android - 用於發布的Paho MQTT服務

Question

我是Android和服務的新手。 我的目標是能夠設置訂閱並在主題字符串上做出版物。 解析文本字段的輸入后，將設置主題字符串和客戶端ID。 我正在使用Paho MQTT服務 （下載源並構建JAR）。

以下內容導致c.publish()處出現空指針異常。 logcat在IMqttDeliveryToken publish(String topic, MqttMessage message, Object userContext, IMqttActionListener callback)中的IMqttDeliveryToken publish(String topic, MqttMessage message, Object userContext, IMqttActionListener callback)方法中顯示MqttAndroidClient ，其中正在獲取傳遞令牌。

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Set locale;
        l = getResources().getConfiguration().locale;
    }

    @Override
    protected void onResume() {
        super.onResume();
        addButtonListener();        
    }

    private void addButtonListener() {
        Button submitButton = (Button) findViewById(R.id.buttonSubmit);

        submitButton.setOnClickListener(new OnClickListener() {
// ...
// validation code for fields in layout
// ...
// Finally, this.

                    MemoryPersistence mPer = new MemoryPersistence();
                    String clientId = UUID.randomUUID().toString();
                    String brokerUrl = "tcp://m2m.eclipse.org:1883";
                    MqttAndroidClient c = new MqttAndroidClient(getApplicationContext(), brokerUrl, clientId, mPer);
                    try {
                        c.connect(); 
                        String topic = "transfers/topic";
                        String msg = "topic payload"
                        MqttMessage m = new MqttMessage();
                        m.setPayload(msg.getBytes());
                        m.setQos(2);
                        m.setRetained(false);
                        c.publish(topic, m); 
                    } catch (MqttException e) {
                        Toast.makeText(getApplicationContext(), e.getMessage(), Toast.LENGTH_SHORT).show();
                    }

您能告訴我如何使用該服務進行發布和訂閱嗎？ 我確實瀏覽了示例項目（來自Paho Android）。 LWT和發布似乎是合並的，因為LWT（ activity_publish.xml ）的布局似乎也用於發布。

Answer 1

NullPointerException是因為connect()調用異步方法，您需要實現ActionListener 。 如果成功，您可以發送消息。

Log.i(LOGTAG, "MQTT Start");

MemoryPersistence memPer = new MemoryPersistence();

final MqttAndroidClient client = new MqttAndroidClient(
    context, "tcp://192.168.0.13:1883", username, memPer);

try {
    client.connect(null, new IMqttActionListener() {

        @Override
        public void onSuccess(IMqttToken mqttToken) {
            Log.i(LOGTAG, "Client connected");
            Log.i(LOGTAG, "Topics="+mqttToken.getTopics());

            MqttMessage message = new MqttMessage("Hello, I am Android Mqtt Client.".getBytes());
            message.setQos(2);
            message.setRetained(false);

            try {
                client.publish("messages", message);
                Log.i(LOGTAG, "Message published");

                client.disconnect();
                Log.i(LOGTAG, "client disconnected");

            } catch (MqttPersistenceException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();

            } catch (MqttException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        @Override
        public void onFailure(IMqttToken arg0, Throwable arg1) {
            // TODO Auto-generated method stub
            Log.i(LOGTAG, "Client connection failed: "+arg1.getMessage());

        }
    });
}

Answer 2

了解您需要調用client.setCallback並實現MqttCallbackHandler以接收有關您訂閱的主題的消息非常重要。

以下是可以發布和訂閱等的代碼示例。

以下代碼最初發布mqtt主題和有效負載：

• 主題： AndroidPhone
• 有效載荷：您好，我是Android Mqtt客戶端。

代碼訂閱主題“tester”。 如果它收到主題為“tester”的消息和“Alarm Activated”的有效負載，則它會發布以下主題和有效負載（通過上面提到的回調）：

