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Multicastsocket keeps receiving same message in infinite loop

I'm writing a java chat application using Multicast.
The clients can communicate among themselves but they can also send predefined messages to the server who always has the same answer to the respective predefined message.
Both client and server can receive and send messages. They are both subscribing recipients to the same host and port, so their sockets are the same, but when a client is sending a predefined message to the server, the server gets stuck in an infinite loop receiving the same message.
Code Server
final Lock lock = new ReentrantLock();
final Condition Rx = lock.newCondition();
final Condition Tx = lock.newCondition();
private volatile boolean msgRead;
private volatile int TypeMsg;
try{
NetworkInterface nif = NetworkInterface.getByName("en1");
MSocket = new MulticastSocket(port_chat);
group = InetAddress.getByName(adresse_chat);
MSocket.joinGroup(new InetSocketAddress(adresse_chat, port_chat), nif);
}catch(IOException se){
System.out.println(this.toString() + " IOException -> " + se.getMessage());
}
/*
Thread Rx
*/
new Thread(){
#Override
public void run(){
while(!isInterrupted()){
lock.lock();
try{
while(msgRead == true)
Rx.await();
byte[] buf = new byte[256];
DatagramPacket packetRx = new DatagramPacket(buf, buf.length);
try{
MSocket.receive(packetRx);
}catch(IOException ioe){
System.out.println(this.toString() + " IOException -> " + ioe.getMessage());
}
String received = new String(packetRx.getData(), 0, packetRx.getLength());
if("end".equals(received))
break;
if(received.contains("WEATHER_FORECAST") == true)
TypeMsg = 1;
else
if(received.contains("ASK_AGE_CAPTAIN") == true)
TypeMsg = 2;
msgRead = true;
Tx.signal();
}catch(InterruptedException ie){
System.out.println("Thread Rx -> " + ie.getMessage());
}
finally{
lock.unlock();
}
}
}
}.start();
/*
Thread Tx
*/
new Thread(){
#Override
public void run(){
while(!isInterrupted()){
lock.lock();
try{
while(msgRead == false)
Tx.await();
byte[] buf = new byte[256];
/* switch(TypeMsg){...} */
buf = text.getBytes();
DatagramPacket packetTx = new DatagramPacket(buf, buf.length, group, port_chat);
try{
MSocket.send(packetTx);
}catch(IOException ioe){
System.out.println(this.toString() + " IOException -> " + ioe.getMessage());
}
msgRead = false;
Rx.signal();
}catch(InterruptedException ie){
System.out.println("Thread Tx -> " + ie.getMessage());
}finally{
lock.unlock();
}
}
}
}.start();
Code Client
try{
NetworkInterface nif = NetworkInterface.getByName("en1");
MSocket = new MulticastSocket(port_chat);
group = InetAddress.getByName(adresse_chat);
MSocket.joinGroup(new InetSocketAddress(adresse_chat, port_chat), nif);
}catch(IOException se){
System.out.println(this.toString() + " IOException -> " + se.getMessage());
}
/*
Thread Rx
*/
new Thread(){
#Override
public void run(){
while(!isInterrupted()){
byte[] buf = new byte[256];
DatagramPacket packetRx = new DatagramPacket(buf, buf.length);
try{
MSocket.receive(packetRx);
}catch(IOException ioe){
System.out.println(this.toString() + " IOException -> " + ioe.getMessage());
}
String received = new String(packetRx.getData(), 0, packetRx.getLength());
if("end".equals(received))
break;
jTextArea_Rx.append(received + "\n");
}
}
}.start();
/*
Tx
*/
private void jButton_SendActionPerformed(java.awt.event.ActionEvent evt) {
byte[] buf = new byte[256];
String text = username + " >> " + jTextArea_Tx.getText();
buf = text.getBytes();
DatagramPacket packetTx = new DatagramPacket(buf, buf.length, group, port_chat);
try{
MSocket.send(packetTx);
}catch(IOException ioe){
System.out.println(this.toString() + " IOException -> " + ioe.getMessage());
}
}

This feels like it has something to do with the IP_MULTICAT_LOOP socket option which is surprisingly enabled by default in Java Multicast Sockets. Basically when this flag is enabled, you will receive messages you send on the multicast socket. So if you also send a message when you receive a message and you have this enabled, then you can create a loop.
Try disabling this socket option and see what happens.

