如何使用MQTT发布数据

Question

I used this docker image to install Mosquitto MQTT. Now it's running and showing the following message in the terminal:

1515680808: mosquitto version 1.4.14 (build date Mon, 10 Jul 2017 23:48:43 +0100) starting
1515680808: Config loaded from /mqtt/config/mosquitto.conf.
1515680808: Opening websockets listen socket on port 9001.
1515680808: Opening ipv4 listen socket on port 1883.
1515680808: Opening ipv6 listen socket on port 1883.

Then I created a simple Maven project:

<dependency>
    <groupId>org.apache.spark</groupId>
    <artifactId>spark-streaming-mqtt_2.11</artifactId>
    <version>1.6.3</version>
</dependency>

I tried to publish some data to a topic using the code shown below. I point to localhost:1883 as the MqttBrokerUrl and a topic test . However, I get this error:

Exception in thread "main" java.lang.NullPointerException at org.eclipse.paho.client.mqttv3.MqttConnectOptions.validateURI(MqttConnectOptions.java:457) at org.eclipse.paho.client.mqttv3.MqttAsyncClient.(MqttAsyncClient.java:273) at org.eclipse.paho.client.mqttv3.MqttAsyncClient.(MqttAsyncClient.java:167) at org.eclipse.paho.client.mqttv3.MqttClient.(MqttClient.java:224) at org.test.MQTTPublisher$.main(MQTTPublisher.scala:37) at org.test.MQTTPublisher.main(MQTTPublisher.scala)

Code:

package org.test

import org.apache.log4j.{Level, Logger}
import org.eclipse.paho.client.mqttv3._
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence
import org.apache.spark.storage.StorageLevel
import org.apache.spark.streaming.{Seconds, StreamingContext}
import org.apache.spark.streaming.mqtt._
import org.apache.spark.SparkConf

object MQTTPublisher {

  def main(args: Array[String]) {
    if (args.length < 2) {
      System.err.println("Usage: MQTTPublisher <MqttBrokerUrl> <topic>")
      System.exit(1)
    }

    // Set logging level if log4j not configured (override by adding log4j.properties to classpath)
    if (!Logger.getRootLogger.getAllAppenders.hasMoreElements) {
      Logger.getRootLogger.setLevel(Level.WARN)
    }

    val Seq(brokerUrl, topic) = args.toSeq

    var client: MqttClient = null

    try {
      val persistence = new MemoryPersistence()
      client = new MqttClient("localhost:1883", MqttClient.generateClientId(), persistence)

      client.connect()

      val msgtopic = client.getTopic(topic)
      val msgContent = "test test test"
      val message = new MqttMessage(msgContent.getBytes("utf-8"))

      while (true) {
        try {
          msgtopic.publish(message)
          println(s"Published data. topic: ${msgtopic.getName()}; Message: $message")
        } catch {
          case e: MqttException if e.getReasonCode == MqttException.REASON_CODE_MAX_INFLIGHT =>
            Thread.sleep(10)
            println("Queue is full, wait for to consume data from the message queue")
        }
      }
    } catch {
      case e: MqttException => println("Exception Caught: " + e)
    } finally {
      if (client != null) {
        client.disconnect()
      }
    }
  }
}

Answer 1

The MqttClient() constructor takes a URI.

What you have provided is just a hostname and port number ( localhost:1883 ), it's missing a protocol section which should be tcp:// (which is what the library is expecting and getting null back. This really should throw a better error.)

You need to change the line to be

client = new MqttClient("tcp://localhost:1883", MqttClient.generateClientId(), persistence);

Answer 2

I think you are giving the wrong Url ie you are not specifying the protocol over which it has to connect that is my hunch.

Try changing the url to :

tcp://localhost:1883

I think it would work ! Rest all seems fine to me.

For a working example See this : https://github.com/shiv4nsh/scala-mqtt-client-rasberrypi-starter-kit/blob/master/src/main/scala/com/knoldus/MQTTPublisher.scala