Google Cloud IoT Core is being retired on August 16, 2023. Contact your Google Cloud account team for more information.

Connect to MQTT server

Connect to MQTT server.

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For detailed documentation that includes this code sample, see the following:

Code sample


For more information, see the Cloud IoT Core Java API reference documentation.

To authenticate to Cloud IoT Core, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

// Build the connection string for Google's Cloud IoT Core MQTT server. Only SSL
// connections are accepted. For server authentication, the JVM's root certificates
// are used.
final String mqttServerAddress =
    String.format("ssl://%s:%s", mqttBridgeHostname, mqttBridgePort);

// Create our MQTT client. The mqttClientId is a unique string that identifies this device. For
// Google Cloud IoT Core, it must be in the format below.
final String mqttClientId =
        projectId, cloudRegion, registryId, gatewayId);

MqttConnectOptions connectOptions = new MqttConnectOptions();
// Note that the Google Cloud IoT Core only supports MQTT 3.1.1, and Paho requires that we
// explictly set this. If you don't set MQTT version, the server will immediately close its
// connection to your device.

Properties sslProps = new Properties();
sslProps.setProperty("", "TLSv1.2");

// With Google Cloud IoT Core, the username field is ignored, however it must be set for the
// Paho client library to send the password field. The password field is used to transmit a JWT
// to authorize the device.

if ("RS256".equals(algorithm)) {
  connectOptions.setPassword(createJwtRsa(projectId, privateKeyFile).toCharArray());
} else if ("ES256".equals(algorithm)) {
  connectOptions.setPassword(createJwtEs(projectId, privateKeyFile).toCharArray());
} else {
  throw new IllegalArgumentException(
      "Invalid algorithm " + algorithm + ". Should be one of 'RS256' or 'ES256'.");

System.out.println(String.format("%s", mqttClientId));

// Create a client, and connect to the Google MQTT bridge.
try (MqttClient client =
    new MqttClient(mqttServerAddress, mqttClientId, new MemoryPersistence())) {
  // Both connect and publish operations may fail. If they do, allow retries but with an
  // exponential backoff time period.
  long initialConnectIntervalMillis = 500L;
  long maxConnectIntervalMillis = 6000L;
  long maxConnectRetryTimeElapsedMillis = 900000L;
  float intervalMultiplier = 1.5f;

  long retryIntervalMs = initialConnectIntervalMillis;
  long totalRetryTimeMs = 0;

  while (totalRetryTimeMs < maxConnectRetryTimeElapsedMillis && !client.isConnected()) {
    try {
    } catch (MqttException e) {
      int reason = e.getReasonCode();

      // If the connection is lost or if the server cannot be connected, allow retries, but with
      // exponential backoff.
      System.out.println("An error occurred: " + e.getMessage());
      if (reason == MqttException.REASON_CODE_CONNECTION_LOST
          || reason == MqttException.REASON_CODE_SERVER_CONNECT_ERROR) {
        System.out.println("Retrying in " + retryIntervalMs / 1000.0 + " seconds.");
        totalRetryTimeMs += retryIntervalMs;
        retryIntervalMs *= intervalMultiplier;
        if (retryIntervalMs > maxConnectIntervalMillis) {
          retryIntervalMs = maxConnectIntervalMillis;
      } else {
        throw e;

  attachCallback(client, gatewayId);

  // The topic gateways receive error updates on. QoS must be 0.
  String errorTopic = String.format("/devices/%s/errors", gatewayId);
  System.out.println(String.format("Listening on %s", errorTopic));

  client.subscribe(errorTopic, 0);

  return client;

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