Question

In my Java Sockets program, I have implemented a client-server Observer pattern. That is, the server subject publishes its server events to client observers that have subscribed to the server. That’s the theory.

In practice, I am unable to send the client observer through the socket to subscribe to the server. The error that comes up is: “java.io.NotSerializableException: java.net.Socket.” I think this means Java is complaining that the client observer contains a Socket which, as we all know, is not Serializable.

However, the Socket is the means of communication between the client and the server!

How can I implement a client-server Observer pattern when the client appears to contain a non-Serializable roadblock?

Here is a code overview to give you an understanding of what is happening:

Server

public class Server implements ServerSocketPublisher {

    // traditional Observer publisher methods implemented here, such as register, 
    // deregister, notifySubscribers

    // ServerSocket implemented here. Waiting on accept()

}

Client

public class Client implements ClientSocketSubscriber, Serializable {

    // traditional Observer subscriber methods implemented here, i.e. updateClient

    Socket connectingSocket = null; //I SUSPECT THIS VARIABLE IS THE PROBLEM

    try {
         connectingSocket = new Socket();
         // set SocketAddress and timeout
         connectingSocket.connect(sockAddr, timeout)
         if (connectingSocket.isConnected()) {
             ObjectOutputStream oos = new ObjectOutputStream     
                  (connectingSocket.getOutputStream());
             oos.writeObject(this); // THIS LINE THROWS THE ERROR in STACKTRACES
             oos.flush();
             oos.close();
         }
    } catch (/*various exceptions*/) {
    }
    // close connectingSocket
}

Answer 1

You have couple of ways to get this fixed:

1. Mark your socket as transient

   transient Socket connectingSocket = null;

2. Instead of implementing Serializable implement Externalizable and then in your implementation of read and write object ignore the Socket.

Along with this you should also read

About transient:

1. Post on SO

About Externalizable :

1. Javabeat