code: public void sendDirect(EventMessage eventMessage) {
  		Assert.notNull(eventMessage.getWebSocketSessionId(),
  				"WebSocket session id must not be null");
  
  		long timeout = this.sendTimeout;
  		boolean sent = timeout >= 0
  				? this.clientOutboundChannel.send(eventMessage, timeout)
  				: this.clientOutboundChannel.send(eventMessage);
  
  		if (!sent) {
  			throw new MessageDeliveryException(eventMessage,
  					"Failed to direct send message with destination '"
  							+ eventMessage.getDestination() + "' within timeout: "
  							+ timeout);
  		}
  	}
docs: Send an EventMessage directly to the client (
  {@link EventMessage#getWebSocketSessionId()}).
  <p>
  In contrast to {@link #send(EventMessage)} this method does not check if the
  receiver is subscribed to the destination. The {@link SimpleBrokerMessageHandler}
  is not involved in sending this message.
  
  @param eventMessage The event message
  
  @see #send(EventMessage)