Develop Reference

Android can't send PrintWriter message twice in socket with Java. It is also possible instead of sending me a Java code to send a Python code

I created a code that sends commands to a server I wrote using Python. The code only works once (the server receives what I sent the first time) but the second time it seems that nothing is sent because the server does not receive new information it keeps waiting for new information. My code:
StringBuilder Data = new StringBuilder(); // The Data to send
public void Send(View view) {
Thread send = new Thread() {
#Override
public void run() {
try {
Socket socket = new Socket("20.7.65.2", 6398); // Create connection to the server
OutputStream output = socket.getOutputStream(); // Get the key to output to server
PrintWriter writer = new PrintWriter(output, true);
writer.println(Data.toString()); // Send the data
Data.setLength(0); // Delete "Data" contant
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
};
send.start();
}
public void Continue(View view) {
Data.append("Hello from client"); // Append to "Data"
Send(null); // Run the send functionn (again)
}
}
My Python Server:
import socket, time
soc = socket.socket()
soc.bind((socket.gethostname(), 6398))
soc.listen(5)
(client, (ipNum, portNum)) = soc.accept()
print("Client connected")
while True:
message = client.recv(1024).decode()
if(message != ""):
print(message)
time.sleep(0.1)
In short, I try to run the run function twice. The first time it sends to the server and it receives the information, and the second time the server is still waiting for the information and not receiving what I sent again. Maybe it's because he's not available to receive all messages sent to him from all clients
It is also possible instead of sending me a Java code to send a Python code that will work and receive all messages from all clients

In your code, the server is only accepting a cnnection once and then it recieves from same client. But according to your question, I think your server should be able to listen to mulitple clients hence, you could use multithreading in the server. Instead of threading the client, I used button which when clicked connects with server. I also can't understand the need of threading the client. If you think, some changes are required in the answer, you could comment.
this is python server
import socket, time
import threading
soc = socket.socket()
# print(socket.)
soc.bind(("192.168.1.5", 6398))
soc.listen(5)
def info(client):
message = client.recv(1024).decode()
if(message != ""):
print(message)
return
while True:
(client, (ipNum, portNum)) = soc.accept()
print("Client connected")
threading.Thread(target=info,args=(client,)).start()
MainActivity.java
package com.example.myapplication;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;
public class MainActivity extends AppCompatActivity {
StringBuilder Data = new StringBuilder(); // The Data to send
private Button btn;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn= findViewById(R.id.connectBtn);
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Continue();
}
});
}
public void Send() {
Thread send = new Thread() {
#Override
public void run() {
try {
Socket socket = new Socket("192.168.1.5", 6398); // Create connection to the server
OutputStream output = socket.getOutputStream(); // Get the key to output to server
PrintWriter writer = new PrintWriter(output, true);
writer.println(Data.toString()); // Send the data
Data.setLength(0); // Delete "Data" contant
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
};
send.start();
}
public void Continue() {
Data.append("Hello from client"); // Append to "Data"
Send(); // Run the send functionn (again)
}
}
Don't forget to add <uses-permission android:name="android.permission.INTERNET"/> in the AndroidManifest.xml.
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<Button
android:id="#+id/connectBtn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

java exit app causes infinite loop

i have made a simple java app to print a text file then should terminate.
I tried to use the return statemend but didn't work at all. So i used the system.exit(0) but instead of exit the app, it goes in an infinite loop...
this is my code: can someone tell me what is wrong ?
package com.example.testprinterdemo;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import com.pda3505.Service.CaptureService;
import com.pda3505.printer.PrinterClassSerialPort;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
public class MainActivity extends Activity {
public static PrinterClassSerialPort printerClass = null;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
init();
printerClass.write(new byte[] { 0x1d, 0x21,0x00,0x00});
printerClass.write(new byte[] { 0x1b, 0x26, 0x00 });
BufferedReader reader = null;
try {reader = new BufferedReader(new FileReader("/mnt/sdcard/temp.txt"));}
catch (FileNotFoundException e) {e.printStackTrace();}
String line = null;
try {line = reader.readLine();}
catch (IOException e) {e.printStackTrace();}
while(line!=null) {
printerClass.printText(line);
printerClass.printText("\n");
try {line = reader.readLine();}
catch (IOException e) {e.printStackTrace();}
}
system.exit(0);
}
private void init() {
printerClass = new PrinterClassSerialPort(new Handler());
printerClass.open(this);
Intent newIntent = new Intent(MainActivity.this, CaptureService.class);
newIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startService(newIntent);
}
}

