docs: @Override
      public Collection<PropertySourceLoader> getPropertySourceLoaders() {
          Collection<PropertySourceLoader> propertySourceLoaderList = this.propertySourceLoaderList;
          if (propertySourceLoaderList == null) {
              synchronized (this) { // double check
                  propertySourceLoaderList = this.propertySourceLoaderList;
                  if (propertySourceLoaderList == null) {
                      propertySourceLoaderList = evaluatePropertySourceLoaders();
                      this.propertySourceLoaderList = propertySourceLoaderList;
                  }
              }
          }
          return propertySourceLoaderList;
      }