Develop Reference

I'm misunderstanding something on how to set up sockets, but I'm not suer if it's client side, server side, or both

I'm setting up a simple program to test starting a server, and I'm getting a silent failure state. My client seems to think it has sent, while my server doesn't think it's recieving. The two are managing the initial connection, it's just sending things after that where it's failing.
I've cut things down to the core of where it's currently failing I think.
Here's part of the Client code
public void Client (int port, String ip)
{
try {
sock = new Socket(ip, port);
System.out.println("Found the server.");
streamInput = new DataInputStream(sock.getInputStream());
// sends output to the socket
streamOutput = new DataOutputStream(
sock.getOutputStream());
streamOutput.writeChars("Client Begining Conversation");
System.out.println(streamInput.readUTF());
}
catch (UnknownHostException u) {
System.out.println(u);
return;
}
catch (IOException i) {
System.out.println(i);
return;
}
}
public static void main(String[] args) throws IOException {
// create the frame
try {
ClientGui main = new ClientGui();
main.Client(8000,"127.0.0.1");
main.show(true);
} catch (Exception e) {e.printStackTrace();}
Here's server code.
public Server(int port) throws Exception
{
ServerSocket gameServer = new ServerSocket(port);
Socket gameSocket = gameServer.accept();
System.out.println("Client has connected");
// to send data to the client
PrintStream dataOutput
= new PrintStream(gameSocket.getOutputStream());
// to read data coming from the client
BufferedReader reader = new BufferedReader( new InputStreamReader(
gameSocket.getInputStream()
));
//play logic
Play(reader,dataOutput);
public void Play(BufferedReader reader, PrintStream dataOutput) throws Exception
{
String received, textSent;
System.out.println("Waiting for response.");
received = reader.readLine();
System.out.println("Client has responded");
//contenue until 'Exit' is sent
while (received != "Exit" || received != "exit") {
System.out.println(received);
textSent = received + "recieved";
// send to client
dataOutput.println(textSent);
}
}
My client gets to here -
Found the server.
and my server gets to here -
Trying to start server.
Client has connected
Waiting for response.
At which point, it just hangs forever, each side waiting for the other. It doesn't throw an error, it just... waits until I force it closed.
So it appears that I'm either doing something wrong when I send with "streamOutput.writeChars" in my client, or I'm doing something wrong when I receive with my server with "reader.readLine();", but I can't figure out what.
Or I could be doing something more fundamentally wrong.

The problem is that reader.readLine() doesn’t return until it sees a new line character, but streamOutput.writeChars("Client Begining Conversation") doesn’t send one.
More generally, mixing a DataOutputStream on the client with a BufferedReader on the server won’t work reliably, as the latter expects plain text, while the former produces formatted binary data. For example, the character encoding might not match. The same applies to communication in the opposite direction with PrintStream and DataInputStream. It’s best to pick either a text based or binary protocol and then be consistent about the pair of classes used on both the client and server.
In the case of a text protocol, an explicit character encoding should be defined, as the default can vary between platforms. As a learning exercise, it might not matter, but it’s a good practice to be explicit about specifying a character encoding whenever handling networked communication. UTF-8 is a good choice unless there’s a specific reason to use another one.
In addition, it is generally preferred to use PrintWriter instead of PrintStream for text output in new code. Read this answer for an explanation.

