vishwanath jayanth vishwanath jayanth - 3 months ago 8
Ajax Question

Get MQTT data using nodejs and java asynchronously

I am trying to develop a publish/subscribe application where a Java program is a publisher and nodejs program is a subscriber. The Java client connects to MQTT server and sends some random data. The NodeJS client should subscribe to the topic the java client has registered. I am getting the data from java client on to the NodeJS console, but I have to print that data asynchronously on a web page.

Here's the code I've written.

MqttPublishSample.java

public class MqttPublishSample {

public static void main(String[] args) {

String topic = "MQTT-Examples";
String content = "HelloWorld";
int qos = 2;
int i =0;
String broker = "tcp://localhost:1883";
String clientId = "JavaSample";
MemoryPersistence persistence = new MemoryPersistence();

try {
MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
MqttConnectOptions connOpts = new MqttConnectOptions();
connOpts.setCleanSession(true);
System.out.println("Connecting to broker: " + broker);
sampleClient.connect(connOpts);
System.out.println("Connected");
while(i < 100)
{
int num = (int) (Math.random() * 2);
String text = content + num;
System.out.println("Publishing message: "+content+ " "+ num);
MqttMessage message = new MqttMessage(text.getBytes());
message.setQos(qos);
sampleClient.publish(topic, message);
System.out.println("Message published");
i++;
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
// System.out.println("Publishing message: " + content);

/*sampleClient.disconnect();
System.out.println("Disconnected");
System.exit(0);*/
} catch (MqttException me) {
System.out.println("reason " + me.getReasonCode());
System.out.println("msg " + me.getMessage());
System.out.println("loc " + me.getLocalizedMessage());
System.out.println("cause " + me.getCause());
System.out.println("excep " + me);
me.printStackTrace();
}
}
}


app.js

var express = require('express');
var app = express();

app.get('/', function (req, res) {
client.on('connect', function() {
console.log("Connection Successful");
client.subscribe('MQTT-Examples');

});
var content;
client.on('message', function (topic, message) {
content = message;
console.log(message.toString());
});
res.render('index', { title: content });
});

app.listen(3000, function () {
console.log('Example app listening on port 3000!');
});


I tried printing the message on the web page but I am unable to see any message.
I need to asynchronously print the data on the web page. Any help with how to proceed. Thanks!

Answer

try moving the line res.render('index', { title: content }); inside the callback for client.on e.g:

var express = require('express');
var app = express();

/*
 * MQTT Client
 */
var content = ""; //buffer

client.on('connect', function() {
  console.log("Connection Successful");
  client.subscribe('MQTT-Examples');
});

client.on('message', function(topic, message) {
  content += message.toString() + "<br>";
  console.log(message.toString());

});

/*
 * Express
 */
app.get('/', function(req, res) {
  res.render('index', {
    title: content
  });
});

app.listen(3000, function() {
  console.log('Example app listening on port 3000!');
});

Update

I did update to code, in the new code you will be able to see all messages received every time you reload the web page.

In the previous implementation was a kind of long polling, the resource will wait for a message from the broker.

Hope it helps.