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Java: cannot read from socket, thread gets stuck on readLine()

Here is my code for the server side:
#Override
public void run(){
String message;
String command;
String[] arguments;
try{
BufferedReader inStream = new BufferedReader(
new InputStreamReader(
clientSocket.getInputStream()));
while(online){
message = inStream.readLine();
if(message == null)
continue;
if(message.charAt(0) == '/'){
int endOfCommandIndex = message.indexOf(' ');
command = message.substring(1, endOfCommandIndex);
arguments = message.substring(endOfCommandIndex + 1).split(" ");
if(command.equals("login")){
setUserName(arguments[0]);
setName(arguments[0]);
sendMessage(this, "Connected");
}
//....
}
}
}
As mentioned in the title, the thread gets stuck reading from the InputStream of the Socket (I checked with JDB and it's not a conditional waiting because it appears to be still "running").
I tried to write a line to the socket but it doesn't change its state at all. I'm trying to build a chat-like local application and I'm quite new to socket and streams. Thanks in advance.
For the client side:
String msg;
try{
while(!((msg = stdIn.readLine()).equals("/quit")))
toServer.println(msg);
}
catch(IOException e){
e.printStackTrace();
}
In case someone wants review my entire code, it is here hosted on github

It looks like the message is never flushed after being written into the socket stream.
Try either call:
toServer.flush();
after println, or enable auto flushing when constructing toServer:
toServer = new PrintWriter(socket.getOutputStream(), true);

How to properly stream data from a socket with Java

I am attempting stream data over a socket with Java in an attempt to write a Kafka producer. I've written a class to pull the data in but I'm not getting the results I'd expect. I've got it set up so the data is being streamed from a Linux box. The source of the data is a csv file that I'm using the nc utility to stream. The class is running on a Windows 10 machine from Eclipse. When I run the class I see two weird things.
The column headers don't get transmitted.
I can only run the class once. If I want to run it again, I have to stop nc and restart it.
Below is my code. Am I missing anything? At this point I'm just trying to connect to the socket and pull the data over.
I run nc with the following command:
$ nc -kl 9999 < uber_data.csv
Below is my class
import java.net.*;
import java.io.*;
public class Client
{
static String userInput;
public static void main(String [] args)
{
try
{
InetAddress serverAddress = InetAddress.getByName("servername");
Socket socket = new Socket(serverAddress, 9999);
BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));
while ((userInput = input.readLine()) != null) {
System.out.println(input.readLine());
}
input.close();
socket.close();
}
catch(UnknownHostException e1)
{
System.out.println("Unknown host exception " + e1.toString());
}
catch(IOException e2)
{
System.out.println("IOException " + e2.toString());
}
catch(IllegalArgumentException e3)
{
System.out.println("Illegal Argument Exception " + e3.toString());
}
catch(Exception e4)
{
System.out.println("Other exceptions " + e4.toString());
}
}
}

You're throwing away every odd-numbered line. It should be:
while ((userInput = input.readLine()) != null) {
System.out.println(userInput);
}
Secondly, you aren't closing the socket. Use a try-with-resources:
try
{
InetAddress serverAddress = InetAddress.getByName("servername");
try (
Socket socket = new Socket(serverAddress, 9999);
BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));
) {
while ((userInput = input.readLine()) != null) {
System.out.println(input.readLine());
}
}
}
catch (...)

First, each call readLine() tries to read line from input stream.
In userInput = input.readLine() you read header, but println(input.readLine()) read body and print in console.
while ((userInput = input.readLine()) != null) {
System.out.println(userInput); //instead input.readLine()
}
Second, I didn't use nc, but I think problem will solve if you will close socket (and reader) in finally statement.

I hope it would be helpful.
For the first question: you were trying to print userInput string. But it's printing the result of another readline() call.
For the second: after the file has been transferred, you have to stop and restart nc; no matter what you do from your side. It's from nc side.
See the nc documentation.