I got it, I had to add
System.setProperty("java.net.preferIPv4Stack", "true");
at the beginning of the application and I also changed this part
try{
NetworkInterface nif = NetworkInterface.getByName("en1");
MSocket = new MulticastSocket(port_chat);
group = InetAddress.getByName(adresse_chat);
MSocket.joinGroup(new InetSocketAddress(adresse_chat, port_chat), nif);
}catch(IOException se){
System.out.println(this.toString() + " IOException -> " + se.getMessage());
}
to
try{
MSocket = new MulticastSocket(port_chat);
group = InetAddress.getByName(adresse_chat);
MSocket.joinGroup(group);
}catch(IOException se){
System.out.println(this.toString() + " IOException -> " + se.getMessage());
}
so no need to explicitly specifying the interface. I found the answer after some time here: Getting `Can't assign requested address` java.net.SocketException using Ehcache multicast

What is the problem of this code...? Java socket programming

I am coding client-server multithread calculator using java, socket programming.
There's any syntax error, but msgs cannot be received from server.
I think
receiveString = inFromServer.readLine()
does not works. This code is in Client program, in the while(true) loop.
What is the problem?
Here is my full code.
SERVER
import java.io.*;
import java.net.*;
public class Server implements Runnable
{
static int max = 5; //maximum thread's number
static int i = 0, count = 0; //i for for-loop, count for count number of threads
public static void main(String args[]) throws IOException
{
ServerSocket serverSocket = new ServerSocket(6789); //open new socket
File file = new File("src/serverinfo.dat"); //make data file to save server info.
System.out.println("Maximum 5 users can be supported.\nWaiting...");
for(i=0; i <= max; i++) { new Connection(serverSocket); } //make sockets - loop for max(=5) times
try //server information file writing
{
String dataString = "Max thread = 5\nServer IP = 127.0.0.1\nServer socket = 6789\n";
#SuppressWarnings("resource")
FileWriter dataFile = new FileWriter(file);
dataFile.write(dataString);
}
catch(FileNotFoundException e) { e.printStackTrace(); }
catch(IOException e) { e.printStackTrace(); }
}
static class Connection extends Thread
{
private ServerSocket serverSocket;
public Connection(ServerSocket serverSock)
{
this.serverSocket = serverSock;
start();
}
public void run()
{
Socket acceptSocket = null;
BufferedReader inFromClient = null;
DataOutputStream msgToClient = null;
String receiveString = null;
String result = "", sys_msg = "";
try
{
while(true)
{
acceptSocket = serverSocket.accept(); // 접속수락 소켓
count++;
inFromClient = new BufferedReader(new InputStreamReader(acceptSocket.getInputStream()));
msgToClient = new DataOutputStream(acceptSocket.getOutputStream());
System.out.println(count + "th client connected: " + acceptSocket.getInetAddress().getHostName() + " " + count + "/" + max);
System.out.println("Waiting response...");
while(true)
{
if (count >= max+1) // if 6th client tries to access
{
System.out.println("Server is too busy. " + max + " clients are already connected. Client access denied.");
sys_msg = "DENIED";
msgToClient.writeBytes(sys_msg);
acceptSocket.close();
count--;
break;
}
try{ msgToClient.writeBytes(result); }
catch(Exception e) {}
try{ receiveString = inFromClient.readLine(); }
catch(Exception e) // if receiveString = null
{
System.out.println("Connection Close");
count--;
break;
}
System.out.println("Input from client : " + receiveString);
try
{
if(receiveString.indexOf("+") != -1) { result = cal("+", receiveString); }
else if(receiveString.indexOf("-") != -1) { result = cal("-", receiveString); }
else if(receiveString.indexOf("/") != -1) { result = cal("/", receiveString); }
else if(receiveString.indexOf("*") != -1) { result = cal("*", receiveString); }
else if(receiveString.indexOf("+") == -1 || receiveString.indexOf("-") == -1 || receiveString.indexOf("*") == -1 || receiveString.indexOf("/") == -1) { result = "No INPUT or Invalid operation"; }
}
catch(Exception e){ result = "Wrong INPUT"; }
try{ msgToClient.writeBytes(result); }
catch(Exception e) {}
}
}
}
catch(IOException e) { e.printStackTrace(); }
}
}
private static String cal(String op, String recv) //function for calculating
{
double digit1, digit2; //first number, second number
String result = null;
digit1 = Integer.parseInt(recv.substring(0, recv.indexOf(op)).trim());
digit2 = Integer.parseInt(recv.substring(recv.indexOf(op)+1, recv.length()).trim());
if(op.equals("+")) { result = digit1 + " + " + digit2 + " = " + (digit1 + digit2); }
else if(op.equals("-")) { result = digit1 + " - " + digit2 + " = " + (digit1 - digit2); }
else if(op.equals("*")) { result = digit1 + " * " + digit2 + " = " + (digit1 * digit2); }
else if(op.equals("/"))
{
if(digit2 == 0){ result = "ERROR OCCURRED: Cannot be divided by ZERO"; }
else{ result = digit1 + " / " + digit2 + " = " + (digit1 / digit2); }
}
return result;
}
#Override
public void run() {
// TODO Auto-generated method stub
}
}
-----------------------------------------------------------------
CLIENT
import java.io.*;
import java.net.*;
public class Client {
public static void main(String args[]) throws IOException
{
Socket clientSocket = null;
BufferedReader userInput = new BufferedReader(new InputStreamReader(System.in));
BufferedReader inFromServer = null;
DataOutputStream msgToServer = null;
String sendString = "", receiveString = "";
try
{
clientSocket = new Socket("127.0.0.1", 6789); //make new clientSocket
inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
msgToServer = new DataOutputStream(clientSocket.getOutputStream());
System.out.println("Input exit to terminate");
System.out.println("Connection Success... Waiting for permission");
while(true)
{
receiveString = inFromServer.readLine();
if(receiveString.equals("DENIED"))
{
System.out.println("Server is full. Try again later.");
break;
}
else { System.out.println("Connection permitted."); }
System.out.print("Input an expression to calculate(ex. 3+1): ");
sendString = userInput.readLine();
if(sendString.equalsIgnoreCase("exit")) //when user input is "exit" -> terminate
{
clientSocket.close();
System.out.println("Program terminated.");
break;
}
try { msgToServer.writeBytes(sendString); }
catch(Exception e) {}
try { receiveString = userInput.readLine(); }
catch(Exception e) {}
System.out.println("Result: " + receiveString); //print result
}
}
catch(IOException e)
{
e.printStackTrace();
}
}
}

