the getSigninKey() is invoked by an application that uses it, the variable key is null.
  How can I do to fill the variable key with the value present on appliaction.properties file, considering the situation explained before?</p>