Develop Reference

Multi-Threaded Server Chat Java - Connecting but no messages

Trying to build a simple multi-threaded chat server that runs via command prompt. The clients do connect to the server and the server will hold multiple clients, however when trying to send messages from one client to the other, or even notifying a client of another user logging in, nothing comes up on the client command prompt.
public class Server {
private static ServerSocket servSock;
private static Socket clientSock;
private static ArrayList<ClientThread> clientList;
private static int IDcount = 0;
public static void main(String args[]){
// Get command line arguments.
if (args.length != 3) {
System.out.println("Required arguments: server port, block duration, timeout");
return;
}
int port = Integer.parseInt(args[0]);
int blockDur = Integer.parseInt(args[1]);
int timeout = Integer.parseInt(args[2]);
try{
servSock = new ServerSocket(port);
clientList = new ArrayList<ClientThread>();
}
catch(IOException ex){
System.err.println(ex);
}
while (true) {
try {
clientSock = servSock.accept();
ClientThread thread = new ClientThread(clientSock);
clientList.add(thread);
thread.start();
} catch (IOException e) {
System.out.println(e);
}
}
}
private synchronized static void broadcast(String msg){
System.out.print(msg);
for(int i = 0; i < clientList.size(); i++){
ClientThread client = clientList.get(i);
client.send(msg);
}
}
synchronized static void unlist(int id){
for(int i = 0; i < clientList.size(); i++){
ClientThread thread = clientList.get(i);
if(thread.id == id){
clientList.remove(i);
return;
}
}
}
static class ClientThread extends Thread {
Socket sock;
BufferedReader tIn;
PrintWriter tOut;
int id;
String username;
String msg;
ClientThread(Socket sock){
id = IDcount++;
this.sock = sock;
try{
tIn = new BufferedReader(new InputStreamReader(sock.getInputStream()));
tOut = new PrintWriter(sock.getOutputStream());
username = tIn.readLine();
broadcast(username + " logged in");
}
catch(IOException ex){
System.err.println(ex);
}
}
public void run(){
boolean loggedIn = true;
while(loggedIn){
try{
msg = tIn.readLine();
}
catch (IOException ex){
System.err.println(ex);
}
String[] parts = msg.split("\\s",2);
String type = parts[0];
Client code is similar
public class Client{
private static Socket clientSock;
private static BufferedReader in;
private static PrintWriter out;
private static Scanner scan;
public static void main(String[] args) throws IOException {
if (args.length != 2) {
System.out.println("Required arguments: server IP, server port");
return;
}
String host = args[0];
int port = Integer.parseInt(args[1]);
clientSock = new Socket(host, port);
in = new BufferedReader(new InputStreamReader(clientSock.getInputStream()));
out = new PrintWriter(clientSock.getOutputStream());
scan = new Scanner(System.in);
new ListenFromServer().start();
boolean online = true;
System.out.println("Enter your username:");
String username = scan.nextLine();
out.println(username);
while(online){
System.out.println("> ");
String msg = scan.nextLine();
String[] parts = msg.split("\\s");
String type = parts[0];
send(msg);
if(type.equalsIgnoreCase("logout")){
online = false;
}
}
logoff();
}
static void send(String msg) throws IOException{
out.println(msg);
}
private static void logoff() throws IOException{
in.close();
out.close();
scan.close();
clientSock.close();
}
static class ListenFromServer extends Thread{
public void run(){
while(true){
try{
String msg = in.readLine();
System.out.println(msg);
}
catch(IOException ex){
System.err.println(ex);
}
}
}
}
}

When you send data with PrintWriter.println(), data write to buffer. You must call PrintWriter.flush() after println to send data to or from the server immediately.
And when you call username = tIn.readLine(); in ClientThread constructor, you block main-thread, because constructor calls in main-thread. So, while connected user won't send username, other clients can't connect.

