File transfer using java sockets : Error on client side "Exception in thread "main" java.lang.NullPointerException"

Question

Guide me through this exception. I have been trying to send a file from client to server - where in client enters the name of the file manually. But i am getting NullPointerException on client side - the possible error as of I know is "before opening the file, I am passing a null argument hence the NPE"

Server.java

import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class Server extends Thread {
    public static final int PORT = 3333;
    public static final int BUFFER_SIZE = 100;

    @Override
    public void run() {
        try {
            ServerSocket serverSocket = new ServerSocket(PORT);

            while (true) {
                Socket s = serverSocket.accept();
                saveFile(s);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void saveFile(Socket socket) throws Exception {
        ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        FileOutputStream fos = null;
        byte [] buffer = new byte[BUFFER_SIZE];

        // 1. Read file name.
        Object o = ois.readObject();

        if (o instanceof String) {
            fos = new FileOutputStream(o.toString());
        } else {
            throwException("Something is wrong");
        }

        // 2. Read file to the end.
        Integer bytesRead = 0;

        do {
            o = ois.readObject();

            if (!(o instanceof Integer)) {
                throwException("Something is wrong");
            }

            bytesRead = (Integer)o;

            o = ois.readObject();

            if (!(o instanceof byte[])) {
                throwException("Something is wrong");
            }

            buffer = (byte[])o;

            // 3. Write data to output file.
            fos.write(buffer, 0, bytesRead);
        } while (bytesRead == BUFFER_SIZE);

        fos.close();

        ois.close();
        oos.close();
    }

    public static void throwException(String message) throws Exception {
        throw new Exception(message);
    }

    public static void main(String[] args) {
        new Server().start();
    }
}  

Client.java

import java.io.File;
import java.io.FileInputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.util.Arrays;
import java.lang.*;

public class Client {
    public static void main(String[] args) throws Exception {
        String fileName = null;

       try {
            fileName = args[0];
        } catch (Exception e) {
            System.out.println("Enter the name of the file :");
        }
        File file = new File(fileName);
        Socket socket = new Socket("localhost", 3333);
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());

        oos.writeObject(file.getName());

        FileInputStream fis = new FileInputStream(file);
        byte [] buffer = new byte[Server.BUFFER_SIZE];
        Integer bytesRead = 0;

        while ((bytesRead = fis.read(buffer)) > 0) {
            oos.writeObject(bytesRead);
            oos.writeObject(Arrays.copyOf(buffer, buffer.length));
        }

        oos.close();
        ois.close();
    }
}  

Answer 1

1. First of all its better to use Scanner for taking the path along with the trim() method, so to prevent any accidental addition of space in the path.

Scanner scan = new Scanner(System.in);
String tmppath = scan.nextLine();
String path = tmppath.trim();

2. But still if you want to go your way...... then try this..

File f = null;   // If its in class scope, then 
                 // no need to initialize it to null, bydefault it will be null.

 try{

     f = args[0];


 }catch(Exception ex){

     System.out.println("file object is null");
}

Answer 2

Your code

String fileName = null


try {
    fileName = args[0];
 } catch (Exception e) {
    System.out.println("Enter the name of the file :");
 }

has got a problem. Suppose if the user has not typed the file name, it ignores the exception saying "Enter the name of the file :" . So your fileName will be NULL always.

Either you try modify it as

try {
    fileName = args[0];
 } catch (Exception e) {
    System.out.println("Usage: java Client <file_name>");
    System.exit(0);
 }

or

try {
 fileName = args[0];
} catch (Exception e) {
  System.out.println("Enter the name of the file :");
  Scanner scanner = new Scanner(System.in);
  String file_name = scanner.nextLine();
}

And whatever the method you are going to use , check for the NULL like

if (fileName == null) {
  System.out.println("File name can not be NULL");
  System.exit(0);
}

This should do. Cheers.

Answer 3

String fileName = null;

       try {
            fileName = args[0];
        } catch (Exception e) {
            System.out.println("Enter the name of the file :");
        }
        File file = new File(fileName);

The above code simply checks if the user has provided a file name or not. Even if the user doesn't enter any file name, it proceeds further, as a result null is being passed on as the argument to File constructor. And hence your NPE.

Put the File instantiation inside a try catch block as well. And secondly, the File constructor takes in the entire path of the file as a parameter, so ensure that the file you are working with is in the current working directory.