Android AsyncTask and networking mayham

Question

I have an app that connects to a remote chat server and then the user is able to chat. I have a form where the user enters hostname and port and then another form where the user sees all messages and types new ones.

For the networking, it seems, I have no other choice but to use an AsyncTask. But there is something that I don't know how to do. I would start the networking in the UI when onCreate() is called which is not a problem. But then when I have to wire the networking and the interface it becomes a mess. I have created two asynchronous tasks – one for sending and one for receiving but yet it doesn't work for me. Here is the code where I first start the SenderTask which is supposed to start communicating by calling the receiver task and if a button shall be pressed a new message is sent:

public class Second extends Activity {

    private SenderTask snd;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second);

        // ...

        final Button sendButton = (Button) findViewById(R.id.button2);

        snd = new SenderTask();
        snd.doInBackground(editTexts);

        sendButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // when the button is clicked the next screen is loaded
                snd.onProgressUpdate(true);
            }
        });

    }   // end of onCreate

}

Then here is the SenderTask class:

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;

import android.os.AsyncTask;
import android.widget.EditText;

public class SenderTask extends AsyncTask<EditText, Boolean, Void> {

    MulticastSocket so;
    InetAddress serverAddress;
    int port;
    EditText eText1;
    EditText eText2;
    EditText eText3;
    EditText messageBoard;

    @Override
    protected Void doInBackground(EditText... eTexts) {
        eText1 = eTexts[0];
        eText2 = eTexts[1];
        eText3 = eTexts[2];
        messageBoard = eTexts[3];
        setUp();

        return null;
    }

    private void setUp() {
        // convert the host name to InetAddress
        try {
            //serverAddress = InetAddress.getByName(eText1.getText().toString());
            serverAddress = InetAddress.getByName("my server name is here");
        } catch (Exception e) {}

        //convert the port to an int
        //port = Integer.parseInt(eText2.getText().toString());
        port = 4456;

        // create socket and start communicating
        try {
            so = new MulticastSocket(port);
            so.joinGroup(serverAddress);
        } catch (IOException e) {}


        // Start listening should be here
        ReceiverTask rec = new ReceiverTask();
        rec.doInBackground(messageBoard);
        rec.onProgressUpdate(so);
    }

    private void sendMessage() {

        // get the text that they contain and add the new messages to the old ones
        String message = eText3.getText().toString();
        String conversation = messageBoard.getText().toString();
        String newConverstion = conversation.concat("\n[You] ").concat(message);

        // make the messages text view editable
        messageBoard.setFocusable(true);
        messageBoard.setText(newConverstion);   // add the new message to the text view
        messageBoard.setFocusable(false);   // make the messages text view not editable

        // erase the text on the second text view that has just been sent
        eText3.setText("");

        // Send message to server

        // convert message to bytes array
        byte[] data = (message).getBytes();

        // create and send a datagram
        DatagramPacket packet = new DatagramPacket(data, data.length, serverAddress, port);

        try {
            so.send(packet);
        } catch (IOException e) {}


    }   // end of sendMessage

    protected void onProgressUpdate(boolean... go) {
        if (go.equals(new Boolean(true))) {
            sendMessage();
        }

    }

}

here is the ReceiverTask class:

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.MulticastSocket;

import android.os.AsyncTask;
import android.widget.EditText;

public class ReceiverTask extends AsyncTask<EditText, MulticastSocket, Void> {

    MulticastSocket so;
    EditText messageBoard;

    @Override
    protected Void doInBackground(EditText... messBo) {
        messageBoard = messBo[0];
        return null;
    }

    @Override
    protected void onProgressUpdate(MulticastSocket... socket) {
        so = socket[0];

        byte[] data = new byte[1024];   // received data container

        while (true) {
            try {
                // create a datagram for receivin
                DatagramPacket packet = new DatagramPacket(data, data.length);

                // wait for the next message
                so.receive(packet);

                String message = new String(data, 0, packet.getLength());