Unresponsive socket read buffer

I am trying to send data to one of my servers and receive an ACK back from it. However, the processing gets hung up when waiting for a response from the server. I know for a fact that there is a connection because I can see the data reaching the server. I also know that the server is outputting data correctly because my C# client is receiving data back from the server. I will note that this client is running on a centOS virtual machine. The server is a remote windows machine. I wouldn't imagine that there would be an issue due to the virtual environment because I am able to use an SNMP java client (SNMP4j package) to make calls to a remote server. I believe my server is outputting raw binary too, but I would expect to see some kind of output either way.
// A Java program for a Client
import java.net.*;
import java.io.*;
public class Client
{
// initialize socket and input output streams
private Socket socket = null;
private DataInputStream input = null;
private DataOutputStream out = null;
private DataInputStream serveroutput= null;
// constructor to put ip address and port
public Client(String address, int port)
{
// establish a connection
try
{
socket = new Socket(address, port);
System.out.println("Connected");
// takes input from terminal
input = new DataInputStream(System.in);
// sends output to the socket
out = new DataOutputStream(socket.getOutputStream());
serveroutput = new DataInputStream(socket.getInputStream());
}
catch(UnknownHostException u)
{
System.out.println(u);
}
catch(IOException i)
{
System.out.println(i);
}
// string to read message from input
String line = "";
// keep reading until "Over" is input
while (!line.equals("Over"))
{
try
{
line = input.readLine();
out.writeUTF(line);
System.out.println(serveroutput.readLine())
}
catch(IOException i)
{
System.out.println(i);
}
}
// close the connection
try
{
input.close();
out.close();
socket.close();
}
catch(IOException i)
{
System.out.println(i);
}
}

Could be great if you would share the otherside codes. (sorry cannot comment yet)
Try use something else over writeUTF(), simply maybe a PrintStream, as mentioned by #marquis-of-lorne (read|write)UTF may be confusing by the peer.
Also this might be a good practice to flush() out the output from both side when there is nothing else to send to make sure data is sent completely.
You may also try BufferedReader over InputDataStream as you are trying to read lines. readLine() from InputDataStream is deprecated.

Java TCP - Client Server help needed

Hi and thanks in advance.
I am currently trying to create a client server program where the server responds to the client based on different requests coming in from the client.
Here is what I am trying to do for the first of the requests (the issue is the same for all of the clients different requests to the server, so fixing just this would help me fix all of them):
1) The client connects to the server
2) The client sends logon information to the server
3) The server checks the logon information for validity
4) The server sends a response saying that login was successful.
5) Client receives and displays response.
Here is the Client (the first try block is located in the client frame, the second within an action listener for a button on the client frame):
try
{
mySocket = new Socket("localhost", 2016);
}
catch (UnknownHostException e1)
{
e1.printStackTrace();
}
catch (IOException e1)
{
e1.printStackTrace();
}
try
{
DOS = new DataOutputStream(mySocket.getOutputStream());
DOS.writeUTF(txtName.getText() + " " + txtPassword.getText());
DOS.flush();
DOS.close();
DataInputStream DISLog = new DataInputStream(mySocket.getInputStream());
JOptionPane.showMessageDialog(null, DISLog.readLine());
}
catch(IOException e1)
{
e1.printStackTrace();
}
Server:
System.out.println("Waiting for client....");
ServerSocket myServerSocket = new ServerSocket(2016);
Socket mySocket = myServerSocket.accept();
myClientHandler = new EZFILEHandler(mySocket);
//Log in check
DataInputStream DISLog = new DataInputStream(mySocket.getInputStream());
StringTokenizer ST = new StringTokenizer(DISLog.readLine());
String Name = ST.nextToken();
String Password = ST.nextToken();
//DISLog.close();
boolean Found = myClientHandler.matchUser(Name,Password);
DataOutputStream DOS = new DataOutputStream(mySocket.getOutputStream());
if (Found == true)
{
DOS.writeUTF("You are logged in.");
DOS.flush();
LoggedIn = true;
}
if (Found == false)
{
DOS.writeUTF("You could not log in.");
DOS.flush();
}
Here is the code for the ClientHandler mentioned in the Server code:
public EZFILEHandler(Socket newConnectionToClient)
{
try
{
mySocket = new Socket("localhost", 2016);
}
catch(Exception e)
{
e.printStackTrace();
}
}
Running the above code will give a Socket is closed exception at this line in the client: DataInputStream DISLog = new DataInputStream(mySocket.getInputStream());, so I then tried commenting out the line: DOS.close(); in the client to keep the socket open, but that causes the button which calls the client code to freeze the program and points to this line if I terminate the program: JOptionPane.showMessageDialog(null, DISLog.readLine());.
So my question is this: How do I allow the client and server to respond to each other in different ways without the socket closing or the jswing freezing from not being allowed to finish processing?
Also, how would it be possible for me to allow multiple clients to logon to the server and all their requests still be processed safely?
Thank you.

