Develop Reference

Android Socket connects to server but doesn't write anything

I'm currently runing a server on Eclipse(local IP 192.168.1.255, listening to port 4567). A client can connect trought sockets and send messages, that will be printed on the terminal by the server.
Part of the server code is the following:
System.out.println("Client connected: " + clientName);
String line;
while (true){
line = in.nextLine();
System.out.println("STRING RECEIVED: " + line + " FROM " + clientName);
}
where in is the input stream of the client socket.
Part of client code, instead, is:
while(true) {
System.out.print("\nEnter your input: ");
line = stdin.next();
socketOut.println(line);
socketOut.flush();
}
So, in example, a possible output on server terminal with two clients connected is the following:
Client connected: Socket[addr=/192.168.1.225,port=54852,localport=4567]
STRING RECEIVED: Hello FROM Socket[addr=/192.168.1.225,port=54852,localport=4567]
STRING RECEIVED: World FROM Socket[addr=/192.168.1.225,port=54852,localport=4567]
Client connected: Socket[addr=/192.168.1.225,port=54945,localport=4567]
STRING RECEIVED: Hello2 FROM Socket[addr=/192.168.1.225,port=54945,localport=4567]
Everything works well, so i'm now trying to access server trough sockets on a simple app developed on Android Studio. The code is:
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new BackgroundTask().execute();
}
private class BackgroundTask extends AsyncTask {
#Override
protected Object doInBackground(Object[] params) {
try {
Socket socket = new Socket("10.0.2.2", 4567);
PrintWriter out = new PrintWriter(socket.getOutputStream());
out.println(new String("Hi from Android!"));
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
}
But the output is just
Client connected: Socket[addr=/192.168.1.225,port=55001,localport=4567]
and nothing else.
Any advice about the println doesn't send anything? The program works perfectly on Eclipse on both client/server side, so i guess the problem is on Android. Also, i enabled the Android network permissions, so the connection should work.
Thanks in advance to everybody.
EDIT: solved, i just changed Android client code to:
try {
Socket socket = new Socket("10.0.2.2", 4567);
if (socket.isConnected()) {
PrintWriter out = new PrintWriter(new BufferedWriter(
new OutputStreamWriter(socket.getOutputStream())), true);
String line = new String("Hi from Android!");
out.println(line);
}
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}

You need to either flush the PrintWriter or construct it to auto-flush. It doesn't by default.

Zebra connection android

Does anyone know if BT_MULTI works on Zebra? I'm using ZebraConnection class in my android application and connection with BT:printerAddress as parameter works but with BT_MULTI:printerAddress throws "Invalid connection type".
I'm trying to use BT_MULTI to connect multiple printers in one time, but if you have other ideas of how I can do that I'll be happy to hear them.
This is the code that throws the exception:
Connection connection;
try {
connection= ConnectionBuilder.build(getConnectionStringForSdk(printerBTAddress));
connection.open();
ZebraPrinter printerdemo = ZebraPrinterFactory.getInstance(connection);
} catch (ConnectionException e) {
System.out.println("Connection could not be opened"+e);
} catch (ZebraPrinterLanguageUnknownException e) {
System.out.println("Unable to create printer"+e);
}
private String getConnectionStringForSdk(String btAddress) {
String selectedPrefix="BT_MULTI:";
final String finalConnectionString = selectedPrefix + btAddress;
return finalConnectionString;
}

Android socket connection refused error

I want to implement socket connection between 2 deceives , client keep sending GPS data to the server and I need both of it run in new thread , the client send first one data then keep show error like this
03-18 16:35:11.805: E/Client run:(8163): java.net.ConnectException: failed to connect to /192.168.2.103 (port 5678): connect failed: ECONNREFUSED (Connection refused)
here is the client code
public class Send implements Runnable{
private boolean Connect = true;
public void Connect(){
Connect = true;
}
public void Disconnect(){
Connect = false;
}
#Override
public void run() {
// TODO Auto-generated method stub
while(Connect){
try {
SocketClient = new Socket("192.168.2.103", 5678);
ObjectOutputStream oos = new ObjectOutputStream(SocketClient.getOutputStream());
oos.writeDouble(GPSinfo[2]);
//ObjectInputStream ois = new ObjectInputStream(SocketClient.getInputStream());
//ois.readInt();
oos.close();
//ois.close();
} catch (Exception e) {
Log.e("Client run: ", e.toString());
}
}
}
}
here is server code
public class Receive implements Runnable{
private boolean CanReceive = true;
private double Data;
public void Connect(){
CanReceive = true;
}
public void Disconnect(){
CanReceive = false;
}
#Override
public void run() {
// TODO Auto-generated method stub
while(CanReceive){
try {
SocketServer = new ServerSocket(5678);
Socket connectedSocket = SocketServer.accept();
ObjectInputStream ois = new ObjectInputStream(connectedSocket.getInputStream());
Data = ois.readDouble();
DataText.setText("" + Data);
//ObjectOutputStream oos = new ObjectOutputStream(connectedSocket.getOutputStream());
//oos.writeInt(1);
//ois.close();
//oos.close();
} catch (Exception e) {
Log.e("Server run: ", e.toString());
}
}
}
}
by the way , the both code is inner class , and INTERNET permission is added.

It's obvious it's not a router-firewall related problem as you are under the same net, so there are only a few possibilities:
There's nothing listening on that port on that IP on the server-side
There's a local firewall on the server-side that is blocking that connection attempt
You are not using WIFI so you're not under the same net.
You should make sure you can open that service some ther way, that would help you debugging where the culprit is. If you've already done this, I'd suggest using some debugging tool to trace TCP packets (I don't know either what kind of operating system you use on the destination machine; if it's some linux distribution, tcpdump might help, in Win environments WireShark works just good).

