我是Android和服务的新手。我的目标是能够设置订阅并在主题字符串上做出版物。解析文本字段的输入后，将设置主题字符串和客户端ID。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在IMqttDeliveryToken publish(String topic, MqttMessage message, Object userContext, IMqttActionListener callback) MqttAndroidClient方法中显示例外，其中正在传递令牌。
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进行发布和订阅吗？我确实浏览了示例项目（来自Paho Android）。 LWT和发布似乎是合并的，因为LWT（activity_publish.xml）的布局似乎也用于发布。
答案 0 :(得分：13)
NullPointerException是因为connect()调用异步方法而您需要实现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());
}
});
}
答案 1 :(得分：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，这一点非常重要。
以下是可以发布和订阅等的代码示例。
以下代码最初发布mqtt主题和有效负载：
代码订阅主题“tester”。如果它收到主题为“tester”的消息和“Alarm Activated”的有效负载，则它会发布以下主题和有效负载（通过上面提到的回调）：
如果您使用的是Mosquitto，则终端中的以下命令会导致此消息被触发：
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) {
}
}