Develop Reference

Android UDP server does not receive packets

I have the following code to receive UDP packets:
public class AsyncReceiveUdp2 extends AsyncTask<String, Void, Boolean> {
#Override
protected Boolean doInBackground(String... f_url) {
int udp=111;
byte[] packet = new byte[2000];
DatagramPacket dp = new DatagramPacket(packet, packet.length);
DatagramSocket ds = null;
try {
ds = new DatagramSocket(udp);
ds.receive(dp);
//...
} catch (SocketException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (ds != null) {
ds.close();
}
}
return null;
}
}
I send UDP data to computer from Android device.
Computer immediately sends response as UDP packet.
I save information to my log file on SD.
And I see, that my app stays on the line "ds.receive(dp);" and does not run after it.
I've tested on the Android device against a program on computer.
As I understand it is tricky to receive UDP packets on Emulator.
I could not do it.
Redirect does not work for me as it is described here
Another important issue is to receive all packets, that were sent to the device. Lossless. How to modify the code for that?
Please help! Thanks!

put your receive inside a while(true) loop. When you receive a packet call an if (pkg_received){break;}... or whatever you want to do...
The problem is that you are probably only be receiving one package and you are getting timeout before receiving it.
Code edited and not tested
while(true)
{
byte[] message = new byte[60*1024];
DatagramPacket recv_packet = new DatagramPacket(message, message.length);
try {
socket.receive(recv_packet);
} catch (IOException e) {
e.printStackTrace();
}
Log.d("UDP", "S: Receiving...listening on port " + recv_packet.getPort() );
String rec_str;
rec_str=new String(recv_packet.getData)
Log.d("PACKAGE LENGTH",Integer.toString(recv_packet.getLength()));
}

android asynctask crash when trying to load an heavy page

I'm programming an app with my brother, and today unfortunately, I encountered with a problem.
When the app load a php page via my asynctask class it works fine. but I would like to program this situation: if the remote server is down, or crash, and doesnt display the right page, the application will show error message. but instead, the app crashes =[
I tried to load this page, for example:
http://alonadoni.com/sql3.php
(I want to simulate that there is a problem with the server. the regular page is sql2.php and it works fine when the server works)
When the app try to load this page (sql3.php) , the app crashes.
I did another experiment : I created a file sql3.php, and wrote "aaaaaaaa" in the page, the app doesn't crash in this situation. it downloaded the data "aaaaa". in this case, the app show jsonexecption error.
Unfortunately, I can't get logcat because my old computer can't run emulators, and my phone also can't connect to my computer on developer mode =[ When I try application I create an apk then transfer the file to my phone and install.
my code is:
in OnCreate:
String serverURL = sss() + "sql3.php?imei=" + imei;
new LongOperation().execute(serverURL);
outside OnCreate:
private class LongOperation extends AsyncTask<String, Void, Void> {
private final HttpClient Client = new DefaultHttpClient();
private String Error = null;
protected void onPreExecute() {
}
protected Void doInBackground(String... urls) {
try {
HttpGet httpget = new HttpGet(urls[0]);
ResponseHandler<String> responseHandler = new BasicResponseHandler();
data[x] = Client.execute(httpget, responseHandler);
} catch (ClientProtocolException e) {
Error = e.getMessage();
Toast.makeText(getApplicationContext(),"error2" , Toast.LENGTH_LONG).show();
cancel(true);
} catch (IOException e) {
Error = e.getMessage();
Toast.makeText(getApplicationContext(),"error34" , Toast.LENGTH_LONG).show();
cancel(true);
}
return null;
}
public void onPostExecute(Void unused) {
if (Error != null) {
} else {
try {
JSONObject json = new JSONObject(data[x]);
name = json.getString("name");
} catch (JSONException e) {
Toast.makeText(getApplicationContext(),"e" + e, Toast.LENGTH_LONG).show();
}
}
x++;
}
}

DoInBackground of asynctask needs to contain only NON UI work , hence referring to context and performing UI operations in UI thread may cause crash.
You can perform UI operations in postexecute of asynctask.
Hence Removing toast from above code which refers to UI operation will solve your issue

