Java request file, send file (Client-server)

Question

I'm making a Client-Server. I've gotten as far as that the server can send a hardcoded file, but not a client specified. I will have to send only text files. As far as I have understood: the clients firstly sends the file name and then, the server sends it, nothing complicated, but I'm getting all kinds of errors, this code is getting a connection reset/socket closed error. The main problem is, that hadn't got much time to research networking.

Ill appreciate any help I can get.

EDIT. I found a work around, closing a stream causes the socket to close, why is that? It shouldn't happen, should it?

Server Side:

    InputStream sin=newCon.getInputStream();
    DataInputStream sdata=new DataInputStream(sin);
    location=sdata.readUTF();   
    //sdata.close();
    //sin.close();

File toSend=new File(location);
byte[] array=new byte[(int)toSend.length()];
FileInputStream fromFile=new FileInputStream(toSend);
BufferedInputStream toBuffer=new BufferedInputStream(fromFile);
toBuffer.read(array,0,array.length);

OutputStream out=newCon.getOutputStream(); //Socket-closed...
out.write(array,0,array.length);
out.flush();
toBuffer.close();
newCon.close();

ClientSide:

int bytesRead;
server=new Socket(host,port);

OutputStream sout=server.getOutputStream();
DataOutputStream sdata=new DataOutputStream(sout);
sdata.writeUTF(interestFile);
//sdata.close();
//sout.close();

InputStream in=server.getInputStream();     //socket closed..
OutputStream out=new FileOutputStream("data.txt");
byte[] buffer=new byte[1024];
while((bytesRead=in.read(buffer))!=-1)
{
    out.write(buffer,0,bytesRead);
}
out.close();
server.close();

Have you tried System.out.println(sdata.available()) before reading? Maybe there's nothing to read yet. — Diego Basch, Dec 12 '12 at 17:14
but does it get to location=sdata.readUTF(), where you say the connection is reset? I mean before that. — Diego Basch, Dec 12 '12 at 18:56
Sorry, but it isnt being reset, i had that solved, in these snippets the sockets are closed when trying to get the input/output streams from the socket. — MustSeeMelons, Dec 12 '12 at 19:32
Suggest calling sdata.flush() on client side after writing the target file to stream. For debugging on server-side you should output the location received. — CodeMonkey, Dec 22 '12 at 01:38

CodeMonkey · Accepted Answer · 2012-12-22T04:12:31.573

Try reading the file in chunks from Server while writing to client output stream rather than creating a temp byte array and reading entire file into memory. What if requested file is large? Also close the new Socket on server-side in a finally block so socket is closed even if an exception is thrown.

Server Side:

    Socket newCon = ss.accept();
    FileInputStream is = null;
    OutputStream out = null;
    try {
        InputStream sin = newCon.getInputStream();
        DataInputStream sdata = new DataInputStream(sin);
        String location = sdata.readUTF();
        System.out.println("location=" + location);
        File toSend = new File(location);
        // TODO: validate file is safe to access here
        if (!toSend.exists()) {
            System.out.println("File does not exist");
            return;
        }
        is = new FileInputStream(toSend);
        out = newCon.getOutputStream();
        int bytesRead;
        byte[] buffer = new byte[4096];
        while ((bytesRead = is.read(buffer)) != -1) {
            out.write(buffer, 0, bytesRead);
        }
        out.flush();
    } finally {
        if (out != null)
            try {
               out.close();
            } catch(IOException e) {
            }
        if (is != null)
            try {
               is.close();
            } catch(IOException e) {
            }
        newCon.close();
    }

If you use Apache Common IOUtils library then you can reduce much of the code to read/write files to streams. Here 5-lines down to one line.

org.apache.commons.io.IOUtils.copy(is, out);

Note that having a server that serves files by absolute path to remote clients is potentially dangerous and the target file should be restricted to a given directory and/or set of file types. Don't want to serve out system-level files to unauthenticated clients.

Java request file, send file (Client-server)

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