Server not receiving bytes written to a socket by Java app

Question

I have the following Java socket client app, that sends same string to socket server:

import java.net.*;
import java.io.*;

public class ServerClient {
public static void main(String[] args) throws IOException {
    System.out.println("Starting a socket server client...");
    Socket client = new Socket("XXX.X.XXX.XX", 12001);
    BufferedOutputStream stream = new BufferedOutputStream(client.getOutputStream());
    String message = "ABC";

    BufferedReader inputReader = new BufferedReader(new InputStreamReader(System.in));      
    String input = null;
    while ( true ) {
        System.out.print("Would you like to send a message to Server? ");
        input = inputReader.readLine();
        if ( !input.equals("Y") ) break;    

        System.out.println("Message to send: " + message);
        System.out.println("Message length is: " + message.length());

        byte[] messageBytes = message.getBytes("US-ASCII");
        stream.write(messageBytes, 0, messageBytes.length);
        stream.flush();
    }
    System.out.println("Shutting down socket server client...");        
    stream.close();
    client.close();
    inputReader.close();
}
}

The first time message is sent, server receives the message; however, every subsequent time I'm trying to send this message, server is not receiving anything. Message simply disappears. I am writing to the socket successfully (no exceptions) but nothing is coming on the other side of the pipe (or so I'm told).

I do not have access to the server app, logs or code, so I'm wondering if there is any approach you can recommend to figure out why server is not receiving subsequent messages. Any ideas would be greatly appreciated!

Clarification :

1. New lines are not expected by the server; otherwise, how would it even receive message the first time? As a trial and error, I did try sending '\\n' and "\\r\\n" and 0x00 characters at the end of the string - all without any luck.

2. I thought flushing was an issue, so I tried various outputstream classes (PrintStream, PrintWriter, FilterOutputStream), but was still running into same exact issues. Then, if "flushing" is an issue, how is it working the first time?

Answer 1

Remember:

• TCP is stream oriented. not message oriented.
• One write on the client could take several reads on the server to .. read
• Multiple writes on the client could get read by the server in one read
• You'll hardly see the above scenarios in a test application on a local network, you will see them very quick in a production environemnt, or when you start to really speed up the sending/receiving.

Following this, if you are sending messages you need a delimiter, or some other way of indicating 'here's one message', eg defining the protocol to be 'the first byte is the length of the following message'. And you'd need to check the receiving end wether it read a partial message, a whole message, and any combination thereof (eeg one read might have read 3 and a half message..).

A quick solution for your test app, write lines. That is, a string followed by a newline character. A bufferedreader's ReadLine() could then take care of the reassembly for you on the receiving end.

Answer 2

Other tests:

1 - use a network sniffer to see what is realy hapening on the network

2 - use some program like TCP Test Tool to send data to the server and simulate your program. (netcat can also be used, but it sends a newline after each line)

Answer 3

It works correctly here... but I am missing a carriage return or some other end of message after sending the message.
Hard to write more without knowing what the server expects (protocol)... Maybe you should try something like

String message = "ABC\n";