Android Udp : How to receive datagramPacket from LAN

I have an UDP server wrote in C langage which broadcasts paquets over my LAN every 5seconds, on port 3001.
i'm creating an android application as UDP client, which is listening on port 3001 (in the AsyncTask thread) and it's running until the receive() method, no data seems to be detected on this port.
Here is my code :
private class ConnectionTask extends AsyncTask<Void, Integer, Void> {
#Override
protected void onPreExecute() {
super.onPreExecute();
}
#Override
protected Void doInBackground(Void... arg0) {
String receivedString = "";
byte[] receiveData = new byte[1024];
DatagramSocket clientSocket;
try {
while(true){
clientSocket = new DatagramSocket(5000);
DatagramPacket receivePacket = new DatagramPacket(receiveData,
receiveData.length);
clientSocket.receive(receivePacket);
receivedString = new String(receivePacket.getData());
}
} catch (SocketException e) {
Log.v("SocketExceptionOccured", e.toString());
e.printStackTrace();
//clientSocket.close();
} catch (IOException e) {
Log.v("IOExceptionOccured", e.toString());
e.printStackTrace();
//clientSocket.close();
}
Toast.makeText(getBaseContext(), receivedString, Toast.LENGTH_LONG)
.show();
return null;
}
}
I test my code with my own device for debug, with USB cable.
I've tested my server with a simple UDP client (in C) running on my computer, and the communication is ok.
I don't know why this code doesn't work. Has someone an idea ?
Thanks,

You're never leaving the while loop. You're message is probably received, and after it, the loop causes the datagramsocket to listen again.

Don't create and close the socket every time around the loop. Create it first and close it afterwards. At present there are windows of time during which the socket doesn't exist, so datagrams to it are dropped: also, all queued datagrams are dropped every time you close it.

I had this same problem. You need to add permissions in the android manifest
<uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE"/>
<uses-permission android:name="android.permission.CHANGE_WIFI_STATE"/>
Also enable broadcasts in the socket.
clientSocket.setBroadcast(true);
Like everyone else before me mentioned, your code also never leaves the while loop, so it never goes to the the line where the toast is displayed.Remember that you CANNOT show Toast messages from doInBackground as this is accessing the UI Thread, you can only do so from the postExecute and preExecute functions. This will cause your application to crash. To check the data you receive you can either debug it or log it.
Your final doInBackground should be something like this
#Override
protected Void doInBackground(Void... arg0) {
String receivedString = "";
byte[] receiveData = new byte[1024];
DatagramSocket clientSocket;
try {
while(true){
clientSocket = new DatagramSocket(5000);
DatagramPacket receivePacket = new DatagramPacket(receiveData,
receiveData.length);
clientSocket.setBroadcast(true);
clientSocket.receive(receivePacket);
receivedString = new String(receivePacket.getData());
Log.i("Received String= "+receivedString);
}
} catch (SocketException e) {
Log.v("SocketExceptionOccured", e.toString());
e.printStackTrace();
//clientSocket.close();
} catch (IOException e) {
Log.v("IOExceptionOccured", e.toString());
e.printStackTrace();
//clientSocket.close();
} finally{
if(clientSocket!=null){
clientSocket.close();
}
}
return null;
}
Now when you check your logs, you should be able to see the value of the string received.

AsyncTask hangs on at beginning

AsyncTask works fine in Android 4.x, but not for Android 2.3.6. I've step-by-step debugged Android 2.3.6 with a physical mobile device.
It hangs on here:
myTask = new GetDataFromServer();
GetDataFromServer is the class of AsyncTask.
What's going on?
Here under is my code, I only used 1 AsyncTask in my code and received messages from server.
that's all.
class GetDataFromServer extends AsyncTask<String, String, String>
{
protected void onPreExecute ()
{
progressDialog1=ProgressDialog.show(MainActivity.this, "Loading data", "Please wait...",true);
}
protected String doInBackground(String... params)
{
String resulttxt="";
try {
serverIp = InetAddress.getByName("192.168.1.123");
int serverPort=31000;
Socket clientSocket=new Socket(serverIp,serverPort);
BufferedWriter bw=new BufferedWriter(new OutputStreamWriter(clientSocket.getOutputStream()));
bw.write(params[0]);
bw.flush();
BufferedReader br=new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
resulttxt=br.readLine();
if(resulttxt.contains("OK"))
{
publishProgress(resulttxt);
}
else
{
publishProgress(resulttxt);
clientSocket.close();
bw.close();
br.close();
return null;
}
resulttxt="";
resulttxt=br.readLine();
resulttxt=resulttxt.trim();
clientSocket.close();
} catch (IOException e) {
if(Status_txt!=null)
Status_txt.append( "Server is done.");
}
catch (NetworkOnMainThreadException e){
if(Status_txt!=null)
Status_txt.append( "NetworkOnMainThreadException");
}
return resulttxt;
}
protected void onProgressUpdate(String...inStr){
String[] strData=inStr[0].split("_");
String szTemp="Last Purchase Date: ";
szTemp+=strData[1];
szTemp+=" ,Valid days: ";
szTemp+=strData[2];
//Status_txt.setText(szTemp);
if(Status_txt!=null)
Status_txt.setText("You Are The Super User");
}
protected void onPostExecute(String data) {
tl_prediction2.removeAllViews();
if (data == null)
{
}
else {
if((data.contains("#")==true) || (data.contains("*")==true)
||data.contains("&")==true)
{
String[] arrayTmp=data.split("#");
for(Integer i=0;i<arrayTmp.length;i++)
{
String[] SubArrayTmp=arrayTmp[i].split("_");
tl_prediction2.addView(generateRow(4,SubArrayTmp));
}
}
}
progressDialog1.dismiss();
}
};

