UDP example gives java.lang.ClassNotFoundException: packagename.Foo Exception; How to fix?

Question

I am new to UDP protocol, and I want to learn from a good example. I finally found a good example but it gives java.lang.ClassNotFoundException: packagename.Student Exception

I run the server first then the client.

The client output:

Message sent from client
Response from server:Thank you for the message□□□□□□□□□□□□□□□□□□□□□

Big line of □□□□ above

The Server output:

java.lang.ClassNotFoundException: ex_udp_client.Student at java.net.URLClassLoader$1.run(URLClassLoader.java:366) at java.net.URLClassLoader$1.run(URLClassLoader.java:355) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:354) at java.lang.ClassLoader.loadClass(ClassLoader.java:423) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308) at java.lang.ClassLoader.loadClass(ClassLoader.java:356) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:266) at java.io.ObjectInputStream.resolveClass(ObjectInputStream.java:622) at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1593) at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1514) at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1750) at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1347) at java.io.ObjectInputStream.readObject(ObjectInputStream.java:369) at ex_udp_server.UDPSocketServer.createAndListenSocket(UDPSocketServer.java:40) at ex_udp_server.UDPSocketServer.main(UDPSocketServer.java:72) BUILD SUCCESSFUL (total time: 6 seconds)

I think the exception thrown at this line:
Student student = (Student) is.readObject();

How to fix?

The Serializable class Copied to both packages of client and server

import java.io.Serializable;

public class Student implements Serializable
{

   public Student(int id, String name, String address1)
   {
      this.id = id;
      this.name = name;
      this.addressLine = address1;
   }
   private static final long serialVersionUID = 1L;
   private int id;
   private String name;
   private String addressLine;

   public int getId()
   {
      return id;
   }

   public void setId(int id)
   {
      this.id = id;
   }

   public String getName()
   {
      return name;
   }

   public void setName(String name)
   {
      this.name = name;
   }

   public String getAddressLine()
   {
      return addressLine;
   }

   public void setAddressLine(String addressLine)
   {
      this.addressLine = addressLine;
   }

   public String toString()
   {
      return "Id = " + getId() + " Name = " + getName() + " Address = " + getAddressLine();
   }
}

The Server class

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;

public class UDPSocketServer
{

   DatagramSocket socket = null;

   public UDPSocketServer()
   {
   }

   public void createAndListenSocket()
   {
      try
      {
         socket = new DatagramSocket(9876);
         byte[] incomingData = new byte[1024];

         while (true)
         {
            DatagramPacket incomingPacket = new DatagramPacket(incomingData, incomingData.length);
            socket.receive(incomingPacket);
            byte[] data = incomingPacket.getData();
            ByteArrayInputStream in = new ByteArrayInputStream(data);
            ObjectInputStream is = new ObjectInputStream(in);
            try
            {
               Student student = (Student) is.readObject();
               System.out.println("Student object received = " + student);
            } catch (ClassNotFoundException e)
            {
               e.printStackTrace();
            }
            InetAddress IPAddress = incomingPacket.getAddress();
            int port = incomingPacket.getPort();
            String reply = "Thank you for the message";
            byte[] replyBytea = reply.getBytes();
            DatagramPacket replyPacket =
                    new DatagramPacket(replyBytea, replyBytea.length, IPAddress, port);
            socket.send(replyPacket);
            Thread.sleep(2000);
            System.exit(0);
         }

      } catch (SocketException e)
      {
         e.printStackTrace();
      } catch (IOException i)
      {
         i.printStackTrace();
      } catch (InterruptedException e)
      {
         e.printStackTrace();
      }
   }

   public static void main(String[] args)
   {
      UDPSocketServer server = new UDPSocketServer();
      server.createAndListenSocket();
   }
}

The Client class

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.net.*;

public class UDPSocketClient
{

   DatagramSocket Socket;

   public UDPSocketClient()
   {
   }

   public void createAndListenSocket()
   {
      try
      {

         Socket = new DatagramSocket();
         InetAddress IPAddress = InetAddress.getByName("localhost");
         byte[] incomingData = new byte[1024];
         Student student = new Student(1, "Bijoy", "Kerala");
         ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
         ObjectOutputStream os = new ObjectOutputStream(outputStream);
         os.writeObject(student);
         byte[] data = outputStream.toByteArray();
         DatagramPacket sendPacket = new DatagramPacket(data, data.length, IPAddress, 9876);
         Socket.send(sendPacket);
         System.out.println("Message sent from client");
         DatagramPacket incomingPacket = new DatagramPacket(incomingData, incomingData.length);
         Socket.receive(incomingPacket);
         String response = new String(incomingPacket.getData());
         System.out.println("Response from server:" + response);
         Thread.sleep(2000);

      } catch (UnknownHostException e)
      {
         e.printStackTrace();
      } catch (SocketException e)
      {
         e.printStackTrace();
      } catch (IOException e)
      {
         e.printStackTrace();
      } catch (InterruptedException e)
      {
         e.printStackTrace();
      }
   }

   public static void main(String[] args)
   {
      UDPSocketClient client = new UDPSocketClient();
      client.createAndListenSocket();
   }
}

Answer 1

The Serializable class Copied to both packages of client and server

That's the problem. A class called client.Student isn't the same thing as server.Student, whatever the source code may say. You need to have one copy of this class, with the .class file deployed to both the server and the client, in the appropriate directory.

Answer 2

Do you get this exception in the server or in the client?
In whichever of the two you got it, it means you don't have
this class at runtime there (in the client or in the server).
You don't have it on the classpath, I mean.