Question

this afternoon I wrote this class whose aim is give a easy way to exchange send a file over TCP Socket.
The problem it that, despite the final file size is correct, the content in wrong: precisely the destination file is made of various copies of the first buffer sent over Socket.
My class is simple: it calculates Q and R based on buffer size and sends this number together original filename to the client. I used a byte array to send data over Socket.

package it.s4sytems.java;  
import java.io.*;  
import java.net.*;  

public class FileOverObjectStream
{
    private File file;
    private int bufferSize = 4*1024*1024; //4MB default, comunque è stabilito dal sender

    private static class Info implements Serializable
    {
        public String fileName;
        public long q;
        public int r;
        public int bufferSize;
    }

    public FileOverObjectStream(File file)
    {
        this.file = file;
    }

    public FileOverObjectStream(File file, int bufferSize)
    {
        this(file);
        this.bufferSize = bufferSize;
    }

    public void sendFile(Socket socket) throws IOException
    {
        socket.getInputStream();
        sendFile( socket.getOutputStream() );
    }

    public void sendFile(OutputStream outStream)throws IOException
    {
        sendFile( new ObjectOutputStream(outStream) );
    }

    public void sendFile(ObjectOutputStream objOutStream) throws IOException
    {
        BufferedInputStream in = new BufferedInputStream( new FileInputStream(file) );
        byte[] buffer = new byte[bufferSize];

        Info info = new Info();
            info.fileName = file.getName();
            info.bufferSize = bufferSize;
            info.q = file.length() / bufferSize;
            info.r = (int) file.length() % bufferSize;
        objOutStream.writeObject(info);

        for(long i=0; i<info.q; i++)
        {
            in.read(buffer);
            objOutStream.writeObject(buffer);
            objOutStream.flush();
        }
        in.read( buffer = new byte[info.r]);
        objOutStream.writeObject(buffer);

        objOutStream.flush();
        in.close();
    }

    public String receiveFile(Socket socket) throws IOException, ClassNotFoundException
    {
        socket.getOutputStream();
        return receiveFile( socket.getInputStream() );
    }

    public String receiveFile(InputStream inStream) throws IOException, ClassNotFoundException
    {
        return receiveFile( new ObjectInputStream(inStream) );
    }

    public String receiveFile(ObjectInputStream objInStream) throws IOException, ClassNotFoundException
    {
        BufferedOutputStream out = new BufferedOutputStream( new FileOutputStream(file) );

        Info info = (Info) objInStream.readObject();
        for(long i=0; i<info.q+1; i++)
        {
            byte[] buffer = (byte[]) objInStream.readObject();
            out.write( buffer );
        }

        out.close();
        return info.fileName;
    }
}

I created two classes to make some try…

import it.s4sytems.java.*;
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

public class Server
{
    public static void main(String arg[]) throws IOException
    {
        ServerSocket ss = new ServerSocket(18000);

        while(true)
        {
            Socket s = ss.accept();

            File file = new File("G:\\HCHCK_72_5.38.part04.rar");
            FileOverObjectStream sender = new FileOverObjectStream(file);
            sender.sendFile(s);
            s.close();
        }
    }
}

and client…

import it.s4sytems.java.*;
import java.io.*;
import java.net.*;

public class Client
{
    public static void main(String arg[]) throws IOException, ClassNotFoundException
    {
        Socket s = new Socket("localhost", 18000);

        String matricola = "616002424";

        File directory = new File(System.getProperty("user.dir") + "\\" + matricola);
        directory.mkdir();

        File file = File.createTempFile("7897_", null, directory);

        String originalName = new FileOverObjectStream(file).receiveFile(s);

        System.out.println(originalName);

        s.close();


        File file2 = new File(directory, originalName);
        System.out.println( file.renameTo( file2 ) );
        System.out.println( file.getAbsoluteFile());
        System.out.println( file2.getAbsoluteFile());
    }
}

Probably it’s a stupid thing, but I can’t see it, so I need your help, please.

Thank you

Answer 1

Your ObjectInput/OutputStream code is flawed in all the ways Alex noted. I wouldn’t use it at all, I would just use raw I/O. The canonical way to copy a stream in Java is as follows:

int count;
byte[] buffer = new byte[8192]; // or more, but megabytes is pointless as the network will packetize anyway
while ((count = in.read(buffer)) > 0)
{
  out.write(buffer, 0, count);
}

Use that same code when both sending and receiving the file. If you want to send > 1 file per connection, you need to prefix all that by sending the file name and length, which you can do with DataOutputStream.writeUTF()/writeLong(), and DataInputStream.readUTF()/readLong() at the receiver, and modify the loop control to read exactly that many bytes:

long remaining = size; // the file size read from the network
while ((count = in.read(buffer, 0, remaining > buffer.length ? buffer.length : (int)remaining)) > 0)
{
    out.write(buffer, 0, count);
    remaining -= count;
}