Transfer log file between client and server while updating files automatically

All this code does is transfer the files. In order to make changes to the file, one has to re-transfer the file. During this time, I stand by to automate the update operation, where the server listens to the files and updates them automatically whenever any changes are made.

fileserver.java
import java.io.*;
import java.net.*;
import java.util.*;
public class FileServer {

private static ServerSocket serverSocket;
private static Socket clientSocket = null;

public static void main(String[] args) throws IOException {

try {
        serverSocket = new ServerSocket(13850);// creating a new serversocket
        System.out.println("Server started.");
} catch (Exception e)// catches errors and display them to the user for eg in case the port is busy we may specify a different port
{
        System.err.println("Port already in use.");
        System.exit(1);
}

    while (true)
{
try {
clientSocket = serverSocket.accept();// establishing the connection
System.out.println("Accepted connection : " + clientSocket);

Thread t = new Thread(new CLIENTConnection(clientSocket));
/*creating thread for clientconnection.java file and sending socket as an object thus everytime a connection is established a new thread/process is generated through which the file is sent and recieved*/
t.start();// executing the thread. here the new process or thread is created and ready to be implemented

} catch (Exception e) // catches error if any and displays it in the terminal/command prompt
{
System.err.println("Error in connection attempt.");
}
}
}
}

clientconnection.java
import java.util.*;
import java.net.*;
import java.io.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.lang.String;



public class CLIENTConnection implements Runnable {

private Socket clientSocket;
private BufferedReader in = null;

public CLIENTConnection(Socket client) {
this.clientSocket = client;
}

@Override
public void run() {
try {
in = new BufferedReader(new InputStreamReader(
clientSocket.getInputStream()));

receiveFile();

in.close();

}


catch (IOException ex) {
Logger.getLogger(CLIENTConnection.class.getName()).log(Level.SEVERE, null, ex);
}
}

public void receiveFile() {
try {
int bytesRead;
DataInputStream clientData = new DataInputStream(clientSocket.getInputStream());

String fileName = clientData.readUTF();
OutputStream output = new FileOutputStream(("received_from_client_" + fileName));
long size = clientData.readLong();
byte[] buffer = new byte[1024];
while (size > 0 && (bytesRead = clientData.read(buffer, 0, (int) Math.min(buffer.length, size))) != -1) {
output.write(buffer, 0, bytesRead);
size -= bytesRead;
}

output.close();
clientData.close();

System.out.println("File "+fileName+" received from client.");
} catch (IOException ex) {
System.err.println("Client error. Connection closed.");
}
}


}

filecliet.java
import java.io.*;
import java.util.*;
import java.net.*;
import java.util.logging.Level;
import java.util.logging.Logger;


public class FileClient {

private static Socket sock;// defining a socket
private static String fileName;// defining a file
private static BufferedReader stdin;// creating a buffered reader object to take and read the input from the users
private static PrintStream os;// creating an object of printstream to display text or output in the terminal/command prompt

public static void main(String[] args) throws IOException {
try {
sock = new Socket("localhost", 13850);// creating a socket specifying the port and the ip address of the server to the client
stdin = new BufferedReader(new InputStreamReader(System.in));
} catch (Exception e) {
System.err.println("Cannot connect to the server, try again later."+e);
        System.exit(1);
}

os = new PrintStream(sock.getOutputStream());

try {

sendFile();

} catch (Exception e) {
System.err.println("not valid input");
}


sock.close();
}



public static void sendFile() {
try {
System.err.print("Enter file name: ");
fileName = stdin.readLine();//reads the file entered by the user 


File myFile = new File(fileName);
byte[] mybytearray = new byte[(int) myFile.length()];//determining the lenght of the file

FileInputStream fis = new FileInputStream(myFile);
BufferedInputStream bis = new BufferedInputStream(fis);//creating an input stream to read the contents of the file
//bis.read(mybytearray, 0, mybytearray.length);

DataInputStream dis = new DataInputStream(bis);
dis.readFully(mybytearray, 0, mybytearray.length);

OutputStream os = sock.getOutputStream();

//Sending file name and file size to the server
DataOutputStream dos = new DataOutputStream(os);//creating an output stream to send the file to the server
dos.writeUTF(myFile.getName());
dos.writeLong(mybytearray.length);
dos.write(mybytearray, 0, mybytearray.length);//writing the contents of the file to the server
dos.flush();
System.out.println("File "+fileName+" sent to Server.");
} catch (Exception e) {                           //reports error if any
System.err.println("File does not exist!"); 
}
}


}