try{
//never hard code the file path in Android.
String filePath = Environment.getExternalStorageDirectory().getAbsolutePath()+File.separator+"temp.txt";
BufferedReader reader = new BufferedReader(new FileReader(filePath));
while (true){
String line = reader.readLine();
if(line.length() > 0){
printerClass.printText(line);
printerClass.printText("\n");
}else{
break;
//you can use return too
}
}
}catch (IOException e){
e.printStackTrace();
}
In your approach, line is never null, it is either "" or it is " " or "\n" etc, but never null. so listen for length and break the loop
Please avoid multiple try catch blocks when you can have your code under single try block. And please scope the variables properly, do not declare variables outside the scope where it is not used, as it causes problems for JAVA Garbage collector/

UDP Server android application wont upload to phone

I have built a upd server listen code to upload on my phone and listen to data packets. This code will compile but for some reason it wont load to my phone. I don't see any errors anywhere. Why could this be ? this is my whole Mainactivity code :
package roman10.tutorial.udpcommserver;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;
import java.net.InetSocketAddress;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;
public class UdpServer extends Activity {
/** Called when the activity is first created. */
private TextView textView;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
textView = (TextView) findViewById(R.id.text1);
runUdpServer();
}
private static final int UDP_SERVER_PORT = 4000;
private static final int MAX_UDP_DATAGRAM_LEN = 1500;
private static final String ipAdd = new String("172.30.42.80");
private void runUdpServer() {
String lText;
// byte[] lMsg = new byte[MAX_UDP_DATAGRAM_LEN];
// DatagramPacket dp = new DatagramPacket(lMsg, lMsg.length);
// DatagramSocket ds = null;
byte buffer[] = new byte[MAX_UDP_DATAGRAM_LEN];
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
try {
// ds = new DatagramSocket(UDP_SERVER_PORT);
// //disable timeout for testing
// //ds.setSoTimeout(100000);
// ds.receive(dp);
//lText = new String(lMsg, 0, dp.getLength());
// Log.i("UDP packet received", lText);
// textView.setText(lText);
DatagramSocket s = new DatagramSocket();
InetSocketAddress address = new InetSocketAddress(ipAdd, UDP_SERVER_PORT);
s.bind(address);
lText = new String(buffer,0,packet.getLength());
Log.i("UDP packet received", lText);
textView.setText(lText);
System.out.println("Waiting...");
s.receive(packet);
if (s != null) {
s.close();
}
// s.close();
System.out.println("Received!");
} catch (SocketException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
System.out.print("we are done ");
}
}
}

1)
You are not returning from your onCreate (until you have received something). The activity can't continue to start (certainly not call onStart, onResume etc.
2)
Then you are doing I/O on the main thread, network I/O even, not recommended and usually detected as not allowed on android and force closed. You will get a NetworkOnMainThreadException
Fortunately both those issues can be solved by creating a separate thread and executing runUdpServer() there.
Short example, instead of:
runUdpServer();
do:
Thread thread = new Thread() {
public void run() {
runUdpServer();
}
};
thread.start();

Unable connecting to a remote socket from OnReceive( )

