code: public static int getfd(FileDescriptor descriptor)
      {
          Field field = FBUtilities.getProtectedField(descriptor.getClass(), "fd");
  
          if (field == null)
              return -1;
  
          try
          {
              return field.getInt(descriptor);
          }
          catch (Exception e)
          {
              JVMStabilityInspector.inspectThrowable(e);
              logger.warn("unable to read fd field from FileDescriptor");
          }
  
          return -1;
      }
docs: Get system file descriptor from FileDescriptor object.
  @param descriptor - FileDescriptor objec to get fd from
  @return file descriptor, -1 or error