Question

I have a client-server application, in the client side i have a button to recieve a frame
with table inside it from the server.
If i pass the frame with other jComponents (JButton,JTextField) it works fine, but when i’m
tryin to pass the frame with a jTable i’m getting null exception in the client side.

Here is my code:

Client side:

private class GetServerData extends Thread
{
    String server_msg = " ";
    Socket the_client;
    ObjectInputStream from_server;

    public GetServerData(Socket client)
    {
        the_client = client;
        try {
            from_server = new ObjectInputStream(the_client.getInputStream());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void run() 
    {
        do {
            try {
                Object obj = from_server.readObject(); // this is the line when the exception reference to 
                                                       // when trying to read the JFRAME 
                if (obj instanceof JFrame) {
                    JFrame window = (JFrame)obj;
                    window.setVisible(true);
                    window.pack();
                }
                else {
                    server_msg = (String)obj;
                    System.out.println(server_msg);
                }
            }
            catch (ClassNotFoundException | IOException e) {
                e.printStackTrace();
            }
        }while(!server_msg.equals("bye"));
    }
}

Server Side:

public void run(){
    while (true){
        try {
            data_from_client = (Vector)from_client.readObject();
            if (data_from_client.elementAt(0)equals("string")) {
                String s = "Hello user";
                to_client.writeObject(s);
                to_client.flush();
            }                   
            else if (data_from_client.elementAt(0).equals("table")) {
                String [][]d = {{"yoyo","jojo"},{"koko","momo"}}; 
                String []h   = {"name","best friend"};
                JTable jtable = new JTable(d,h);
                JScrollPane scroll = new JScrollPane(jtable);
                JPanel panel = new JPanel();
                panel.add(scroll,BorderLayout.CENTER);
                JFrame frame = new JFrame("Im from the server!!");
                frame.add(panel);
                to_client.writeObject(frame);
                to_client.flush();
            }
            //else if (data_from_client.elementAt(0).equals("bye")) {
            //    to_client.println("bye");
            //    to_client.flush();
            //    socket.close();
            //    socket = null;
            //}
        }
        catch(IOException | ClassNotFoundException ioe) {
            break;
            // error in reading streams from client
        }
    }
    close();
}

The exception is:

Exception in thread "Thread-21" java.lang.NullPointerException
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.access$300(Unknown Source)
at java.io.ObjectInputStream$GetFieldImpl.readFields(Unknown Source)
at java.io.ObjectInputStream.readFields(Unknown Source)
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.access$300(Unknown Source)
at java.io.ObjectInputStream$GetFieldImpl.readFields(Unknown Source)
at java.io.ObjectInputStream.readFields(Unknown Source)
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.access$300(Unknown Source)
at java.io.ObjectInputStream$GetFieldImpl.readFields(Unknown Source)
at java.io.ObjectInputStream.readFields(Unknown Source)
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.access$300(Unknown Source)
at java.io.ObjectInputStream$GetFieldImpl.readFields(Unknown Source)
at java.io.ObjectInputStream.readFields(Unknown Source)
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.access$300(Unknown Source)
at java.io.ObjectInputStream$GetFieldImpl.readFields(Unknown Source)
at java.io.ObjectInputStream.readFields(Unknown Source)
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.access$300(Unknown Source)
at java.io.ObjectInputStream$GetFieldImpl.readFields(Unknown Source)
at java.io.ObjectInputStream.readFields(Unknown Source)
at java.awt.Container.readObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at pack.connect_to_server.ServerConnection$GetServerData.run(ServerConnection.java:92)

I’ve commented in the client side, where the exception occurs.

Answer 1

Passing JFrames (or any Window for that matter) via Serialization is a bad idea, in my experience, this seems to come down to the connection it has to the systems native peer, once transferred, this is lost and generally causes lots of nasty problems, in my experience.

I could be missing really obvious and some has over come these limitations, but I’ve not found away.

If you can, you should only transfer the data content instead, it’s generally safer and a lot less messy 😉