Client Server File Transfer Java

Question

I am trying to write an application using Java that will allow me to transfer files between a server and a client that requests the file. I plan to do it using sockets. My algorithm is somewhat like this:

On Server: Create the connection between client and server. Once connected find the file u need to send to client. Then send the size of file to client. Then send file broken down in parts.

On Client After connection is created, ask for the file. Receive the file size, then accept data till u reach file size. Stop.

Please correct me if i am wrong somewhere in the algorithm

Answer 1

Here is the method that I use, it uses the socket's input and output streams to send and receive the files, and when it's done, it will automatically restart the server and reconnect to it from the client.

Server Code:

package app.server;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Functions 
{
    private static ServerSocket server;
    private static Socket socket;

    public static void startServer(int port)
    {   
        try 
        {
            server = new ServerSocket(port);
            socket = server.accept();
        }
        catch (IOException ex) 
        {
            Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    private static void restartServer()
    {
        new Thread()
        {
            @Override
            public void run()
            {
                try 
                {
                    socket = server.accept();
                } 
                catch (IOException ex) 
                {
                    Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }.start();
    }

    public static void sendFile(String inputFilePath)
    {
        FileInputStream fis;
        BufferedInputStream bis;
        OutputStream os;
        BufferedOutputStream bos;
        try 
        {
            File input = new File(inputFilePath);
            fis = new FileInputStream(input);
            bis = new BufferedInputStream(fis);
            os = socket.getOutputStream();
            bos = new BufferedOutputStream(os);
            byte[] buffer = new byte[1024];
            int data;
            while(true)
            {
                data = bis.read(buffer);
                if(data != -1)
                {
                    bos.write(buffer, 0, 1024);
                }
                else
                {
                    bis.close();
                    bos.close();
                    break;
                }
            }
        } 
        catch (FileNotFoundException ex) 
        {
            Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
        } 
        catch (IOException ex) 
        {
            Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
        }
        restartServer();
    }
}

Client Code:

package app.client;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.Socket;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Functions 
{
    private static Socket socket;
    private static String hostName;
    private static int portNumber;

    public static void connectToServer(String host, int port)
    {
        new Thread()
        {
            @Override
            public void run()
            {
                try 
                {
                    hostName = host;
                    portNumber = port;
                    socket = new Socket(host, port);
                } 
                catch (IOException ex) 
                {
                    Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }.start();
    }

    private static void reconnectToServer()
    {
        try 
        {
            socket = new Socket(hostName, portNumber);
        } 
        catch (IOException ex) 
        {
            Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public static void receiveFile(String outputFilePath)
    {
        InputStream is;
        BufferedInputStream bis;
        FileOutputStream fos;
        BufferedOutputStream bos;
        try 
        {
            File output = new File(outputFilePath);
            is = socket.getInputStream();
            bis = new BufferedInputStream(is);
            fos = new FileOutputStream(output);
            bos = new BufferedOutputStream(fos);
            byte[] buffer = new byte[1024];
            int data;
            while(true)
            {
                data = bis.read(buffer);
                if(data != -1)
                {
                    bos.write(buffer, 0, 1024);
                }
                else
                {
                    bis.close();
                    bos.close();
                    break;
                }
            }
        } 
        catch (IOException ex) 
        {
            Logger.getLogger(Functions.class.getName()).log(Level.SEVERE, null, ex);
        }
        reconnectToServer();
    }
}

This method works very well, I use it for my server and client file transfer program, all you need to do is enter the Server Host's IP address and choose a port number (I use 8888).

Answer 2

This isn't really an "algorithm" question; you're designing a (simple) protocol. What you've described sounds reasonable, but it's too vague to implement. You need to be more specific. For example, some things you need to decide:

• How does the receiving program know what filename it should save to? Should that be sent through the socket, or should it just ask the user?
• How is the file size transmitted?
  • Is it a character string? If so, how is its length indicated? (With a null terminator? A newline?)
  • Is it a binary value? If so, how big? (32 bits or 64?) What endianness?
• What does "broken down in parts" mean? If you're writing to a TCP socket, you don't need to worry about packet boundaries; TCP takes care of that.
• Does the recipient send anything back, like a success or failure indication?
• What happens when the whole file has been transmitted?
  • Should both ends assume that the connection must be closed?
  • Or can you send multiple files through a single connection? If so, how does the sender indicate that another file will follow?

Also, you're using the terms "client" and "server" backward. Typically the "client" is the machine that initiates a connection to a server, and the "server" is the machine that waits for connections from clients.

Answer 3

You can also add Acknowledgement from server once a particular part of the file is recieved, similar to what we have in HTTP protocol , that would ensure proper delivery of the file has been received on the server.