Java TCP byteArray transfering using buffer and threading

So basically what I want to do is: Create a very simple multithreaded TCP server that can connect to several clients at once. This using threads and transferring messages through Byte[] and returning an echo of the message.
I have never touched anything related to server programming or TCP before, so I expect to have made a lot of mistakes. I am open for improvement and suggestions.
I made a simple Server class:
public class TCPEchoServer {
public static final int SERVERPORT = 4950;
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(SERVERPORT);
while (true) {
Socket clientSocket = serverSocket.accept();
Runnable connectionHandler = new TCPConnectionHandler(clientSocket);
new Thread(connectionHandler).start();
}
}
}
And the connection handler class:
public class TCPConnectionHandler implements Runnable {
private final Socket clientSocket;
private int msgLength = 0;
private byte[] data;
public TCPConnectionHandler(Socket clientSocket) {
this.clientSocket = clientSocket;
}
#Override
public void run() {
try {
BufferedReader inFromClient = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
DataOutputStream outToClient = new DataOutputStream(clientSocket.getOutputStream());
InputStream in = clientSocket.getInputStream();
DataInputStream dis = new DataInputStream(in);
msgLength = dis.readInt();
data = new byte[msgLength];
if (msgLength > 0) {
dis.readFully(data);
}
String message = inFromClient.readLine();
System.out.println("Message recieved: " + message);
outToClient.writeBytes(String.valueOf(data));
clientSocket.close();
}
catch (IOException e) {
System.out.printf("Could not listen on port: " + clientSocket.getLocalPort());
}
}
}
And the Client class:
public class TCPEchoClient {
public static final int MYPORT = 0;
public static int BUFFSIZE = 0;
public static Socket socket;
public static final String MSG = "An Echo Message! LOL";
public static String RETURNMSG = "";
public static final byte[] messageByteArr = MSG.getBytes(Charset.forName("UTF-8"));
private static final Pattern PATTERN = Pattern.compile(
"^(([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.){3}([01]?\\d\\d?|2[0-4]\\d|25[0-5])$");
public static void main(String[] args) throws IOException, InterruptedException {
if (args.length != 4) {
System.err.printf("ERROR: The EchoClient expects 4 parameter inputs:");
System.out.printf("IP address, Port number, message rate (msg/second).");
System.exit(1);
}
if (!isValidIP(args[0])) {
System.out.printf("ERROR: The entered IP address is not a valid IPv4 address.");
System.out.printf("Please enter a valid IPv4 address as the first argument.");
System.exit(1);
}
if (Integer.parseInt(args[1]) < 0 || Integer.parseInt(args[1]) > 65535) { //If the portnumber is negative or bigger than the highest portnumber (unsigned 16-bit integer)
System.out.printf("ERROR: The chosen portnumber is outside the available range.");
System.out.printf("Expected portnumbers: 0-65535");
System.exit(1);
}
if (Integer.parseInt(args[3]) < messageByteArr.length) {
System.out.println("Buffer size can not be smaller than the message size.");
System.out.println("Current message size: " + messageByteArr.length);
System.exit(1);
}
BUFFSIZE = Integer.parseInt(args[3]);
byte[] buf = new byte[BUFFSIZE];
DataOutputStream outToServer = null;
BufferedReader serverEcho = null;
try {
socket = new Socket(args[0], Integer.parseInt(args[1]));
outToServer = new DataOutputStream(socket.getOutputStream());
serverEcho = new BufferedReader(new InputStreamReader(socket.getInputStream()));
} catch (UnknownHostException e) {
System.out.println("Unknown host address: " + args[0]);
System.exit(1);
} catch (IOException e) {
System.out.println("Could not access port " + Integer.parseInt(args[1]));
System.exit(1);
}
int msgLength = 0;
int msgStart = 0;
if (msgLength < 0) {
throw new IllegalArgumentException("Negative length not allowed.");
}
if (msgStart < 0 || msgStart >= messageByteArr.length) {
throw new IndexOutOfBoundsException("Out of bounds: " + msgStart);
}
for (int i = 1; i <= Integer.parseInt(args[2]); i++) {
outToServer.writeInt(msgLength);
if (msgLength > 0) {
outToServer.write(messageByteArr, msgStart, msgLength);
System.out.println("Message sent: " + MSG);
}
RETURNMSG = serverEcho.readLine();
System.out.println("ECHO MESSAGE: " + RETURNMSG);
}
socket.close();
}
private static boolean isValidIP(final String ip) {
return PATTERN.matcher(ip).matches();
}
}
I ran into a problem just now as well, the print outs worked before on the client and sever, but now when a message is sent. Nothing happens at all.
My main question is how I can incorporate a buffer and use it when sending and receiving messages.

