android - 用于发布的 Paho MQTT 服务

Question

我是 Android 和服务的新手。我的目标是能够设置订阅并发布主题字符串。主题字符串和客户端 ID 是在解析文本字段的输入后设置的。我正在使用Paho MQTT service（下载了源代码并构建了 JAR）。

以下导致c.publish() 处出现空指针异常。 logcat 显示了 MqttAndroidClient 中的 IMqttDeliveryToken publish(String topic, MqttMessage message, Object userContext, IMqttActionListener callback) 方法的异常，其中正在获取传递令牌。

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();
                    }

您能告诉我如何使用该服务发布和订阅吗？我确实浏览了示例项目（来自 Paho Android）。 LWT 和发布似乎已合并，因为 LWT (activity_publish.xml) 的布局似乎也用于发布。

Answer 1

NullPointerException 是因为connect() 调用了一个异步方法，你需要实现一个ActionListener。 如果成功，您可以发送消息。

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

了解您需要调用 client.setCallback 并实现 MqttCallbackHandler 以接收有关您订阅的主题的消息非常重要。

这是一个可以发布和订阅等的代码示例。

以下代码最初发布mqtt主题和payload：

• 主题： AndroidPhone
• Payload： 大家好，我是 Android Mqtt 客户端。

代码订阅主题“tester”。如果它接收到主题为“tester”且负载为“Alarm Activated”的消息，则它会发布以下主题和负载（通过上述回调）：

• 主题： Fitlet
• 有效负载： 您好，Mosquitto Broker 收到您的消息，说 警报已激活。

如果您使用的是 Mosquitto，那么终端中的以下命令将导致此消息被触发：

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

我的 Mosquitto 用户名是 fred，密码是 1234

守则：

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) {

    }
}