@Scheduled(cron = "${scheduled.jobs.cron}")
    public void sendMessages() {
        List<ScheduledJob> foundJobsToExecute = scheduledJobRepository.findByAusfuehrungsZeitpunktLessThanEqualAndStatus(LocalDateTime.now(), JobStatus.ANSTEHEND);
        log.info("started cron job, found {} jobs to execute",  foundJobsToExecute.size());
        foundJobsToExecute.forEach(job -> asyncService.executeSendAndSubscribePerJob(job));
    }

 public void executeSendAndSubscribePerJob(ScheduledJob job) {

        try {
            mqttPahoClient.connect();

            setupSecurityContextForBatchJob(job);

            setJobInAusfuehrung(job.getId());

            String sendTopic = String.format(VERSION_TOPIC, job.getRessourcenUniqueIdentifier());
            String responseTopic = "reply_" + job.getCorrelationId();
            log.info("sending message {} to topic {} ", job.getMessagePayload(), sendTopic);


            mqttPahoClient.subscribe(responseTopic, message -> {
                tempService.doStuff(message, job.getId(), responseTopic);
            });


            MqttMessage messageToSend = new MqttMessage(job.getMessagePayload().getBytes(), 2, true, createMqttProperties(responseTopic));
            mqttPahoClient.send(sendTopic, messageToSend);


        } catch (MqttException e) {
            e.printStackTrace();
        }

    }

Caused by: Untranslated MqttException - RC: 0 (0) - java.util.ConcurrentModificationException
	at org.eclipse.paho.mqttv5.client.internal.CommsCallback.run(CommsCallback.java:238)
	... 1 more
Caused by: java.util.ConcurrentModificationException
	at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1493)
	at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1526)
	at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1524)
	at org.eclipse.paho.mqttv5.client.internal.CommsCallback.deliverMessage(CommsCallback.java:610)
	at org.eclipse.paho.mqttv5.client.internal.CommsCallback.handleMessage(CommsCallback.java:481)
	at org.eclipse.paho.mqttv5.client.internal.CommsCallback.run(CommsCallback.java:224)
	... 1 more