You have a loop in the client that will wait args[2] times the RETURNMSG, the server will send this message only once. The RETURNMSG reading code should be outside (after) the for loop.
You should flush the buffer once you've finished writing in it.
This should be done in both client and server, since your client will wait the RETURNMSG forever depending how your protocol will evolve in the future.

Connection Refused - Java Client/Server

I'm trying to create a java push server model for testing on my own machine, this won't be used to connect external clients as the server and client will be on the same machine. After the client connects on the port specified by he/she, they should send a message of their choice that will be sent back to them, and any other clients connected to the same port.
The problem I'm having is i receive a java.net.ConnectException: Connection refused: connect when this is attempted. Below is the client and the server.
Edit: I've taken the time to ensure that the necessary ports are open too, and even disabled my firewall, but to no avail.
Server:
class MainServer {
// port that oir server is going to operate on
final static int port = 1234;
public static void main(String[] args) {
// this is going to model the server for the moment
System.out.println("Server has been started...");
Buffer<Messages> store = new Buffer<Messages>(10);
new Writer(store).start();
try {
ServerSocket serve = new ServerSocket(port);
while(true) {
// wait for server request
Socket socket = serve.accept();
// start thread to service request
new ServerThread(socket,store).start();
}
} catch(IOException e) {e.printStackTrace();}
}
}
class ServerThread extends Thread {
Socket socket;
Buffer<Messages> buffer;
public ServerThread(Socket s, Buffer<Messages> b) {
socket = s;buffer = b;
}
public void run() {
try {
DataInputStream in = new DataInputStream(socket.getInputStream());
int port = in.readInt();
System.out.println("Port: "+port);
Messages ms = new Messages(port);
// Read message as string from user
String message = in.readUTF();
int k = in.readInt();
while(k != -1) {
// Add message to array
// read next message
ms.add(message);
message = in.readUTF();
}
// close connection
socket.close();
// add message to buffer
buffer.put(ms);
} catch(IOException e) {e.printStackTrace();}
}
}
class Writer extends Thread {
Buffer<Messages> buffer;
public Writer(Buffer<Messages> m) {buffer = m;}
public void run() {
while(true) {
Messages dp = buffer.get();
dp.write();
}
}
}
class Buffer <E> {
/**
* Producer & Consumer Buffer
*/
private int max;
private int size = 0;
private ArrayList<E> buffer;
private Semaphore empty; //control consumer
private Semaphore full; // control producer
private Lock lock = new ReentrantLock();
public Buffer(int s) {
buffer = new ArrayList<E>();
max = s;
empty = new Semaphore(0);
full = new Semaphore(max);
}
// add data to our array
public void put(E x) {
try {
full.acquire();
} catch(InterruptedException e) {}
// sync update to buffer
lock.lock();
try {
buffer.add(x);
size++;
empty.release();
} finally {lock.unlock();}
}
public E get() {
try {
empty.acquire();
} catch(InterruptedException e) {}
// sync uodate on buffer
lock.lock();
try {
E temp = buffer.get(0);
buffer.remove(0);
size--;
full.release();
return temp;
} finally {lock.unlock();}
}
}
final class Messages {
private final int port;
private final ArrayList<String> data = new ArrayList<String>();
public Messages(int p) {port = p;}
void add(String message) {
data.add(message);
}
void write() {
try {
Socket socket;
socket = new Socket(InetAddress.getLocalHost(),port);
DataOutputStream out = new DataOutputStream(socket.getOutputStream());
// write message
out.write(data.size());
for(String k : data) {out.writeUTF(k);}
out.flush();
socket.close();
} catch(IOException e) {e.printStackTrace();}
}
}
Client:
class Client {
final static int nPort = 1234;
static int serverPort;
public static void main(String[] args) {
// this class and those present in it
// will model the client for assignment 8
Scanner in = new Scanner(System.in);
System.out.println("Please enter the messageboard number: ");
serverPort = in.nextInt();
System.out.println("Please type your message: ");
String msg = in.next();
Listener lis = new Listener(serverPort);
lis.start();
boolean go = true;
while(go) {
try {
Thread.sleep(2000);
} catch(InterruptedException e) {}
write(serverPort, msg);
System.out.println("Continue: 0/1");
int x = in.nextInt();
if(x == 0)go = false;
}
System.exit(0);
}
static void write(int port, String msg) {
try {
Socket socket;
socket = new Socket(InetAddress.getLocalHost(),port);
DataOutputStream out = new DataOutputStream(socket.getOutputStream());
// send message to port
out.writeInt(port);
out.writeUTF(msg);
// write sentinal after message has been written and close socket
out.writeInt(-1);
out.flush();
socket.close();
} catch(IOException e) {System.out.println(e);}
}
}
class Listener extends Thread {
private int port;
volatile boolean go;
public Listener(int p) {p = port;}
public void run() {
try {
ServerSocket serversock = new ServerSocket(port);
while(go) {
Socket socket = serversock.accept();
DataInputStream in = new DataInputStream(socket.getInputStream());
// Read the message
while(in.available() > 0) {
String k = in.readUTF();
System.out.print(k+" ");
}
System.out.println();
socket.close();
}
} catch(IOException e) {go = false;}
}
}

