ObjectInputStream readObject(): ClassNotFoundException

Question

In both client and server classes, I have an exact same inner class called Data. This Data object is being sent from the server using:

ObjectOutputStream output= new ObjectOutputStream(socket.getOutputStream());
output.writeObject(d);

(where d is a Data object)

This object is received on the client side and cast to a Data object:

ObjectInputStream input = new ObjectInputStream(socket.getInputStream());
Object receiveObject = input.readObject();
if (receiveObject instanceof Data){
    Data receiveData = (Data) receiveObject;
    // some code here... 
}

I'm getting a java.lang.ClassNotFoundException: TCPServer$Data on this line Object receiveObject = input.readObject();

My guess is that it's trying to to look for the Data class in the Server side and can't find it, but I'm not sure... How do I fix this?

Answer 1

What you were trying to do is something along the lines of the following:

class TCPServer {
    /* some code */

    class Data {

    }
}

class TCPClient {
    /* some code */

    class Data {

    }
}

Then you are serializing a TCPServer$Data and trying to unserialize it as a TCPClient$Data. Instead you are going to want to be doing this:

class TCPServer {
    /* some code */

}

class TCPClient {
    /* some code */

}

class Data {
    /* some code */

}

Then make sure the Data class is available to both the client and the server programs.

Answer 2

When you use some class in two different JVMs, and you are marshalling/unmarshalling the class then the class should be exported to a common library and shared between both server and client. Having different class wont work any time.

What you are trying to do is marshall TCPServer$Data and unmarshall as TCPClient$Data. That is incompatible.