If you are writing a Swing program, your socket communication should be on its own SwingWorker thread.
Let's ignore that, and pretend it is just debugging code, not intended to stay.
DISLog.readLine() will read until it reaches the newline character, or the end of the input stream. The server is executing (say) the following:
DOS.writeUTF("You are logged in.");
DOS.flush();
No newline there. And flush() does not close the stream; it simply ensures all the characters are sent, not left in the buffer to be combine with the next set of characters to send.
Catch-22? Yup! The client is waiting for \n, and the server is waiting for more commands from the client.
Change the client to use readUTF().
Update Looks like you need to change the server to use readUTF() as well. The server is probably not getting past the new StringTokenizer(DISLog.readLine()) call unless the client closes the socket.

Communication between Client and Server using Sockets

Okay this is a revised question from earlier today, I have included code to help explain the problem. I am sending two messages from the client to the server. The server then picks the messages up and processes them. The server finally attempts to send a message back to the client(please note in the server code "testmessage"), it is here I am having problems. Either I am not recieving the message at the client side or sending it incorrectly from the server side.
public class ClientConnection {
String address, language, message;
int portNumber;
Socket clientSocket = null;
public ClientConnection(String lan, String mes, String add, int pn) throws IOException{
address = add;
portNumber = pn;
language = lan;
message = mes;
}
public String createAndSend() throws IOException{
// Create and connect the socket
Socket clientSocket = null;
clientSocket = new Socket(address, portNumber);
PrintWriter pw = new PrintWriter(clientSocket.getOutputStream(),true);
BufferedReader br = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
// Send first message - Message is being correctly received
pw.write(language+"\n");
pw.flush();
// Send off the data
// Send the second message - Message is being correctly received
pw.write(message);
pw.flush();
pw.close();
// Send off the data
// NOTE: Either I am not receiving the message correctly or I am not sending it from the server properly.
String translatedMessage = br.readLine();
br.close();
//Log.d("application_name",translatedMessage); Trying to check the contents begin returned from the server.
return translatedMessage;
}
Server Code:
public class ServerConnection {
public static void main(String[] args) throws Exception {
// Delete - Using while loop to keep connection open permanently.
boolean status = false;
while( !status){
ServerSocket serverSocket = null;
try {
serverSocket = new ServerSocket(4444);
} catch (IOException e) {
System.err.println("Could not listen on port: 4444.");
System.exit(1);
}
Socket clientSocket = null;
try {
clientSocket = serverSocket.accept();
} catch (IOException e) {
System.err.println("Accept failed.");
System.exit(1);
}
// Delete - Working as of here, connection is established and program runs awaiting connection on 4444
BufferedReader br = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String language = br.readLine();
String message = br.readLine();
// Test - Works
System.out.println(language);
// Test - Works
System.out.println(message);
// Delete - Working as of here, both messages are passed and applied. Messages are received as sent from client.
TranslateMessage tm = new TranslateMessage();
String translatedMessage = tm.translateMessage(language, message);
// NOTE: This seems to be where I am going wrong, either I am not sending the message correctly or I am not receiving it correctly..
// PrintWriter writer = new PrintWriter(new BufferedOutputStream(clientSocket.getOutputStream()));
PrintWriter pw = new PrintWriter(clientSocket.getOutputStream(),true);
// Send translation back
System.out.println(translatedMessage);
// pw.write(translatedMessage+"\n");
pw.write("Return test"); // Test message!
pw.flush();
// Send off the data
pw.close();
br.close();
clientSocket.close();
serverSocket.close();
}
}
}
The code is a bit of a mess and I can see a few duplicates, I have commented where I feel the problems occour.
Thanks for any help!