Turns out i had a wrong assignment in my Listener class. I had p = port instead of port = p.
Solved. Through my own stupidity.

The hostname you are connecting to is localhost so you are assuming the server is on the same machine. If you need to connect to a different machine, you need to specify the host you want it to connect to.

Try changing socket = new Socket(InetAddress.getLocalHost(),port);
to
socket = new Socket(*IP OF SERVER (127.0.0.1 if same machine)*, port);

Java DataInputStream sometimes returns empty

I currently am making a messaging application which I will also add encryption to later.
I have a server which opens a socket and waits for clients to connect. Once client connects, it creates a new thread for that client. and then waits for messages etc.
The problem I am having is that when I try to send the public key exponent and modulus to the client, the Client is unable to use datainputstream(clientSocket.getInputStream()).readLine.
The sending of the exponent and Modulus happen on line 173 and 174.The receiving on the client side happens on line 125 and 136.
Server sending the data.
os.println(modulusMap.get(requestName));
os.println(exponentMap.get(requestName));
Client attempting to write the data to a variable:
pubModulusString = is.readLine().trim();
BigInteger pubModulus = new BigInteger(pubModulusString);
HashMap<String, BigInteger> modulusMap = new HashMap<String, BigInteger>();
modulusMap.put(message.substring(1), pubModulus);
String pubExponentString = is.readLine();
I have tested the sending of data as just a print stream and it is working perfectly. It is only when I try to read the data and save it into a variable that the problem occurs(line 125 and 136).
Majority of the time the readLine() reads the modulus as just blank, but sometimes it will successfully read it. I cannot find a pattern to when it is successful and when it is not.
What is also very weird is that I use the exact same code to send data from client to server. This can be seen on Client side line 89 and 91. And the server receives the data on lines 113 and 119.
Thank you for any help or ideas in advance.
Posted below is the full code for any 1 that is kind enough and has the time to run it.
Server:
import java.io.*;
import java.math.*;
import java.net.Socket;
import java.net.ServerSocket;
import java.util.*;
/*
* A chat server that delivers public and private messages.
*/
public class MultiThreadChatServerSync {
// The server socket.
private static ServerSocket serverSocket = null;
// The client socket.
private static Socket clientSocket = null;
// This chat server can accept up to maxClientsCount clients' connections.
private static final int maxClientsCount = 10;
private static final clientThread[] threads = new clientThread[maxClientsCount];
public static void main(String args[]) {
// The default port number.
int portNumber = 2222;
if (args.length < 1) {
System.out.println("Usage: java MultiThreadChatServerSync <portNumber>\n"
+ "Now using port number=" + portNumber);
} else {
portNumber = Integer.valueOf(args[0]).intValue();
}
/*
* Open a server socket on the portNumber (default 2222). Note that we can
* not choose a port less than 1023 if we are not privileged users (root).
*/
try {
serverSocket = new ServerSocket(portNumber);
} catch (IOException e) {
System.out.println(e);
}
/*
* Create a client socket for each connection and pass it to a new client
* thread.
*/
while (true) {
try {
clientSocket = serverSocket.accept();
int i = 0;
for (i = 0; i < maxClientsCount; i++) {
if (threads[i] == null) {
(threads[i] = new clientThread(clientSocket, threads)).start();
break;
}
}
if (i == maxClientsCount) {
PrintStream os = new PrintStream(clientSocket.getOutputStream());
os.println("Server too busy. Try later.");
os.close();
clientSocket.close();
}
} catch (IOException e) {
System.out.println(e);
}
}
}
}
/*
* The chat client thread. This client thread opens the input and the output
* streams for a particular client, ask the client's name, informs all the
* clients connected to the server about the fact that a new client has joined
* the chat room, and as long as it receive data, echos that data back to all
* other clients. The thread broadcast the incoming messages to all clients and
* routes the private message to the particular client. When a client leaves the