How to automatically update server and client side in java

I'm learning distributed systems basics and currently I'm trying to do a simple yet realistic messenger between one server and one client. What I do intend is that on each endpoint socket side (Server and Client) text automatically updates (like a real "messaging app"). In other words, I want that the moment I write and "send" the message, it automatically appears on recipient side. What I have now follows this schema:
I send a message (let's assume from client)
To see that message on Server's side I need to reply first (because Server's BufferedReader / Client's PrintWriter is only read after asking for the answer)
My code:
public class ClientSide {
public static void main(String [] args){
String host_name = args[0];
int port_number = Integer.parseInt(args[1]);
try {
Socket s = new Socket(host_name, port_number);
PrintWriter out =
new PrintWriter(s.getOutputStream(), true);
BufferedReader in =
new BufferedReader(
new InputStreamReader(s.getInputStream()));
BufferedReader stdIn =
new BufferedReader(
new InputStreamReader(System.in));
String answer;
while ((answer = stdIn.readLine()) != null) {
out.println(answer);
System.out.println("\nlocalhost said\n\t" + in.readLine());
}
} catch (IOException ex) {
Logger.getLogger(ClientSide.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
public class ServerSide {
public static void main(String [] args){
int port_number = Integer.parseInt(args[0]);
try {
ServerSocket ss = new ServerSocket(port_number);
Socket tcp = ss.accept();
PrintWriter out =
new PrintWriter(tcp.getOutputStream(), true);
BufferedReader in =
new BufferedReader(
new InputStreamReader(tcp.getInputStream()));
BufferedReader stdIn =
new BufferedReader(
new InputStreamReader(System.in));
String answer;
while ((answer = stdIn.readLine()) != null){
out.println(answer);
System.out.println("\nClient said\n\t" + in.readLine());
}
} catch (IOException ex) {
Logger.getLogger(ServerSide.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
How can I do this? Does it involve advanced knowledge on the matter?
Thanks in advance.

The core problem is that you want to wait for two events concurrently -- either a message from the socket, or input from the user!
You want to wait on both at the same time -- you don't want to be stuck waiting for a message in the socket if the user types a message; nor to be waiting for the user message while you have a new message from the network.
To 'wait' for messages from multiple streams, you have java.nio. I believe it is the most correct way of doing it.
But if you want to keep using the BufferedReader, there is a ready() method that returns true if and only if there is a message waiting to be read.
Your code after the in and stdIn declarations would then look something like (I didn't test it!!):
while(true) {
if(stdIn.ready()) {
System.out.println("I said " + stdIn.readLine());
}
if(in.ready()) (
System.out.println("He said " + in.readLine());
}
}
A few somewhat useful random links:
Java - Reading from a buffered reader (from a socket) is pausing the thread
Is there epoll equivalent in Java?

Java chat server client issue

I followed this tutorial to make a chat with multiples client and one server: http://inetjava.sourceforge.net/lectures/part1_sockets/InetJava-1.9-Chat-Client-Server-Example.html
but I have a problem, I want the client to send his username when he starts the app via the command prompt like this:
java -jar Client.jar Jonny
but I don't know how to do this.
If someone can explain me..
Thanks for your answers.

If you input your parameters like java -jar Client.jar Jonny, you can get the argument in the Client class' main method as a String array.
For example you can print out the first argument like this:
public static void main(String[] args)
{
//This will output: "The first argument is: Jonny"
System.out.println("The first argument is: "+args[0]);
}
All you have to do now is send this to the server. If you use the NakovChat example it could be something like this:
public static void main(String[] args)
{
BufferedReader in = null;
PrintWriter out = null;
try {
// Connect to Nakov Chat Server
Socket socket = new Socket(SERVER_HOSTNAME, SERVER_PORT);
in = new BufferedReader(
new InputStreamReader(socket.getInputStream()));
out = new PrintWriter(
new OutputStreamWriter(socket.getOutputStream()));
System.out.println("Connected to server " +
SERVER_HOSTNAME + ":" + SERVER_PORT);
//We print out the first argument on the socket's outputstream and then flush it
out.println(args[0]);
out.flush();
} catch (IOException ioe) {
System.err.println("Can not establish connection to " +
SERVER_HOSTNAME + ":" + SERVER_PORT);
ioe.printStackTrace();
System.exit(-1);
}
// Create and start Sender thread
Sender sender = new Sender(out);
sender.setDaemon(true);
sender.start();
try {
// Read messages from the server and print them
String message;
while ((message=in.readLine()) != null) {
System.out.println(message);
}
} catch (IOException ioe) {
System.err.println("Connection to server broken.");
ioe.printStackTrace();
}
}
}