                // add the new messages to the old ones
                String conversation = messageBoard.getText().toString();
                String newConverstion = conversation.concat("\n[Remote] ").concat(message);

                // make the messages text view editable
                messageBoard.setFocusable(true);
                messageBoard.setText(newConverstion);   // add the new message to the text view
                messageBoard.setFocusable(false);   // make the messages text view not editable


            } catch (IOException ioe) {}
        }
    }

}

The error is:

01-25 14:26:09.739: E/AndroidRuntime(661): FATAL EXCEPTION: main
01-25 14:26:09.739: E/AndroidRuntime(661): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.regeduseroox.projx/com.regeduseroox.projx.Second}: java.lang.NullPointerException
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.ActivityThread.access$600(ActivityThread.java:123)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.os.Looper.loop(Looper.java:137)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.ActivityThread.main(ActivityThread.java:4424)
01-25 14:26:09.739: E/AndroidRuntime(661):  at java.lang.reflect.Method.invokeNative(Native Method)
01-25 14:26:09.739: E/AndroidRuntime(661):  at java.lang.reflect.Method.invoke(Method.java:511)
01-25 14:26:09.739: E/AndroidRuntime(661):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
01-25 14:26:09.739: E/AndroidRuntime(661):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
01-25 14:26:09.739: E/AndroidRuntime(661):  at dalvik.system.NativeStart.main(Native Method)
01-25 14:26:09.739: E/AndroidRuntime(661): Caused by: java.lang.NullPointerException
01-25 14:26:09.739: E/AndroidRuntime(661):  at com.regeduseroox.projx.ReceiverTask.onProgressUpdate(ReceiverTask.java:33)
01-25 14:26:09.739: E/AndroidRuntime(661):  at com.regeduseroox.projx.SenderTask.setUp(SenderTask.java:53)
01-25 14:26:09.739: E/AndroidRuntime(661):  at com.regeduseroox.projx.SenderTask.doInBackground(SenderTask.java:27)
01-25 14:26:09.739: E/AndroidRuntime(661):  at com.regeduseroox.projx.Second.onCreate(Second.java:38)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.Activity.performCreate(Activity.java:4465)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
01-25 14:26:09.739: E/AndroidRuntime(661):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
01-25 14:26:09.739: E/AndroidRuntime(661):  ... 11 more

ReceiverTask.java:33 is so.receive(packet);

SenderTask.java:53 is rec.onProgressUpdate(so);

SenderTask.java:27 is setUp();

Second.java:38 is snd.doInBackground(editTexts);

I don't know what is wrong now and I really don't have anymore ideas how to solve this. Any help will be much appreciated.

Answer 1

Replace

snd.doInBackground(editTexts);

with

snd.execute(editTexts);

EDIT

You should not be directly calling onProgressUpdate() or doInBackground() to initiate an AsyncTask, only execute() . Furthermore, onProgressUpdate() does its work on the UI thread, which is really only intended to be used to update a ProgressBar . You should move all of the work from onProgressUpdate() to doInBackground() , and return the data in onPostExecute() .

My suggestion is to create a custom class that includes the String (conversation) and MulticastSocket (so) as its fields. Send this object in your AsyncTask.execute() call, and retrieve from the varargs.

Eg:

public class MyObject {
     MulticastSocket so;
     EditText messageBoard;

     //... Getters and Setters omitted for brevity
}

Then in your SenderTask:

ReceiverTask rec = new ReceiverTask();
MyObject obj = new MyObject(so, messageBoard);
rec.execute(obj);

You should use this same idea for your Second/SenderTask (take the work out of onProgressUpdate() ).

Let me know if this is all clear, and be sure to re-read the docs on the proper usage of AsyncTask.

Answer 2

It says there is a NullPointerException in Second.java. Set a breakpoint in your code, use the debugger, step through your code line by line, and see where it breaks. I can't tell enough from the error but to me it looks like editText could possibly be null in

snd.doInBackground(editTexts);

Also if you are using multicast sockets you should be using a multicast address not the IP address of the server