Java TCP socket: data transfer is slow

You do not want to write single bytes when you are transferring large amounts of data.

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class Transfer {

    public static void main(String[] args) {
        final String largeFile = "/home/dr/test.dat"; // REPLACE
        final int BUFFER_SIZE = 65536;
        new Thread(new Runnable() {
            public void run() {
                try {
                    ServerSocket serverSocket = new ServerSocket(12345);
                    Socket clientSocket = serverSocket.accept();
                    long startTime = System.currentTimeMillis();
                    byte[] buffer = new byte[BUFFER_SIZE];
                    int read;
                    int totalRead = 0;
                    InputStream clientInputStream = clientSocket.getInputStream();
                    while ((read = clientInputStream.read(buffer)) != -1) {
                        totalRead += read;
                    }
                    long endTime = System.currentTimeMillis();
                    System.out.println(totalRead + " bytes read in " + (endTime - startTime) + " ms.");
                } catch (IOException e) {
                }
            }
        }).start();
        new Thread(new Runnable() {
            public void run() {
                try {
                    Thread.sleep(1000);
                    Socket socket = new Socket("localhost", 12345);
                    FileInputStream fileInputStream = new FileInputStream(largeFile);
                    OutputStream socketOutputStream = socket.getOutputStream();
                    long startTime = System.currentTimeMillis();
                    byte[] buffer = new byte[BUFFER_SIZE];
                    int read;
                    int readTotal = 0;
                    while ((read = fileInputStream.read(buffer)) != -1) {
                        socketOutputStream.write(buffer, 0, read);
                        readTotal += read;
                    }
                    socketOutputStream.close();
                    fileInputStream.close();
                    socket.close();
                    long endTime = System.currentTimeMillis();
                    System.out.println(readTotal + " bytes written in " + (endTime - startTime) + " ms.");
                } catch (Exception e) {
                }
            }
        }).start();
    }
}

This copies 1 GiB of data in short over 19 seconds on my machine. The key here is using the InputStream.read and OutputStream.write methods that accept a byte array as parameter. The size of the buffer is not really important, it just should be a bit larger than, say, 5. Experiment with BUFFER_SIZE above to see how it effects the speed but also keep in mind that it probably is different for every machine you are running this program on. 64 KiB seem to be a good compromise.