Question

I am trying to send a serialized Java object from a client to a server via socket, but there is a cast error on the server side and I can’t figure out why.
The Class being serialized is below. (I am using Eclipse, and this class is in both the client
package and the server package.)

package client; //it is "package server;" on server side

import java.io.Serializable;

public class Simple implements Serializable {
    public String msg;
    public Simple(){
        msg = "simple";
    }
}

on client side, I did:

Simple obj = new Simple();
Socket socket = new Socket(hostname, portNum);
ObjectOutputStream objOut = new ObjectOutputStream(socket.getOutputStream());
objOut.writeObject(obj);
objOut.flush();
objOut.close();

on the server side, I did:

//already accepted clientSocket
ObjectInputStream objIn = new ObjectInputStream(clientSocket.getInputStream());
Simple anObj = (Simple) objIn.readObject();
System.out.println("from server: " + anObj.msg);
objIn.close();
clientSocket.close();

However, I got the error:

Exception in thread "main" java.lang.ClassCastException: client.Simple cannot be cast to server.Simple at server.Server.main

I tried something similar with the Server and Client in a single package, and both the serialization and inflation worked. I would really appreciate it if you can help me find out why it doesn’t work here.

Answer 1

Simply put; to avoid two types in different namespaces from colliding, the package name is a part of the type name.

What happens is that the client serializes a client.Simple, which the server also deserializes as a client.Simple. Then you proceed by casting it to a server.Simple, which is an entirely different type.

It can be shown by doing;

ObjectInputStream objIn = new ObjectInputStream(clientSocket.getInputStream());
Object anObj = objIn.readObject();
System.out.println(anObj);               // Output "client.Simple@39617189"
obj = (server.Simple)anObj;              // Throws ClassCastException

The code shows that the server knows that the type of the deserialized object is client.Simple, and it simply can’t cast it to the separate type server.Simple.