* chat room this thread informs also all the clients about that and terminates.
*/
class clientThread extends Thread {
private String clientName = null;
private DataInputStream is = null;
private BufferedReader inputLine = null;
private PrintStream os = null;
private Socket clientSocket = null;
private final clientThread[] threads;
private int maxClientsCount;
public clientThread(Socket clientSocket, clientThread[] threads) {
this.clientSocket = clientSocket;
this.threads = threads;
maxClientsCount = threads.length;
}
public void run() {
int maxClientsCount = this.maxClientsCount;
clientThread[] threads = this.threads;
HashMap<String, BigInteger> modulusMap = new HashMap<String, BigInteger>();
HashMap<String, BigInteger> exponentMap = new HashMap<String, BigInteger>();
try {
/*
* Create input and output streams for this client.
*/
is = new DataInputStream(clientSocket.getInputStream());
os = new PrintStream(clientSocket.getOutputStream());
inputLine = new BufferedReader (new InputStreamReader(System.in));
String name;
while (true) {
os.println("Enter your name.");
name = is.readLine().trim();
String pubModulusString = is.readLine().trim();
BigInteger pubModulus = new BigInteger(pubModulusString);
modulusMap.put(name, pubModulus);
String pubExponentString = is.readLine().trim();
BigInteger pubExponent = new BigInteger(pubExponentString);
exponentMap.put(name, pubExponent);
if (name.indexOf('#') == -1) {
break;
} else {
os.println("The name should not contain '#' character.");
}
}
/* Welcome the new the client. */
os.println("Welcome " + name
+ " to our chat room.\nTo leave enter /quit in a new line.");
synchronized (this) {
for (int i = 0; i < maxClientsCount; i++) {
if (threads[i] != null && threads[i] == this) {
clientName = "#" + name;
break;
}
}
for (int i = 0; i < maxClientsCount; i++) {
if (threads[i] != null && threads[i] != this) {
threads[i].os.println("*** A new user " + name
+ " entered the chat room !!! ***");
}
}
}
/* Start the conversation. */
while (true) {
String line = is.readLine();
if (line.startsWith("/quit")) {
break;
}
if (line.startsWith("!")){
String requestName;
requestName = line;
requestName = requestName.substring(1);
os.println(modulusMap.get(requestName));
os.println(exponentMap.get(requestName));
}
else {
/* If the message is private sent it to the given client. */
if (line.startsWith("#")) {
String[] words = line.split("\\s", 2);
if (words.length > 1 && words[1] != null) {
words[1] = words[1].trim();
if (!words[1].isEmpty()) {
synchronized (this) {
for (int i = 0; i < maxClientsCount; i++) {
if (threads[i] != null && threads[i] != this
&& threads[i].clientName != null
&& threads[i].clientName.equals(words[0])) {
threads[i].os.println("<" + name + "> " + words[1]);
/*
* Echo this message to let the client know the private
* message was sent.
*/
this.os.println(">" + name + "> " + words[1]);
break;
}
}
}
}
}
} else {
/* The message is public, broadcast it to all other clients. */
synchronized (this) {
this.os.println("Please select who you wish to message by typing '#username *Your Message*'");
}
}
}
}
synchronized (this) {
for (int i = 0; i < maxClientsCount; i++) {
if (threads[i] != null && threads[i] != this
&& threads[i].clientName != null) {
threads[i].os.println("*** The user " + name
+ " is leaving the chat room !!! ***");
}
}
}
os.println("*** Bye " + name + " ***");
/*
* Clean up. Set the current thread variable to null so that a new client
* could be accepted by the server.
*/
synchronized (this) {
for (int i = 0; i < maxClientsCount; i++) {
if (threads[i] == this) {
threads[i] = null;
}
}
}
/*
* Close the output stream, close the input stream, close the socket.
*/
is.close();
os.close();
clientSocket.close();
} catch (IOException e) {
}
}
}
Client
import java.io.DataInputStream;
import java.io.*;
import java.math.*;
import java.math.BigInteger;
import java.net.Socket;
import java.net.ServerSocket;
import java.net.UnknownHostException;
import java.security.*;
import java.security.spec.*;
import java.util.*;
import javax.crypto.*;
public class MultiThreadChatClient implements Runnable {
// The client socket
private static Socket clientSocket = null;
// The output stream
private static PrintStream os = null;
// The input stream