Server Client communication fails at Outputstreams

My program is basically:
Client sends a String to Server,
Based on this String, Server is creating an ArrayList,
ArrayList is sent back to the Client.
What is failing here is:
After Client sends a String, the Server receives it and doesn't do anything else. In this time Client keeps on working and gets a NullPointer.
Client side:
public static ArrayList<String> sendStringToServer(String report) {
Socket socket;
ArrayList<String> fieldsList = new ArrayList<String>();
try {
socket = new Socket("localhost", 2345);
OutputStream os = socket.getOutputStream();
PrintStream ps = new PrintStream(os, true);
ps.println(report);
ps.flush();
//Here the debugger should stop and wait for server to create a List
//at this point there is no answer, code breaks
ObjectInputStream objectInput = new ObjectInputStream(socket.getInputStream());
Object object = objectInput.readObject();
fieldsList = (ArrayList<String>) object;
socket.close();
return fieldsList;
} catch (IOException e1) {
e1.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
}
Server side:
public class Server {
private ServerSocket serverSocket;
private Socket clientSocket;
private String telegram;
private StringBuilder telegramSB;
public static void main(String[] args) throws IOException, JRException {
new Server();
}
public Server() {
try {
serverSocket = new ServerSocket(2345);
while (true) {
clientSocket = serverSocket.accept();
InputStream is = clientSocket.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
try {
//debugger goes to here and then stops
telegram = br.readLine();
int counter = 0;
boolean startSeq = false;
for (char ch : telegram.toCharArray()) {
if (counter == 0 && ch == '/') {
startSeq = true;
}
if (startSeq == true) {
telegramSB = new StringBuilder();
telegramSB.append(ch);
}
if (ch == '\n') {
if (telegram.length() < 255) {
sendListWithFields();
} else {
new Launcher(telegram).run();
}
}
counter++;
}
} catch (JRException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
} catch (IOException e) {
System.out.println(e);
}
}

My guess here would be that the BufferedReader is waiting to fill its buffer and you haven't sent enough data for it to do that and return so its waiting for more data to come through that never does (because your clients stops writing and starts to read). You could test this theory temporarily by dumping a load more data into the OutputStream on the client and flushing it.
If the above is the case then you probably want to not use BufferedReader but you have other issues here which also mean you probably want to avoid using PrintStream and BufferedReader for communication and serialisation anyway. For example the default character encoding on two different machines and JVMs could be different. When you create your PrintStream and InputStreamReader you don't specify a character encoding so they could end up being mismatched and the string that you write (including the newline character) could end up being understood completely differently by the remote side, this could also be a reason why its blocking (the client side encodes the newline character in one way but the server is expecting it to be encoded a completely different way), though less likely I think .
If you don't have to use PrintStream then I would suggest instead using DataOutputStream / DataInputStream:
//Client
BufferedOutputStream bufout = new BufferedOutputStream(socket.getOutputStream());
DataOutputStream dout = new DataOutputStream(bufout);
dout.writeUTF(report);
dout.flush();
//Server
BufferedInputStream bufin = new BufferedInputStream(socket.getInputStream());
DataInputStream din = new DataInputStream(bufin);
String report = din.readUTF();
You still get buffering from the BufferedIn/OutputStreams so it will be performant but the DataIn/OutputStreams will manage termination of variable length objects for you - they will send a length prefixing the string to tell the other side exactly how many bytes to read, so you don't need to use a special character to terminate the string you wrote, and this also means it doesn't matter what the content of your String is. In your example above even if it was working if your String had a newline character in it the server would read up until that first newline character, not to the end of the string you sent and that would put them out of sync for the next send/receive along that stream.
Using write/readUTF also specifies an encoding (UTF-8) so there is no mismatch there either.