Question

I’m trying to create an MqttClient in Android, using an Android emulator (Nexus7) on ADT (Eclipse 3.8.0 on a Linux Fedora17.
I found this question answered (Android MQTT unable to create client), but it failed to solve my problem.

I created an activity that allows the user to enter the directory where to store the presistence file, but there is no way to avoid catching the exception.
I tried with “/mnt/sdcard/”, “/mnt/”, “/mnt/sdcard/tmp/” and similar.

Do I need to pay attention to particular settings in Android emulator on in Eclipse project?
Are there any uses-permissions to be included in the application?

I looked at the various mqtt resources mentioned in the quoted answered question, but I couldn’t manage to find an answer.

This is my code:

package com.storassa.android.mqttsample;

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttDefaultFilePersistence;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.AutoCompleteTextView;
import android.widget.Button;
import android.widget.TextView;

public class MqttSample extends Activity {

    Button okButton = null;
    AutoCompleteTextView inputTextView = null;
    TextView errorTextView = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_mqtt_sample);

        okButton = (Button)findViewById(R.id.okButton);
        inputTextView = (AutoCompleteTextView)findViewById(R.id.InputText);
        errorTextView = (TextView)findViewById(R.id.errorText);

        okButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                try {
                    String dir = inputTextView.getText().toString();
                    MqttDefaultFilePersistence mdfp = new MqttDefaultFilePersistence(
                            dir);
                    MqttClient client = new MqttClient("tcp://127.0.0.1:1833",
                            "0001", mdfp);
                }
                catch (Exception e) {
                    String errorText = "";
                    for (StackTraceElement error : e.getStackTrace())
                        errorText += error.toString() + "\n";
                    errorTextView.setText(errorText);
                }
            }
        });


    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_mqtt_sample, menu);
        return true;
    }

}

and this is the Stack:

MqttException(0) at
org.eclipse.paho.client.mqttv3.MqttDefaultFilePersistence.open(MqttDefaultFilePersistence.java:74)
org.eclipse.paho.client.mqttv3.MqttClient.<init>(MqttClient.java:183)
com.storassa.android.mqttsample.MqttSample$1.onClick(MqttSample.java:38)
android.view.View.performClick(View.java:4202)
android.view.View$PerformClick.run(View.java:17340)
android.os.Handler.handleCallback(Handler.java:725)
android.os.Handler.dispatchMessage(Handler.java:92)
android.os.Looper.loop(Looper.java:137)
android.app.ActivityThread.main(ActivityThread.java:5039)
java.lang.reflect.Method.invokeNative(Native Method)
java.lang.reflect.Method.invoke(Method.java:511)
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
dalvik.system.NativeStart.main(Native Method)    

Answer 1

After a whole day spent in understanding the issue and in googling anywhere to find it, eventually I got it: very simply a permission has to be added to the manifest

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

I don’t know whether internal directories can be used instead of the sdcard.

As this is the first time I answer my own question, let me know whether it is a good etiquette to flag as correct answer my own answer (I found different opinions on this topics)