Question

Alright, I’m trying to set a program I have to receive data from a socket, and send data to a socket. I’m stumped on how to get the client side of the socket to send specific data, then have the server side send specific data. Here’s what I currently have, it’s only my server-side because I’m really lost on the client part as of now.

To further evaluate, I would like to do as listed in the following, yet I can’t figure out what to research to write the client-side of the socket, and if there was any code I would need to rewrite in the server side?

package sockets;
import java.net.*;
import java.io.*;

public class SocketMain {
    private int port = 0;
    public ServerSocket socket;
    public Socket clientSock;

    public SocketMain() {
        init();
    }

    public static void main(String[] args) {
        new SocketMain();
    }

    private void init() {
        try { 
            socket = new ServerSocket(port);
            System.out.println("Server started, bound to port: "+port);
            clientSock = socket.accept();
            File directory = new File("./Storage/");
            if (!directory.exists()) {
                directory.mkdirs();
            }
            File file = new File(directory + "/Store.dat");
            if (!file.exists()) {
                file.createNewFile();
            }
            DataInputStream in = new DataInputStream(clientSock.getInputStream());  
            FileWriter fw = new FileWriter(file);
            BufferedWriter bw = new BufferedWriter(fw);
            String line;
            while ((line = in.readLine()) != null) {
                System.out.println(line);
                bw.write(line+"\n");
                bw.flush();
                bw.close();
            }
            socket.close();
            clientSock.close();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

Answer 1

About what you currently have:

The first thing that jumped in my eye was this loop:

while ((line = in.readLine()) != null) {
    System.out.println(line);
    bw.write(line+"\n");
    bw.flush();
    bw.close(); // <- Problem
}

You’re closing the writer every time you write a line. Now, as the documentation of Writer.close() states:

Closes the stream, flushing it first. Once the stream has been
closed, further write() or flush() invocations will cause an
IOException to be thrown. Closing a previously closed stream has no
effect.

You should be getting IOExceptions for every line after the first one. You program will however not crash, since you’re catching the exceptions.

---

Second, you use DataInputStream to read from your client, but write out using a BufferedWriter. As the former states in it’s documentation:

A data input stream lets an application read primitive Java data types
from an underlying input stream in a machine-independent way. An
application uses a data output stream to write data that can later be
read by a data input stream.

The class includes multiple methods for boolean, char, int, whatever primitive data-type you can think of. But for the DataInputStream.readLine()-method, it clearly states:

Deprecated. This method does not properly convert bytes to characters.
As of JDK 1.1, the preferred way to read lines of text is via the
BufferedReader.readLine() method.

So, for reading strings, you should use a BufferedReader.

---

About what you don’t yet have:

The communication over sockets is established on a “ask-answer” base. The workflow should be something like this:

1. Client opens connection
2. Server accepts connection
3. Client asks for something (using the server-sockets OutputStream)
4. Server reads the request (using the client-sockets InputStream)
5. Server answers (using the client-sockets OutputStream)
6. Client reads the answer (using the server-sockets InputStream)
7. Repeat steps 3-6 if necessary.
8. Connection is closed (either by client, server or both).