• 主題： Fitlet
• 有效負載：您好，Mosquitto Broker收到消息說警報已激活。

如果您正在使用Mosquitto，則終端中的以下命令將導致此消息被觸發：

mosquitto_pub -h 192.168.9.100 -t tester -m "Alarm Activated" -u fred -P 1234

我的Mosquitto用戶名是fred，密碼是1234

編碼：

package colin.android.mqtt;    
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;
import org.eclipse.paho.android.service.MqttAndroidClient;
import org.eclipse.paho.client.mqttv3.IMqttActionListener;
import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.IMqttToken;
import org.eclipse.paho.client.mqttv3.MqttCallbackExtended;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttPersistenceException;
import java.io.UnsupportedEncodingException;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String clientId = MqttClient.generateClientId();

        //The URL of the Mosquitto Broker is 192.168.9.100:1883
        final  MqttAndroidClient client = new MqttAndroidClient(this.getApplicationContext(), "tcp://192.168.9.100:1883", clientId);

        client.setCallback(new MqttCallbackHandler(client));//This is here for when a message is received

        MqttConnectOptions options = new MqttConnectOptions();

        try {
            options.setUserName("fred");
            options.setPassword("1234".toCharArray());
            IMqttToken token = client.connect(options);

            token.setActionCallback(new IMqttActionListener() {
                @Override
                public void onSuccess(IMqttToken asyncActionToken) {
                    // We are connected
                    Log.d("mqtt", "onSuccess");
//-----------------------------------------------------------------------------------------------
                    //PUBLISH THE MESSAGE                    
                    MqttMessage message = new MqttMessage("Hello, I am an Android Mqtt Client.".getBytes());
                    message.setQos(2);
                    message.setRetained(false);

                    String topic = "AndroidPhone";

                    try {
                        client.publish(topic, message);
                        Log.i("mqtt", "Message published");

                        // client.disconnect();
                        //Log.i("mqtt", "client disconnected");

                    } catch (MqttPersistenceException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();

                    } catch (MqttException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
//-----------------------------------------------------------------------------------------------

                    String subtopic = "tester";
                    int qos = 1;
                    try {
                        IMqttToken subToken = client.subscribe(subtopic, qos);
                        subToken.setActionCallback(new IMqttActionListener() {
                            @Override
                            public void onSuccess(IMqttToken asyncActionToken) {
                                // The message was published
                                Log.i("mqtt", "subscription success");
                            }

                            @Override
                            public void onFailure(IMqttToken asyncActionToken,
                                                  Throwable exception) {
                                // The subscription could not be performed, maybe the user was not
                                // authorized to subscribe on the specified topic e.g. using wildcards
                                Log.i("mqtt", "subscription failed");

                            }
                        });



                    } catch (MqttException e) {
                        e.printStackTrace();
                    }

//---------------------------------------------------------------------------

                }

                @Override
                public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
                    // Something went wrong e.g. connection timeout or firewall problems
                    Log.d("mqtt", "onFailure");

                }

            });


        } catch (MqttException e) {
            e.printStackTrace();
        }

    }


}//End of Activity class

//-----------------------------------------------------------------------------

class MqttCallbackHandler implements MqttCallbackExtended {

    private final MqttAndroidClient client;

    public MqttCallbackHandler (MqttAndroidClient client)
    {
        this.client=client;
    }

    @Override
    public void connectComplete(boolean b, String s) {
        Log.w("mqtt", s);
    }

    @Override
    public void connectionLost(Throwable throwable) {

    }

    public void AlarmActivatedMessageReceived()
    {
        MqttMessage msg= new MqttMessage("Hello, the Mosquitto Broker got your message saying that the Alarm is Activated.".getBytes());
        try {
            this.client.publish("Fitlet", msg);
            Log.i("mqtt", "Message published");

        } catch (MqttPersistenceException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();

        } catch (MqttException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    @Override
    public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception {
        Log.w("mqtt", mqttMessage.toString());

        if (mqttMessage.toString().contains("Alarm Activated"))
        {
            AlarmActivatedMessageReceived();
        }

    }

    @Override
    public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {

    }
}