code: private URL getStaticFileFromAssetsDir(String fileName) {
  
          URL url = null;
  
          if (ninjaProperties.isDev() 
                  // Testing that the file exists is important because
                  // on some dev environments we do not get the correct asset dir
                  // via System.getPropery("user.dir").
                  // In that case we fall back to trying to load from classpath
                  && new File(assetsDirInDevModeWithoutTrailingSlash()).exists()) {
              String finalNameWithoutLeadingSlash = assetsControllerHelper.normalizePathWithoutLeadingSlash(fileName, false);
              File possibleFile = new File(
                      assetsDirInDevModeWithoutTrailingSlash() 
                              + File.separator 
                              + finalNameWithoutLeadingSlash);
              url = getUrlForFile(possibleFile);
          } else {
              String finalNameWithoutLeadingSlash = assetsControllerHelper.normalizePathWithoutLeadingSlash(fileName, true);
              url = this.getClass().getClassLoader()
                      .getResource(ASSETS_DIR
                                   + "/"
                                   + finalNameWithoutLeadingSlash);
          }
  
          return url;
      }
docs: Loads files from assets directory. This is the default directory
  of Ninja where to store stuff. Usually in src/main/java/assets/.