You are using BufferedReader.readLine() to read the response from the server, but in the test case you are sending a string that is not terminated with a \n or \r\n, so it will not get the line as far as I can tell from the docs...
public String readLine()
throws IOException
Read a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.
Returns:
A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached
An additional suggestion...
When writing request response protocols like this I would not rely on line endings to terminate the requests or responses. Typically I would use either a fully formatted JSON string, or my preference is for a binary protocol where all requests and response are prepended with a binary count (usually 4 bytes bigendian/network byte order). Then the client and server reads the 4 bytes then reads the number of bytes that follow. This handles the packet fragmentation that typically happens over network connections, also it helps avoid DOS attacks by malicious users sending long strings that never terminate.
In Java you can use ByteBuffer.order() to handle bigendian numbers.

How do I read a server reply without it blocking me?

I'm writing a proxy and have the following code:
Socket conUser;
Socket conDest;
try{
ServerSocket ss = new ServerSocket(Integer.parseInt(p.getProperty("proxy.port")));
while(true){
//Connect to user
conUser = ss.accept();
BufferedReader inputFromUser = new BufferedReader(new InputStreamReader(conUser.getInputStream()));
BufferedWriter outputToUser = new BufferedWriter(new OutputStreamWriter(conUser.getOutputStream(), "UTF8"));
//Get user request
StringBuffer req = new StringBuffer();
getUserRequest(inputFromUser, req);
System.out.println("User requested the following:");
System.out.println(req);
//Connect to server
InetAddress a = InetAddress.getByName(determineHost(req));
conDest = new Socket(a,80);
//Send request to server
BufferedWriter outputToServer = new BufferedWriter(new OutputStreamWriter(conDest.getOutputStream(), "UTF8"));
InputStreamReader inputFromServer = new InputStreamReader(conDest.getInputStream(), "UTF8");
outputToServer.write(req.toString());
outputToServer.flush();
System.out.println("==============================");
System.out.println("Server replied with the following:");
//Read reply from the server
//=========================================
int chars;
while ((chars = inputFromServer.read()) != -1) {
System.out.print((char)chars);
outputToUser.write(chars);
outputToUser.flush();
//serverReply.append(chars);
}
//Relay reply to user
//outputToUser.write(serverReply.toString());
//System.out.println(serverReply);
//outputToUser.flush();
conUser.close();
conDest.close();
}
}
catch (Exception e) {
System.err.println(e);
}
What happens is: I make a connection and it succeeds. I also send the request, and that succeeds too. I also get a reply, and am able to load the entire page's HTML, except that the read doesn't seem to terminate when it reaches the end of the content.
Specifically, I was attempting to load Google's homepage and the chunked transfer reached 0 (that is- end of chanked transfer), and thus there should've been no more input to read, but this did not cause the loop to stop reading. What's also strange to me is that pretty much all code examples of proxies do use this loop, and assuming they work, I don't see much differences between their code and mine.
How do I make the loop terminate correctly?
EDIT: for the record, yes- I know that the TCP connection should be kept open to handle further connections. This is not relevant to the problem I'm having. I need to get this loop to terminate per response.

In general the connection is not closed at the end of each response. Creating TCP connections is relatively time-consuming so the connection is left open, ready for you to send your next request.
Here are a couple of explanatory links:
http://en.wikipedia.org/wiki/HTTP_persistent_connection
http://en.wikipedia.org/wiki/HTTP_pipelining

If you want to terminate connection correctly after receiving HTTP response, your simple loop is not enough. You have to determine the end of message as described in section 4.4 Message Length of RFC 2616 and then close the connection.
However, it would be better to use existing libraries, such as built-in URLConnection.