Since you haven't posted any code, I could only give you some random probable solutions:
May be your AsyncTask is taking a lot of time to download. Trying increasing its priority using android.os.Process.setThreadPriority(9) inside doInBackground()
Check if you have other previous running long AsyncTask in your code. AsyncTask by default operates on a single background thread. That means your AsyncTask task wouldn't be executed unless your previous AsyncTask are done. To allow parallel execution use executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, params). You can read more here
Check for Internet and other permissions in Manifest. This is mostly where people make mistake.

AsyncTask works with ThreadPool. If there too many synctasks are executing, the later AsyncTask will be blocked by others. I think you can use the thread tool in DDMS to check the How many ayncTasks are executing.

Android asynctask reading from socket

Im having some trouble reading/writing to a tcp server for which im building an app. In a recent thread I was suggested to use a service instead but this is a project for school which suggested asyncTask so I might aswell go for that.
So the classes ive got are my activity class and async, nothing interesting is going on in activity but sending a string which is working so ill get on with the async one.
class ServerTask extends AsyncTask<Void, Void, Void>{
public static String ip = "10.0.2.2";
public static int port = 2002;
Socket socket;
public DataInputStream dis;
public DataOutputStream dos;
public String message;
#Override
protected Void doInBackground(Void... params) {
try {
socket = new Socket(ip, port);
dis = new DataInputStream(socket.getInputStream());
dos = new DataOutputStream(socket.getOutputStream());
} catch (Exception e) {
Log.i("AsyncTank", "Cannot create Socket");
}
while(socket.isConnected()){
read();
}
}
}
return null;
}
public void write(String message) {
try {
if (socket.isConnected()){
dos.writeUTF(message);
dos.flush();
} else {
Log.i("AsynkTask", "Socket appears to be closed");
}
} catch (Exception e) {
Log.i("AsynkTask", "Writing failed");
}
}
public String read() {
try {
if (socket.isConnected()) {
message = dis.readLine();
} else {
Log.i("AsyncTask", "Cannot read");
}
} catch (Exception e) {
Log.i("AsyncTask", "Cannot read from stream");
}
return message;
}
}
Things I do know, the server DOES get the messages but it doesnt update until I restart the server which leads me to believe that im not pushing a new line or something which makes it all appear as one line after its closed. This however might aswell be the server for which im not reponsible so ill have to read up in that.
The read part however does not want to work, im not sure on how to call the method to have it constantly listen and react to the servers sockt? I tried make a thread just before the return in doInBackGround but then the application starts works for a couple of seconds the force closes due to lack of memory? Do I need a thread to keep constantly listen?
The whole point of this as you might guess is to make a chat so the read method is eventually supposed to update a textView in my activity class. The send method is "working" but not as it should though this might be as I said earlier the server doing some funky buisness.
Another one, is it even possible to have the read as a method like I have or does something have to react when the server sends data and then call the method?
Edit
I have now moved the read part, or atleast some of it to doInBackGround so its now
dis = new BufferedReader(new InputStreamReader(socket.getInputStream()));
message = dis.readLine();
Log.i("AsynkTask", "Read : "+message+" this is doInBackGround!");
This along with a change to simply hardcode a printline in the server made me read that line in the client so im guessing its working realtively good for now.
How is it looking? Is it utter crap this code and should be done some other way? Got my functionality but never bad to learn to do it better so to speak :).

You should do both your writing and reading to the Socket in an AsyncTask's doInBackground() method, as both take time and could block the main (UI) thread. I don't know how you are calling your write() method above but you might also want to take a look at this question that might be related.