I am writing an Android Application which has a broadcast receiver secured with permissions. The broadcast receiver on receiving requests from authenticated application scans port 80 of a hardcoded IP address. It returns the result of the socket connection attempt. In my code I have the socket connection called from OnReceive ( ) method. I am able to get to the receiver as i can see the toast messages. However, socket connection is caught by Exception E. Here is my code for the receiver :
package com.example.naveenbreceive;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.UnknownHostException;
import android.os.Bundle;
import android.app.Activity;
import android.content.ActivityNotFoundException;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.util.Log;
import android.view.Menu;
import android.widget.Toast;
public class breceiver extends Activity {
BroadcastReceiver mReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
String Target="107.20.112.24";
int i=80;
int timeout=1000;
Toast.makeText(getApplicationContext(),"Intent Received",Toast.LENGTH_LONG).show();
Log.i("INFO","toast launched");
try {
Socket socket = new Socket();
socket.connect(new InetSocketAddress(Target, i), timeout);
if (socket.isConnected())
{Toast.makeText(getApplicationContext(), "Connected to port 80", Toast.LENGTH_LONG).show();
socket.close();
}
else
{Toast.makeText(getApplicationContext(), "Unable to Connect port 80", Toast.LENGTH_LONG).show();
socket.close();
}
}
catch (UnknownHostException e) {
// TODO Auto-generated catch block
Toast.makeText(getApplicationContext(),"UnknownHost Exception",Toast.LENGTH_LONG).show();
e.printStackTrace();
}
catch (IOException e) {
// TODO Auto-generated catch block
Toast.makeText(getApplicationContext(),"IO Exception",Toast.LENGTH_LONG).show();
e.printStackTrace();
}
catch (Exception e) {
// TODO Auto-generated catch block
Toast.makeText(getApplicationContext(),"Exception",Toast.LENGTH_LONG).show();
e.printStackTrace();
}
}
};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// TODO Auto-generated method stub
IntentFilter filterView = new IntentFilter();
filterView.addAction("com.example.HACKING");
registerReceiver(mReceiver, filterView,"com.example.permission.naveen", null);
Toast.makeText(getApplicationContext(),"Inside Receiver",Toast.LENGTH_LONG).show();
}

The broadcast receiver is on the UI thread. You can't do networking on the UI thread. You need to do it in a Thread or AsyncTask.
Also, BroadcastReceivers are supposed to be very quick and do very little work. Their design is supposed to be a quick notification and then you do the heavy lifting elsewhere, so the next app can get the notification. Networking IO is way too much work for a receiver.

Problem with MulticastSocket on Java-Android

I'm starting to code with MulticastSocket, trying to make a simple app with a client and a server to send messages.
The code I have for the server:
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.net.SocketException;
public class Servidor {
private static MulticastSocket ms;
public static void main(String[] args) throws IOException{
InetAddress sessAddr = InetAddress.getByName("224.2.76.24");
try{
sessAddr = InetAddress.getByName("224.2.76.24");
ms = new MulticastSocket(5500);
ms.joinGroup(sessAddr);
while (true)
{
byte[] mensaje = new byte[1024];
mensaje = "aa".getBytes();
DatagramPacket dp = new DatagramPacket(mensaje, mensaje.length,sessAddr,5500);
ms.send(dp);
}
}
catch (SocketException se) {
System.err.println(se);
}
ms.leaveGroup(sessAddr);
}
}
And this on the client:
package com.example;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.net.UnknownHostException;
import android.app.Activity;
import android.os.Bundle;
import android.widget.EditText;
import android.widget.TextView;
public class ClienteMultiCast extends Activity {
/** Called when the activity is first created. */
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
TextView Mensaje;
Mensaje =(TextView)findViewById(R.id.Mensaje);
InetAddress ia = null;
byte[] buffer = new byte[65535];
MulticastSocket ms = null;
int port = 5500;
try {
ia = InetAddress.getByName("224.2.76.24");
DatagramPacket dp = new DatagramPacket(buffer, buffer.length,ia,port);
ms = new MulticastSocket(port);
ms.joinGroup(ia);
while (true) {
ms.receive(dp);
String s = new String(dp.getData(),0,dp.getLength());
Mensaje.setText(s);
}
} catch (UnknownHostException e) {Mensaje.setText(e.getMessage());} catch (IOException e) {Mensaje.setText(e.getMessage()); }
try {
ms.leaveGroup(ia);
} catch (IOException e) {
Mensaje.setText(e.getMessage());
}
}
}
The problem is that when I start both, nothing happens. The client doesn't get any message.
Any idea what's wrong?

Diego,
By default, the Android WiFi stack filters out multicast packets. Take a look at http://developer.android.com/reference/android/net/wifi/WifiManager.MulticastLock.html.
You need something along the lines of:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
/* Turn off multicast filter */
MulticastLock mcastLock = new MulticastLock();
mcastLock.acquire();
/* Process Multicast Packets */
}

It appears that Multicast support in Android is not as solid as some of use might hope. See http://codeisland.org/2012/udp-multicast-on-android/
Ie whether it actually works out or may be device dependent. It is not working on my Nexus5.
https://code.google.com/p/android/issues/detail?id=51195