private static DataInputStream is = null;
private static BufferedReader inputLine = null;
private static boolean closed = false;
private static final String PUBLIC_KEY_FILE = "Public.key";
private static final String PRIVATE_KEY_FILE = "Private.key";
public static void main(String[] args) {
// The default port.
int portNumber = 2222;
// The default host.
String host = "192.168.0.16";
if (args.length < 2) {
System.out
.println("Usage: java MultiThreadChatClient <host> <portNumber>\n"
+ "Now using host=" + host + ", portNumber=" + portNumber);
} else {
host = args[0];
portNumber = Integer.valueOf(args[1]).intValue();
}
/*
* Open a socket on a given host and port. Open input and output streams.
*/
try {
clientSocket = new Socket(host, portNumber);
inputLine = new BufferedReader(new InputStreamReader(System.in));
os = new PrintStream(clientSocket.getOutputStream());
is = new DataInputStream(clientSocket.getInputStream());
} catch (UnknownHostException e) {
System.err.println("Don't know about host " + host);
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection to the host "
+ host);
}
/*
* If everything has been initialized then we want to write some data to the
* socket we have opened a connection to on the port portNumber.
*/
if (clientSocket != null && os != null && is != null) {
try {
/* Create a thread to read from the server. */
new Thread(new MultiThreadChatClient()).start();
String myName = inputLine.readLine();
os.println(myName.trim());
try {
KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
keyPairGenerator.initialize(2048); //1024 used for normal securities
KeyPair keyPair = keyPairGenerator.generateKeyPair();
PublicKey publicKey = keyPair.getPublic();
PrivateKey privateKey = keyPair.getPrivate();
//Pullingout parameters which makes up Key
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
RSAPublicKeySpec rsaPubKeySpec = (RSAPublicKeySpec) keyFactory.getKeySpec(publicKey, RSAPublicKeySpec.class);
RSAPrivateKeySpec rsaPrivKeySpec = (RSAPrivateKeySpec) keyFactory.getKeySpec(privateKey, RSAPrivateKeySpec.class);
os.println(rsaPubKeySpec.getModulus());
System.out.println(rsaPubKeySpec.getModulus());
os.println(rsaPubKeySpec.getPublicExponent());
FileOutputStream fos = null;
ObjectOutputStream oos = null;
fos = new FileOutputStream(PUBLIC_KEY_FILE);
oos = new ObjectOutputStream(new BufferedOutputStream(fos));
oos.writeObject(rsaPubKeySpec.getModulus());
oos.writeObject(rsaPubKeySpec.getPublicExponent());
oos.close();
fos.close();
fos = new FileOutputStream(PRIVATE_KEY_FILE);
oos = new ObjectOutputStream(new BufferedOutputStream(fos));
oos.writeObject(rsaPrivKeySpec.getModulus());
oos.writeObject(rsaPrivKeySpec.getPrivateExponent());
oos.close();
fos.close();
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
}catch (InvalidKeySpecException e) {
e.printStackTrace();
}
while (!closed) {
String message = inputLine.readLine();
if (message.startsWith("!")){
os.println(message.trim());
String pubModulusString = is.readLine().trim();
BigInteger pubModulus = new BigInteger(pubModulusString);
HashMap<String, BigInteger> modulusMap = new HashMap<String, BigInteger>();
modulusMap.put(message.substring(1), pubModulus);
String pubExponentString = is.readLine();
BigInteger pubExponent = new BigInteger(pubExponentString);
HashMap<String, BigInteger> exponentMap = new HashMap<String, BigInteger>();
exponentMap.put(message.substring(1), pubExponent);
System.out.println(modulusMap.get(message.substring(1)) + " Hi");
System.out.println(exponentMap.get(message.substring(1)) + "asdk");
}
else{
// INSERT ENCRYPTION ON "message" here
os.println(message.trim());
}
}
/*
* Close the output stream, close the input stream, close the socket.
*/
os.close();
is.close();
clientSocket.close();
} catch (IOException e) {
System.err.println("IOException: " + e);
}
}
}
/*
* Create a thread to read from the server. (non-Javadoc)
*
* #see java.lang.Runnable#run()
*/
public void run() {
/*
* Keep on reading from the socket till we receive "Bye" from the
* server. Once we received that then we want to break.
*/
String responseLine;
try {
while ((responseLine = is.readLine()) != null) {
System.out.println(responseLine);
if (responseLine.indexOf("*** Bye") != -1)
break;
}
closed = true;
} catch (IOException e) {
System.err.println("IOException: " + e);
}
}
}

