Question

Hello there im trying to send files using client-server classes in java. For some reason when the method that sends the file is called the socket closes. here is the code :

FileInputStream fIn = new FileInputStream(file);
out = new BufferedOutputStream(clientSocket.getOutputStream());
byte fileContent[] = new byte[(int) file.length()];
fIn.read(fileContent);
for (byte b : fileContent) {
    out.write(b);
}

and the code from the client :

FileOutputStream fIn = new FileOutputStream("testing");
BufferedInputStream inAout = new BufferedInputStream(clientSocket.getInputStream());
byte fileContent[] = new byte[1000000];
inAout.read(fileContent);
fIn.write(fileContent);

and the error message i get : SEVERE: null
java.net.SocketException: Socket closed

Im not really experienced with this so if any can help it would be great.

Answer 1

The InputStream.read(byte[]) method returns an int for the number of bytes it actually read. It’s not guaranteed to read as many bytes as you requested from the byte array. It’ll often return the size of the underlying buffer and you’ll have to call it many times.

You can use this to be more efficient by streaming the bytes from the socket to the file instead of buffering the whole byte array in memory. Likewise on the server side you can do the same thing to save memory and be faster than writing a byte at a time.

Here’s a working example of a server and client in one that connects to itself to transfer a file:

public class SocketFileExample {
    static void server() throws IOException {
        ServerSocket ss = new ServerSocket(3434);
        Socket socket = ss.accept();
        InputStream in = new FileInputStream("send.jpg");
        OutputStream out = socket.getOutputStream();
        copy(in, out);
        out.close();
        in.close();
    }

    static void client() throws IOException {
        Socket socket = new Socket("localhost", 3434);
        InputStream in = socket.getInputStream();
        OutputStream out = new FileOutputStream("recv.jpg");
        copy(in, out);
        out.close();
        in.close();
    }

    static void copy(InputStream in, OutputStream out) throws IOException {
        byte[] buf = new byte[8192];
        int len = 0;
        while ((len = in.read(buf)) != -1) {
            out.write(buf, 0, len);
        }
    }

    public static void main(String[] args) throws IOException {
        new Thread() {
            public void run() {
                try {
                    server();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }.start();

        client();
    }
}