Question

I got here client and server side programs. Client talks to server by sending a string, the server then converts the string into capital letters and sends back. The problem is that the client does not receive any string from the server. Only the server prints 2 passed in strings, then the server throws IOException. I guess that because client closed connection. But why does client does not receive any message from server? How to overcome this issue?
Thanks

Client:
package solutions;

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

class SocketExampleClient {

    public static void main(String [] args) throws Exception {

    String host = "localhost"; // hostname of server
    int port = 5678;           // port of server
    Socket s = new Socket(host, port);
    DataOutputStream dos = new DataOutputStream(s.getOutputStream());
    DataInputStream dis = new DataInputStream(s.getInputStream());

    dos.writeUTF("Hello World!");
    System.out.println(dis.readUTF());

    dos.writeUTF("Happy new year!");
    System.out.println(dis.readUTF());

    dos.writeUTF("What's the problem?!");
    System.out.println(dis.readUTF());

    }
}

Server:

package solutions;

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

class SocketExampleServer {

    public static void main(String [] args) throws Exception {

    int port = 5678;
    ServerSocket ss = new ServerSocket(port);
    System.out.println("Waiting incoming connection...");

    Socket s = ss.accept();
    DataInputStream dis = new DataInputStream(s.getInputStream());
    DataOutputStream dos = new DataOutputStream(s.getOutputStream());

    String x = null;

    try {
        while ((x = dis.readUTF()) != null) {

        System.out.println(x);

        dos.writeUTF(x.toUpperCase());
        }
    }
    catch(IOException e) {
        System.err.println("Client closed its connection.");
    }
    }
}

Output:

Waiting incoming connection...
Hello World!
Happy new year!
What's the problem?!
Client closed its connection.

Answer 1

Your main program is exiting before it has a chance to read the response from the server. If you add the following code it will work fine. 🙂 UPDATE- I just realised your code is working fine on my computer – and it does output the string as expected. DataInputStream.readUTF() is blocking correctly and receiving the response. Are you still having the problem?

Thread t = new Thread(){
public void run()
{
    for(;;)
    {
        String s = null;
    try 
        {
        s = dis.readUTF();
    } 
        catch (IOException e) 
        {
        e.printStackTrace();
        }
        while(s!=null)
        {
            System.out.println("Output: " + s);
        try 
        {
        s = dis.readUTF();
    } 
        catch (IOException e) 
        {
        e.printStackTrace();
    }
    }}}};
   t.start();