code: public File getHudsonWar() {
          String war = SystemProperties.getString("executable-war");
          if(war!=null && new File(war).exists())
              return new File(war);
          return null;
      }
docs: If the location of {@code jenkins.war} is known in this life cycle,
  return it location. Otherwise return null to indicate that it is unknown.
  
  <p>
  When a non-null value is returned, Hudson will offer an upgrade UI
  to a newer version.