If readLine() returns a blank line, it is because the sender sent a blank line. If it returns null, the sender has closed the connection.
You're looking in the wrong place.

The method readLine of DataInputStream is deprecated. I don't know why you use it, normally if you use an IDE like eclipse you'll see this method crossed out.
You should use a BufferedReader instead as explained in the Java documentation itself, here is a link.

Server socket always return null to the client , Java

I, doing some computer network homework and I have to develop some sort of distributed DBMS which are connected to each other with peer to peer network, so I have a TCP client and a TCP server in one .java file which running next to each other by threads. the TCP Server of the class always listen to the other TCP client from others and give them service, the problem is when I System.out the String which I have to send back to the client on the Server side it's in the way which it's supposed to be but after sending , the client gets nothing and prints null. I wrote my code based on tutorials I found on the net and I when I test them they worked well but it's not working in my own code. could you see where my problem is? thanks
class CommandComp
{
int PORT = 1210;
int PORT2 = 1211;
String IPLocal = "";
String IPdest = "";
InetAddress IPAD;
InetAddress IPAD2;
int numOfNodes;
int numOfNodesnonchanged;
String[] Nodes;
Random rand = new Random();
int max = 2000;
int min = 1000;
String command;
Socket clientSocket;
CommandComp(String[] IPdest, String IPLocal, int numOfNodes, String command)
{
try
{
this.numOfNodes = numOfNodes;
numOfNodesnonchanged = numOfNodes;
this.IPLocal = IPLocal;
this.Nodes = IPdest;
this.command = command;
// this.IPAD = InetAddress.getByName(this.IPdest);
this.IPAD2 = InetAddress.getByName(this.IPLocal);
// clientSocket = new Socket(this.IPAD , PORT ,this.IPAD2 , PORT2 );
}
catch (Exception e)
{
// //e.printStackTrace();
}
}
public String call()
{
int i = 0;
while (numOfNodes > 0)
{
String response = "";
try
{
Thread.sleep(rand.nextInt(max - min + 1) + min);
i = numOfNodes - 1;
int max2 = 50;
int min2 = 10;
this.IPAD = InetAddress.getByName(Nodes[i]);
clientSocket = new Socket(this.IPAD, PORT, this.IPAD2, PORT2 + rand.nextInt(max2 - min2 + 1) + min2);
PrintWriter outToServer = new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
outToServer.println(command);
System.out.println(inFromServer.readLine());
response = inFromServer.readLine();
outToServer.close();
inFromServer.close();
clientSocket.close();
numOfNodes--;