You've set up your server socket stack wrong.
Your code will make 5 threads, each calling accept on a serversocket.
The idea is to have a single ServerSocket (and not 5, as in your example). Then, this single serversocket (running in a single thread that handles incoming sockets flowing out of this serversocket) will call .accept which will block (freeze the thread) until a connection is made, and will then return a Socket object. You'd then spin off a thread to handle the socket object, and go right back to the accept call. If you want to 'pool' (which is not a bad idea), then disassociate the notion of 'handles connections' from 'extends Thread'. For example, implement Runnable instead. Then pre-create the entire pool (for example, 10 threads), have some code that lets you 'grab a thread' from the pool and 'return a thread' to the pool, and now the serversocket thread will, upon accept returning a socket object, grab a thread from the pool (which will block, thus also blocking any incoming clients, if every thread in the pool is already taken out and busy handling a connection), until a thread returns to the pool. Alternatively, the serversocket code checks if the pool is completely drained and if so, will put on a final thread the job of responding to that client 'no can do, we are full right now'.
I'm not sure if you actually want that; just.. make 1 thread per incoming socket is a lot simpler. I wouldn't dive into pool concepts until you really need them, and if you do, I'd look for libraries that help manage them. I think further advice on that goes beyond the scope of this question, so I'll leave the first paragraph as an outlay of how ServerSocket code ought to work, for context.

