This project provides three Java examples for learning MQTT using the Eclipse Paho MQTT client and the public test broker at test.mosquitto.org.

🚀 Features

• Simple Publisher, Subscriber, and Publish–Subscriber (Handler)
• Uses the Eclipse Paho MQTT Client
• Demonstrates core MQTT concepts (topics, QoS, callbacks)
• Ready for students to explore and extend

📦Maven Dependency

<dependency>
    <groupId>org.eclipse.paho</groupId>
    <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
    <version>1.2.5</version>
</dependency>

📨 Broker and Topics

All communication goes through the public MQTT broker:

• Broker URL: tcp://test.mosquitto.org:1883

The handler uses two topics:

• example/toIgnore
• example/toRead

The subscriber listens to both, but you can imagine example/toIgnore as background or noisy data and example/toRead as the “interesting” stream. You are encouraged to change these topic strings and observe how behavior changes.

📘 Class Overview

Handler.java

This is the only class you need to run to see the full publish/subscribe demo.

• Defines the broker URL and the topics (example/toIgnore, example/toRead)

• Creates a Subscriber and a Publisher, each with the same broker and topics

• Starts both as independent threads so they run concurrently

• How to run:

javac *.java
java Handler

Publisher.java

A Runnable that connects to the broker and periodically publishes messages to the two topics given by Handler.

Conceptual behavior:

• Connects to tcp://test.mosquitto.org:1883
• For a fixed number of iterations:
  • Builds one message for example/toIgnore
  • Builds one message for example/toRead
  • Publishes both with a chosen QoS (e.g., 2)
• Logs each published message to the console
• Disconnects cleanly at the end

Subscriber.java

A Runnable and MqttCallback that connects to the broker and subscribes to the given topics.

Conceptual behavior:

• Connects to tcp://test.mosquitto.org:1883
• Subscribes to both:
  • example/toIgnore
  • example/toRead
• Implements messageArrived(...) to print incoming messages:

🧪 Example Console Output

Subscriber Output Example

📥 Connected to broker: tcp://test.mosquitto.org:1883
📥 Subscribed to: example/toIgnore
📥 Subscribed to: example/toRead
📥 Delivery :: [example/toRead : 2] :: this is message to read 0
📥 Delivery :: [example/toIgnore : 2] :: this is message to ignore 0

Publisher Output Example

↗️ Connected to broker: tcp://test.mosquitto.org:1883
↗️ Published to example/toIgnore: this is message to ignore 0
↗️ Published to example/toRead: this is message to read 0

🏞️ Class Diagram

A drafted class diagram of the current release is as follows:

🎯 Notes & Tips

• test.mosquitto.org is a public broker:
  • Do not send sensitive or private data.
  • Anyone can publish/subscribe on the same topics.
• If you don’t see messages:
  • Make sure Handler is running (both publisher and subscriber threads are started).
  • Check your network and any firewalls that might block port 1883.
  • Confirm that the broker is online (the public test broker can occasionally restart).