Connect Java to an MQTT broker with the Eclipse Paho Java client. Start on the free public broker, then point the same code at a secure private broker on ConnectMQ.
mqtt-code-samples/javaClone the complete, runnable examples. Stars and pull requests welcome. View on GitHub No account needed: connect to broker.connectmq.com on port 1883 and start publishing in seconds.
import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
public class Example {
public static void main(String[] args) throws Exception {
String topic = "your-project/sensors/temperature"; // Use a unique topic prefix: everyone can read the public broker
// Connect to the free public broker (no account needed)
MqttClient client = new MqttClient("tcp://broker.connectmq.com:1883",
MqttClient.generateClientId(), new MemoryPersistence());
MqttConnectOptions options = new MqttConnectOptions();
// Reconnect automatically if the connection drops
options.setAutomaticReconnect(true);
client.connect(options);
// Subscribe to the temperature topic
client.subscribe(topic, 1, (t, message) ->
System.out.println(t + ": " + new String(message.getPayload())));
// Publish a JSON reading
client.publish(topic, "{\"temperature\":21.5}".getBytes(), 1, false);
}
}
The public broker is shared: anyone can subscribe to your topics. Never send passwords, personal data or anything confidential through it.
About the free public MQTT brokerFor real devices, use a private serverless MQTT broker on ConnectMQ instead of the public one: TLS encryption, a username and password for every device and per-topic access control, fully managed. Start on the Free plan — no card required.
Replace YOUR_BROKER_HOST, YOUR_USERNAME and YOUR_PASSWORD with the values from your ConnectMQ dashboard.
import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
public class Example {
public static void main(String[] args) throws Exception {
String topic = "your-project/sensors/temperature";
// Connect to your private ConnectMQ broker on port 8883 (TLS: the system CA store verifies the broker certificate)
MqttClient client = new MqttClient("ssl://YOUR_BROKER_HOST:8883",
MqttClient.generateClientId(), new MemoryPersistence());
MqttConnectOptions options = new MqttConnectOptions();
// Username and password from your ConnectMQ dashboard
options.setUserName("YOUR_USERNAME");
options.setPassword("YOUR_PASSWORD".toCharArray());
// Reconnect automatically if the connection drops
options.setAutomaticReconnect(true);
client.connect(options);
// Subscribe to the temperature topic
client.subscribe(topic, 1, (t, message) ->
System.out.println(t + ": " + new String(message.getPayload())));
// Publish a JSON reading
client.publish(topic, "{\"temperature\":21.5}".getBytes(), 1, false);
}
}
Install Eclipse Paho Java Client, connect to the broker host on port 1883 (or 8883 with TLS), then subscribe to a topic and publish messages to it. The examples on this page connect to the free public broker broker.connectmq.com and to a private ConnectMQ broker.
Yes. broker.connectmq.com is a free public MQTT broker that needs no account. Anyone can read its topics, so use it only for tests and use a unique topic prefix.
Connect over TLS on port 8883 with a username and password from your ConnectMQ dashboard, and give every device its own credentials and topic permissions instead of using the public broker.
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View exampleConnect Home Assistant to an MQTT broker and turn MQTT messages into sensors.
View exampleConnect Node-RED to an MQTT broker with an importable flow.
View exampleTest any example on broker.connectmq.com — no sign-up, MQTT 3.1.1 and 5, TCP, TLS and WebSocket.
About the free public MQTT brokerWhen it is time for real devices, run the same code on a private, fully managed ConnectMQ broker with TLS and per-device credentials.
Start for freeCreate an account, get your broker credentials and connect your first device — no servers to install, patch or scale.
