code: public void addConnection(ServerSentEventConnection connection) {
          Objects.requireNonNull(connection, Required.CONNECTION.toString());
  
          final String url = RequestUtils.getServerSentEventURL(connection);
          Set<ServerSentEventConnection> uriConnections = getConnections(url);
          if (uriConnections == null) {
              uriConnections = new HashSet<>();
              uriConnections.add(connection);
          } else {
              uriConnections.add(connection);
          }
          setConnections(url, uriConnections);
      }
docs: Adds a new connection to the manager
  
  @param connection The connection to put