code: private static void appendThrown(StringBuilder message, LogRecord event)
      {
          final Throwable thrown = event.getThrown();
          if (thrown != null)
          {
              final StringWriter sw = new StringWriter();
              thrown.printStackTrace(new PrintWriter(sw));
              message.append(sw);
          }
      }
docs: Append thrown exception trace.
  
  @param message The message builder.
  @param event The log record.