This isn't a 'data transfer error'. This is a 'connection refused' error. It means the server you want to transfer the data to or from isn't running at the IP:port you specified.

Try killing the adb service before you begin the connection. I had a similar problem and killing the adb service before the connection resolved the issue.

I had the same error. I simply used ServerSocket and it worked well.
ServerSocket socket = new ServerSocket(8888);

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.

WIFI to WIFI Connectivity using Android

I want to transfer messages from the android device to desktop application. My question is that can i connect the android WiFi device with the desktop WiFi device without any use of internet connection. I want to use it just like the Bluetooth. is this possible or not? if it is possible then how can i implement it?
Thanks and Regards
Amit Thaper

Here is an implementation of mreichelt's suggestion. i looked this up when i had the same problem and figured i'd just post my implementation of the solution. it's really simple. i also built a java server that listens for incoming requests from the android device (for debugging purposes mostly). here's the code to send stuff over the wireless:
import java.net.*;
import java.io.*;
import java.util.*;
import android.app.Activity;
import android.content.Context;
import android.content.ContentValues;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import android.os.Bundle;
import android.util.Log;
public class SMSConnection {
/* The socket to the server */
private Socket connection;
/* Streams for reading and writing the socket */
private BufferedReader fromServer;
private DataOutputStream toServer;
/* application context */
Context mCtx;
private static final String CRLF = "\r\n";
/* Create an SMSConnection object. Create the socket and the
associated streams. Initialize SMS connection. */
public SMSConnection(Context ctx) throws IOException {
mCtx=ctx;
this.open();
/* may anticipate problems with readers being initialized before connection is opened? */
fromServer = new BufferedReader(new InputStreamReader(connection.getInputStream()));
toServer = new DataOutputStream(connection.getOutputStream());
}
public boolean open(String host, int port) {
try {
connection = new Socket(host, port);
return true;
} catch(IOException e) {
Log.v("smswifi", "cannot open connection: " + e.toString());
}
return false;
}
/* Close the connection. */
public void close() {
try {
connection.close();
} catch (IOException e) {
Log.v("smswifi","Unable to close connection: " + e.toString());
}
}
/* Send an SMS command to the server. Check that the reply code
is what is is supposed to be according to RFC 821. */
public void sendCommand(String command) throws IOException {
/* Write command to server. */
this.toServer.writeBytes(command+this.CRLF);
/* read reply */
String reply = this.fromServer.readLine();
}
}
that's a basic skeleton for a connection class. you simply instantiate the class, and call open on the instance you create with the host and port (don't forget to close the connection when you're done) and you can change the body of sendCommand to your liking. i've included a read/write operation in the function body as an example.
here is the code to run a server on a remote machine that listens for connections and spawns a thread to handle each request. it can easily interact with the above code for debugging (or any use).
import java.io.*;
import java.net.*;
import java.util.*;
public final class smsd {
///////MEMBER VARIABLES
ServerSocket server=null;
Socket client=null;
///////MEMBER FUNCTIONS
public boolean createSocket(int port) {
try{
server = new ServerSocket(port);
} catch (IOException e) {
System.out.println("Could not listen on port "+port);
System.exit(-1);
}
return true;
}
public boolean listenSocket(){
try{
client = server.accept();
} catch (IOException e) {
System.out.println("Accept failed: ");
System.exit(-1);
}
return true;
}
public static void main(String argv[]) throws Exception {
//
smsd mySock=new smsd();
//establish the listen socket
mySock.createSocket(3005);
while(true) {
if(mySock.listenSocket()) {
//make new thread
// Construct an object to process the SMS request message.
SMSRequest request = new SMSRequest(mySock.client);
// Create a new thread to process the request.
Thread thread = new Thread(request);
// Start the thread.
thread.start();
}
}
//process SMS service requests in an infinite loop
}
///////////end class smsd/////////
}
final class SMSRequest implements Runnable {
//
final static String CRLF = "\r\n";
Socket socket;
// Constructor
public SMSRequest(Socket socket) throws Exception
{
this.socket = socket;
}
// Implement the run() method of the Runnable interface.
public void run()
{
try {
processRequest();
} catch (Exception e) {
System.out.println(e);
}
}
private static void sendBytes(FileInputStream fis, OutputStream os) throws Exception
{
// Construct a 1K buffer to hold bytes on their way to the socket.
byte[] buffer = new byte[1024];
int bytes = 0;
// Copy requested file into the socket's output stream.
while((bytes = fis.read(buffer)) != -1 ) {
os.write(buffer, 0, bytes);
}
}
private void processRequest() throws Exception
{
// Get a reference to the socket's input and output streams.
InputStream is = this.socket.getInputStream();
DataOutputStream os = new DataOutputStream(this.socket.getOutputStream());
// Set up input stream filters.
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
// Get the request line of the SMS request message.
String requestLine = br.readLine();
//print message to screen
System.out.println(requestLine);
//send a reply
os.writeBytes("200");
// Close streams and socket.
os.close();
br.close();
socket.close();
}
}
nb4namingconventions.
almost forgot. you will need to set these permissions inside the tags in your AndroidManifest.xml in order to use wireless.
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />

This is easily possible if both devices are using the same wifi network and can ping each other. You may just create a Java application on your desktop which creates a ServerSocket. Then you can open a Socket in your Android app using the desktop's IP address and send data through the OutputStream.

I believe that Amit is referring to having the machines connect directly to each other using wireless.
There is the development currently of the Wifi-direct specification to allow for Plug-in-Play setup of Access Points. The issue currently is ensuring one of the machines is an AP that other machine(s) can establish connection to.
I'm interested in how this relates to Ad-Hoc networks. I don't have a solution, however I am quite interested in this question too ! (Assuming this is your question Amit).