System.out.println(Nodes[i] + " Remote DBMS");
System.out.println(response);
}
catch (Exception e)
{
e.printStackTrace();
try
{
clientSocket.close();
}
catch (Exception e2)
{
// TODO: handle exception
}
}
}
return command;
}
}
class TCPListnerService
implements Callable<Object>
{
String from;
String to;
ServerSocket Server;
String IP = "";
int numOfNodes;
int numofNodesUnchanged;
static clientThread t[];
TCPListnerService(String IP, int numOfNodes)
{
try
{
this.IP = IP;
this.numOfNodes = numOfNodes;
numofNodesUnchanged = numOfNodes * 2;
this.t = new clientThread[numofNodesUnchanged];
InetAddress IPAD = InetAddress.getByName(IP);
Server = new ServerSocket(1210, 20, IPAD);
}
catch (Exception e)
{
e.printStackTrace();
}
}
public String call()
throws Exception
{
String gotten = "";
while (numOfNodes > 0)
{
Socket connected = Server.accept();
for (int i = 0; i < numofNodesUnchanged; i++)
{
if (t[i] == null)
{
(t[i] = new clientThread(connected)).start();
break;
}
}
}
return gotten;
}
}
class clientThread
extends Thread
{
Socket clientSocket = null;
sqlite DB = new sqlite();
String response = "";
String fromclient;
String delims = "[ =)(',\n\t\r]+";
String[] tokens;
public clientThread(Socket clientSocket)
{
this.clientSocket = clientSocket;
}
public void run()
{
try
{
BufferedReader inFromClient = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
PrintWriter outToClient = new PrintWriter(clientSocket.getOutputStream(), true);
fromclient = inFromClient.readLine();
tokens = fromclient.split(delims);
if (tokens[0].equalsIgnoreCase("create")
|| tokens[0].equalsIgnoreCase("drop")
|| tokens[0].equalsIgnoreCase("delete")
|| tokens[0].equalsIgnoreCase("insert")
|| tokens[0].equalsIgnoreCase("update")
|| tokens[0].equalsIgnoreCase("select"))
{
response = DB.RunQuery(fromclient);
System.out.println(response);
outToClient.print(response);
clientS.close();
}
else if (tokens[0].equalsIgnoreCase("shut"))
{
System.exit(0);
}
inFromClient.close();
outToClient.close();
clientSocket.close();
}
catch (Exception e)
{
}
;
}
}

The problem is here:
inFromClient.close();
outToClient.close();
clientSocket.close();
You are closing (1) the input stream, which closes the socket, (2) the PrintWriter, which flushes it and closes the socket, and (3) the socket. Obviously (2) cannot succeed if the socket is already closed. The data you sent to the client is still buffered, never got flushed, so it never got sent.
Just close the PrintWriter, and close the socket in a finally block in case (2) fails somehow. No need to close the input at all.