Problems with Applet/Servlet communication with serialization

Hi,

I have a test JApplet with a tread running. Every X seconds, the tread sends a serialized Vector to the servlet as a http request and receives a serialized Vector as a http response. I have two questions:

1)  In my applet, I have a Thread with the inner class UpdateHandler as its runnable target. In UpdateHandler, the above mentioned applet-servlet communication takes place.
When running my applet, I got an exception: java.io.NotSerializedException for UpdateHandler, and when I let UpdateHanler implement Serializable, that error seemed to be fixed. Then I got the same error for the Tread _updateThread. I created an inner class UpdateTread which extends Thread and implements Serializable. Now both errors were gone. Seems strange to me that the objects containing the code for sending an object to/from the servlet, also need to be Serializable, not only the passed Vector. Why is it so?

2)  When the NotSerializedExceptions are gone, I get another error when trying to read the response from servlet, when instantiating new ObjectInputStream(servletConnection.getInputStream())   - see posted code. The exception is:

java.io.FileNotFoundException: http://hostIP:portNo./emilion/servlet/result/TestServlet
      at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:707)
      at sun.plugin.net.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:384)
      at result.TestApplet$UpdateHandler._requestResultsFromServlet(TestApplet.java:158)
      at result.TestApplet$UpdateHandler._requestResults(TestApplet.java:117)
      at result.TestApplet$UpdateHandler.run(TestApplet.java:99)
      at java.lang.Thread.run(Thread.java:536)


Any ideas what causes the error? Help greatly appreciated!



package result;

import java.awt.*;
import java.util.*;
import java.net.URL;
import java.net.URLConnection;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.io.Serializable;
import javax.swing.*;

public class TestApplet extends JApplet implements {

  private UpdateHandler _updateHandler;
  private Thread _updateThread;
  private URL _resultServlet;

  public void init() {
    super.init();

    try {
      String servletPath = "/emilion/servlet/result/TestServlet";
      _resultServlet = new URL("http", getCodeBase().getHost(), getCodeBase().getPort(), servletPath);
    } catch(Exception x) {
      x.printStackTrace();
    }

    // Set up the GUI  
    QCPanel bigViewPanel = new QCPanel();
    QCLabel label = new QCLabel("TEST TEST");
    bigViewPanel.add(label);
    getContentPane().add(bigViewPanel, BorderLayout.CENTER);

    // Updating thread
    _updateHandler = new UpdateHandler();
    _updateThread = new UpdateTread(_updateHandler);
    _updateThread.start();
  }

  private class UpdateTread extends Thread implements Serializable {
    public UpdateTread(UpdateHandler updateHandler) {
      super(updateHandler);
    }
  }

  private class UpdateHandler implements Runnable, Serializable {
    public final static int UPDATE_INTERVAL = 10;

    public UpdateHandler() {
    }

    public void run() {
      try {
          _requestFromServlet();
          // Update GUI with responded data from servlet
          //…..
      } catch (InterruptedException ie) {
      }
    }

    private synchronized void _ requestFromServlet () throws InterruptedException {
      while(true) {
         _request();    
        wait(UPDATE_INTERVAL*1000);
      }
    }

    public Vector _request() {
      ObjectOutputStream outputToServlet;
      ObjectInputStream inputFromServlet;

      Vector testVector = new Vector();
      testVector.add("AAA");
      testVector.add("BBB");
      testVector.add("CCC");

      try {
        // Set up servlet connection
        URLConnection servletConnection = _resultServlet.openConnection();
        servletConnection.setDoInput(true);
        servletConnection.setDoOutput(true);
        servletConnection.setUseCaches(false);
        servletConnection.setDefaultUseCaches(false);
        servletConnection.setRequestProperty("Content-Type", "application/octet-stream");

        // REQUEST
        // Send testVector to servlet
        outputToServlet = new ObjectOutputStream(servletConnection.getOutputStream());
        outputToServlet.writeObject(testVector);
        outputToServlet.flush();
        outputToServlet.close();

        // RESPONSE
        // Read the response from the servlet. The servlet will return a sericalized Vector containing
        // Objects (just for testing)
        inputFromServlet = new ObjectInputStream(servletConnection.getInputStream()); //THIS IS WHERE THE EXCEPTION OCCURS
        Vector vectorFromServlet = (Vector)inputFromServlet.readObject();
        inputFromServlet.close();

        return vectorFromServlet;

      } catch(Exception x) {
        x.printStackTrace();
        return null;
      }
    }

  }
 
}




package result;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.Vector;

public class TestServlet extends HttpServlet {

  private Vector _viewModels;
  private Vector _viewsFromApplet;

  public void init() {
  }

  public void doPost(HttpServletRequest request, HttpServletResponse response)
          throws ServletException, IOException {

    ObjectInputStream inputFromApplet = null;
    _viewsFromApplet = null;

    try {
      // Get an input stream from the applet
      inputFromApplet = new ObjectInputStream(request.getInputStream());
      _viewsFromApplet = (Vector)inputFromApplet.readObject();
      inputFromApplet.close();
    } catch(Exception x) {
      x.printStackTrace();
    }

    //Respond to applet
    try {
      _respondResultsToApplet(response, _getObjectVector());
    } catch(Exception x) {
      x.printStackTrace();
    }
  }

  public void doGet(HttpServletRequest req, HttpServletResponse resp)
          throws ServletException, IOException {
    doPost(req, resp);
  }

  private void _respondResultsToApplet(HttpServletResponse response, Vector objects) throws Exception {

    if (objects != null) {
      ObjectOutputStream outputToApplet = new ObjectOutputStream(response.getOutputStream());

      outputToApplet.writeObject(objects);
      outputToApplet.flush();
      outputToApplet.close();

    } else {

    }
  }

  private Vector _getObjectVector() {
    // Handle input data from applet
    ///...
    // Objects to be passed back to applet
    Vector v = new Vector();
    v.add(new Object());
    v.add(new Object());
    return v;

  }
}