Reliable UDP in java

I am working on my assignment to make UDP reliable using java. How can i add Timeout and re-transmission to handle data-grams that are discarded and add Sequence numbers so the client can verify that a reply is for the appropriate request ??
this is client code
import java.net.*;
import java.io.*;
public class EchoClient {
// UDP port to which service is bound
public static final int SERVICE_PORT = 7;
// Max size of packet
public static final int BUFSIZE = 256;
public static void main(String args[]){
if (args.length != 1)
{
System.err.println ("Syntax - java EchoClient hostname");
return;
}
String hostname = args[0];
// Get an InetAddress for the specified hostname
InetAddress addr = null;
try
{
// Resolve the hostname to an InetAddr
addr = InetAddress.getByName(hostname);
}
catch (UnknownHostException uhe)
{
System.err.println ("Unable to resolve host");
return;
}
try
{
// Bind to any free port
DatagramSocket socket = new DatagramSocket();
// Set a timeout value of two seconds
socket.setSoTimeout (2 * 1000);
for (int i = 1 ; i <= 10; i++)
{
// Copy some data to our packet
String message = "Packet number " + i ;
char[] cArray = message.toCharArray();
byte[] sendbuf = new byte[cArray.length];
for (int offset = 0; offset < cArray.length ; offset++)
{
sendbuf[offset] = (byte) cArray[offset];
}
// Create a packet to send to the UDP server
DatagramPacket sendPacket = new DatagramPacket(sendbuf, cArray.length, addr, SERVICE_PORT);
System.out.println ("Sending packet to " + hostname);
// Send the packet
socket.send (sendPacket);
System.out.print ("Waiting for packet.... ");
// Create a small packet for receiving UDP packets
byte[] recbuf = new byte[BUFSIZE];
DatagramPacket receivePacket = new DatagramPacket(recbuf, BUFSIZE);
// Declare a timeout flag
boolean timeout = false;
// Catch any InterruptedIOException that is thrown
// while waiting to receive a UDP packet
try
{
socket.receive (receivePacket);
}
catch (InterruptedIOException ioe)
{
timeout = true;
}
if (!timeout)
{
System.out.println ("packet received!");
System.out.println ("Details : " + receivePacket.getAddress() );
// Obtain a byte input stream to read the UDP packet
ByteArrayInputStream bin = new ByteArrayInputStream (
receivePacket.getData(), 0, receivePacket.getLength() );
// Connect a reader for easier access
BufferedReader reader = new BufferedReader (
new InputStreamReader ( bin ) );
// Loop indefinitely
for (;;)
{
String line = reader.readLine();
// Check for end of data
if (line == null)
break;
else
System.out.println (line);
}
}
else
{
System.out.println ("packet lost!");
}
// Sleep for a second, to allow user to see packet
try
{
Thread.sleep(1000);
}catch (InterruptedException ie) {}
}
}
catch (IOException ioe)
{
System.err.println ("Socket error " + ioe);
}
}
}

What you can do is adding import TCP headers like sequence number, windows into the UDP message body to make it more like TCP. Here is the a solution that might help you.

