code: public void sendKeepAlive(final Application _application)
      {
          final long reference = new Date().getTime();
          final ConcurrentMap<String, IKey> sessionId2key = _application.getMetaData(ConnectionRegistry.SESSION2KEY);
          final ConcurrentMap<String, Long> keepalive = _application.getMetaData(ConnectionRegistry.KEEPALIVE);
          if (keepalive != null) {
              for (final Entry<String, Long> entry : keepalive.entrySet()) {
                  if (reference - entry.getValue()
                                  > Configuration.getAttributeAsInteger(ConfigAttribute.WEBSOCKET_KATH) * 1000) {
                      final IKey key = sessionId2key.get(entry.getKey());
                      if (key != null) {
                          final IWebSocketConnectionRegistry registry = WebSocketSettings.Holder.get(
                                          _application).getConnectionRegistry();
                          final IWebSocketConnection conn = registry.getConnection(_application, entry.getKey(), key);
                          if (conn != null) {
                              try {
                                  conn.sendMessage(KeepAliveBehavior.MSG);
                                  ConnectionRegistry.LOG.debug("Send KeepAlive for Session: {}", entry.getKey());
                              } catch (final IOException e) {
                                  ConnectionRegistry.LOG.error("Catched error", e);
                              }
                          }
                      }
                  }
              }
          }
      }
docs: Send the KeepAlive.
  @param _application Application the KeepAlive will be send for