NullPointerException in the attack method.

Question

Hi Guys i getting a NullPointerException on the attack() method,i've been trying to figure it out for some hours but i failed.Pls help.
* Intruder.java
* Created on March 2, 2007, 7:31 AM
* To change this template, choose Tools | Template Manager
* and open the template in the editor.
package org.biyela.security;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.*;
public class Intruder
    private String host_name = "localhost";
    private static final int PORT = 8000;
    private ObjectInputStream objectReader;
    private ObjectOutputStream objectWriter;
    private Socket socket;
    public Intruder()
    public void connect()
        try
                socket = new Socket(host_name,PORT);
                System.out.println("Creating the client socket......");
                objectReader = new ObjectInputStream(socket.getInputStream());
                System.out.println("Opening the input channel for object data.....");
                objectWriter = new ObjectOutputStream(socket.getOutputStream());
                System.out.println("Opening the output channel for object data.....");
                //return true;
        catch(Exception ex)
            ex.printStackTrace();
           // return false;
    public void attack()
            Sender s = new Sender();
            s.setIdentity("Alice");
            s.setSenderChallenge("encryptthis");
            try
                objectWriter.writeObject(s);
                objectWriter.flush();
                objectWriter.close();
                System.out.println("<Intruder Instance>.............");
                System.out.println("Starting reflection attack......");
               // ObjectInputStream objectReader = new ObjectInputStream(socket.getInputStream());
                Sender sender = (Sender)objectReader.readObject();
                System.out.println("Ecrypted challenge:"+sender.getSenderChallenge());
                System.out.println("Bob challenge:"+sender.getReceiverChallenge());
               // sender.setSenderChallenge(sender.getReceiverChallenge());
                //sender.setReceiverChallenge(null);
               // objectWriter.writeObject(sender);
            catch(Exception ioex)
                ioex.printStackTrace();
//    public Object []  handleServerResponse()
//            Object [] objects = null;
//            Object obj = null;
//            int i = 0;
//            try
//                    int length =  objectReader.available();
//                    objects = new Object[length];
//                    while((obj = objectReader.readObject()) != null)
//                            objects[i] = obj;
//                            i++;
//            catch(Exception ex)
//                ex.printStackTrace();
//            return objects;
    public void disconnect()
        try
            //Checking if the client socket is connected
            if(socket != null)
                if(objectReader != null && objectWriter != null)
                    objectReader = null;
                    System.out.println("Closing the object input stream.......");
                    objectWriter = null;
                    System.out.println("Closing the object output stream.......");
                socket = null;
                System.out.println("Closing the client socket......");
           // return true;
        catch(Exception ex)
            ex.printStackTrace();
           // return false;
}

Answer 1

You only have to look at the stack trace to get the line number where the NPE is happening so there's no reason why you can't solve it yourself.
However there are several other problems with your code:
(a) You are closing the object output stream in the attack() method, which will disconnect you completely. Closing an input stream or output stream of a socket closes the other stream and the socket. You should just flush the object output stream here.
(b) You are creating the ObjectInputStream before the ObjectOutputStream. Reverse this, and flush the ObjectOutputStream before you create the ObjectInputStream.
(c) Your disconnect() method doesn't disconnect. It sets things to null instead of closing something. It should close the object output stream. Because of (a), closing anything else is redundant.
(d) Your commented-out code based on using available() to determine the size of an array will never work because available() can't do that, and it can't do too much else either. Just read objects in from the stream until you get an EOFException and put them into some form of a List.