How to implement file transfer with Java sockets?

I am working on a chat implementation with Java sockets. I have focused on few functionalities, like authentication, one person chat and a group chat. I was thinking about adding file transfer functionality, and I wonder what's the good practice about this. Should I have separate socket on the server with different port listening just for file transfers? Right now input and output streams that I开发者_如何学Python get from server socket are binded to Scanner and PrintWriter objects respectively, so I find it hard to use that for file transfer.

Any patterns you guys could recommend or give me good recommendations are very appreciated.

Thanks,

ZeKoU

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Well, FTP creates a new socket for each file transfer. Note that the connection can be established on the same port as the one used for chat, but with a different initialization dialog.

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If you can use a native lib do that, it will be faster. Failing that unless there is a really good reason use a library which will have had all the kinks worked out.

If you really need to send a file in pure Java. Here's one way based on an old project of mine. Note however that there is some serialisation overhead however much of this could be removed via use of the NIO equivalents as discussed here or you could go slightly further and use Zero Copy which lets you tell the OS to copy a file direct to a socket without any intermediary copies. This only works with files though not other data.

You should do this in a separate thread so that the chat still works while the file is transferring. Create a socket and give it some standard port (you can give it port 0 which just chooses the next available port but then you will need to send that information to the other recipient so just using a standard port is easier). Then split your file into blocks and transmit it using the socket:

//Send file (Server)
//Put this in a thread
Socket socket = new Socket(destinationIP, destinationPort);
ObjectOutputStream sender = new ObjectOutputStream(socket.getOutputStream());
sender.writeObject(dataToSend);

//Receive File (Client)
//Kick off a new thread to receive the file to preserve the liveness of the program
ServerSocket serverSocket = new ServerSocket(ListenPort);
socket = new Socket();
while (true) {
        new Thread(new TCPReceiver(socket)).start();
}

//Receive file thread TCPReceiver(Socket socket)
//Get the stream where the object is to be sent
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());

/*  Where type is the type of data you send... String or anything really...
    read part of the file into something and send it then at this end use the same data                                                       
    type to recieve it and it will magically pull the entire object across.
*/
while(fileIsIncomplete){
    type recievedData = (type) objectInputStream.readObject();
    //Reconstruct file
}

Hope that is enough for you to get a quick file sender up and running :)

Edit: Removed nonsense statement. Added native point and zero copy mention.