java socket not accepting byte array

I am writing a simple socket program as shown below. I want to send a byte array to a server socket and readi it at the server end.
This is the server
package com.java;
//File Name GreetingServer.java
import java.net.*;
import java.io.*;
public class GreetingsServer extends Thread
{
private ServerSocket serverSocket;
public GreetingsServer(int port) throws IOException
{
serverSocket = new ServerSocket(port);
serverSocket.setSoTimeout(10000);
}
public void run()
{
while(true)
{
try
{
System.out.println("Waiting for client on port " +
serverSocket.getLocalPort() + "...");
Socket server = serverSocket.accept();
System.out.println("Just connected to "
+ server.getRemoteSocketAddress());
DataInputStream in =
new DataInputStream(server.getInputStream());
System.out.println("--"+ server.getReceiveBufferSize());
byte[] b1 = new byte[4] ;
int i =in.read(b1);
System.out.println("i = " + i + "b1 = " +b1);
/* DataOutputStream out =
new DataOutputStream(server.getOutputStream());
out.writeUTF("Thank you for connecting to "
+ server.getLocalSocketAddress() + "\nGoodbye!");*/
server.close();
}catch(SocketTimeoutException s)
{
System.out.println("Socket timed out!");
break;
}catch(IOException e)
{
e.printStackTrace();
break;
}
}
}
public static void main(String [] args)
{
int port = Integer.parseInt(args[0]);
try
{
Thread t = new GreetingsServer(port);
t.start();
}catch(IOException e)
{
e.printStackTrace();
}
}
}
This is the client
package com.java;
import java.net.*;
import java.io.*;
public class GreetingClient
{
public static void main(String [] args)
{
String serverName = args[0];
int port = Integer.parseInt(args[1]);
try
{
System.out.println("Connecting to " + serverName
+ " on port " + port);
Socket client = new Socket(serverName, port);
System.out.println("Just connected to "
+ client.getRemoteSocketAddress());
OutputStream outToServer = client.getOutputStream();
byte[] b = {1,2,3,4};
DataOutputStream out =
new DataOutputStream(outToServer);
/*BufferedOutputStream out = new BufferedOutputStream(outToServer);*/
System.out.println("**" + b);
out.write(b);
/*out.writeUTF("Hello from "
+ client.getLocalSocketAddress());*/
client.close();
}catch(IOException e)
{
e.printStackTrace();
}
}
}
The output I get is this for the server.
Waiting for client on port 6066...
Just connected to /127.0.0.1:52349
--65536
i = 4b1 = [B#48270664
Waiting for client on port 6066...
Socket timed out!
There two things wrong. The first is that the bytearray is not coming entirely to the server. And secondly the server is calling thread twice.
Any input will be helpful. Please let me know if anyone finds a problem!

The problem isn't what is being sent/received but rather how you're trying to output it to the console.
Arrays don't override toString(). In your server you have:
System.out.println("i = " + i + "b1 = " +b1);
The output of b1.toString() is the default from Object.toString() which as noted in the Javdoc for Object is:
a string consisting of the name of the class of which the object is an instance, the at-sign character `#', and the unsigned hexadecimal representation of the hash code of the object.
You need to use Arrays.toString(b1) to get a String that shows the actual bytes in that array as [ x, x, x ,x ] (where each x is the numeric value of the byte)

Java UDP socket client not blocking

I'm trying to code a UDP client to receive packets from a server that is broadcasting on the local network. The problem is the receive method isn't blocking and waiting for a packet to arrive.
Instead, it's receiving null or empty packets.
I've tried to use .setSoTimeout(0), which supposedly will tell the receive to block until it receives a packet, but it doesn't.
Does anyone know how to fix this?
Here's the code:
while (search == true) {
InetAddress addr = InetAddress.getByName("0.0.0.0");
DatagramSocket sock = new DatagramSocket(1355);
sock.setSoTimeout(0);
byte[] recebe = new byte[1024];
sock.setBroadcast(true);
System.out.println("entrou1");
DatagramPacket packet = new DatagramPacket(recebe, recebe.length);
System.out.println("entrou2");
sock.receive(packet);
String info = new String(packet.getData());
System.out.println("tamanho: " + info.length());
if (info.trim().equals("") == false && info != null) {
System.out.println("entrou aqui");
System.out.println("info recebida:" + info + ":fsadfsfs");
String servs[] = info.split("\n");
list1.clear();
servidores.clear();
for (int i = 0; i < servs.length; i++) {
System.out.println("vec: " + servs[i]);
if (servs[i].trim().equals("")) {
System.out.println("break;");
break;
} else {
String aux = servs[i].substring(0, servs[i].lastIndexOf("->"));
System.out.println("aux: " + aux);
list1.add(aux);
servidores.add(servs[i]);
}
}
}
System.out.println("info:\n" + info);
sock.close();
synchronized (obj) {
try {
obj.wait();
} catch (InterruptedException ex) {
Logger.getLogger(AcederPartilhaGUI.class.getName()).log(Level.SEVERE, null, ex);
}
}
}