MQTT：Android代理在向代理发布消息时崩溃

Question

我正在尝试向我的MQTT代理（我的Raspberry Pi）发布一条消息，用于主题rpi / gpio。 发送该消息的目的是打开/关闭连接到我的Raspberry Pi的LED（就像在本教程中一样，但该教程适用于Swift而不是Android）。 因此，当我打开Android应用程序上的切换小部件时，消息“on”将发布到Raspberry Pi代理以打开它（通过也连接到Raspberry Pi上的MQTT的python脚本）。 没有所有MQTT代码，下面的片段通过显示开关很好地工作。 但是，使用以下代码，它会崩溃。

public class ControlsFragment extends Fragment {

    private ControlsFragment.OnFragmentInteractionListener listener;

    public static ControlsFragment newInstance() {
        return new ControlsFragment();
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view=inflater.inflate(R.layout.fragment_controls, container, false);

        Switch switch1= (Switch)view.findViewById(R.id.tempSwitch);
        switch1.setChecked(true);
        switch1.setTextOn("On");
        switch1.setTextOff("Off");

        String clientId = MqttClient.generateClientId();
        MqttAndroidClient client =
                new MqttAndroidClient(this.getActivity(), "tcp://192.168.100.8:1883",
                        clientId);

        try {
            IMqttToken token = client.connect();
            token.setActionCallback(new IMqttActionListener() {
                @Override
                public void onSuccess(IMqttToken asyncActionToken) {
                    // We are connected
                    Log.d(TAG, "onSuccess");
                }

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

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

        String topic = "rpi/gpio";
        String payload;


        if(switch1.isChecked()){
            payload="on";
        }

        else{
            payload="off";
        }
        byte[] encodedPayload = new byte[0];
        try {
            encodedPayload = payload.getBytes("UTF-8");
            MqttMessage message = new MqttMessage(encodedPayload);
            client.publish(topic, message);
        } catch (UnsupportedEncodingException | MqttException e) {
            e.printStackTrace();
        }


        return view;
    }

    @Override
    public void onAttach(android.content.Context context) {
        super.onAttach(context);
        if (context instanceof ControlsFragment.OnFragmentInteractionListener) {
            listener = (ControlsFragment.OnFragmentInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString() + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        listener = null;
    }

    public interface OnFragmentInteractionListener {
    }


}

Logcat错误：

03-07 00:40:37.537 32415-32415/com.example.teerna.smartagriculturev5 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                       java.lang.NullPointerException
                                                                                           at org.eclipse.paho.android.service.MqttAndroidClient.publish(MqttAndroidClient.java:815)
                                                                                           at org.eclipse.paho.android.service.MqttAndroidClient.publish(MqttAndroidClient.java:673)
                                                                                           at com.example.teerna.smartagriculturev5.ControlsFragment.onCreateView(ControlsFragment.java:90)
                                                                                           at android.support.v4.app.Fragment.performCreateView(Fragment.java:2354)
                                                                                           at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1419)
                                                                                           at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740)
                                                                                           at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809)
                                                                                           at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799)
                                                                                           at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580)
                                                                                           at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
                                                                                           at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
                                                                                           at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2229)
                                                                                           at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:700)
                                                                                           at android.os.Handler.handleCallback(Handler.java:725)
                                                                                           at android.os.Handler.dispatchMessage(Handler.java:92)
                                                                                           at android.os.Looper.loop(Looper.java:137)
                                                                                           at android.app.ActivityThread.main(ActivityThread.java:5283)
                                                                                           at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                           at java.lang.reflect.Method.invoke(Method.java:511)
                                                                                           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
                                                                                           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
                                                                                           at dalvik.system.NativeStart.main(Native Method)

com.example.teerna.smartagriculturev5.ControlsFragment.onCreateView(ControlsFragment.java:90)指向以下行：

client.publish(topic, message);

这是在try块中找到的。

使用以下参考：

https://www.hivemq.com/blog/mqtt-client-library-enyclopedia-paho-android-service

http://androidkt.com/android-mqtt/

Answer 1

如果您遵循paho源代码中的错误，您会发现NPE是由客户端的mqttService为null 。 你需要等待在发布消息之前建立与服务的连接，正确的位置是onSuccess of the callback，你基本上忽略了

token.setActionCallback(new IMqttActionListener() {
    @Override
    public void onSuccess(IMqttToken asyncActionToken) {
        // after this is called you can publish messages
    }

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

    }
});