java.io.EOFException between applet and servlet

[may Lee]

I use the following code to pass an object from applet to servlet, then receive another object from servlet to applet. Got the
java.io.EOFException error.
It is Signals that an end of file or end of stream has been reached unexpectedly during input. What could be the reason?

datato=new dataToservlet(year,month,day,hour,minute,timelength);
datafrom=new dataFromservlet(area,channelnum);

try{
URL url=new URL(servletPath);
URLConnection conn=url.openConnection();
conn.setDoInput(true);
conn.setDoOutput(true);
conn.setUseCaches(false);
conn.setDefaultUseCaches(false);
/****send object from applet to servlet******************/
conn.setRequestProperty("Content-Type","application/octet-stream");
ObjectOutputStream toServlet=new ObjectOutputStream(conn.getOutputStream());
toServlet.writeObject(datato);
toServlet.flush();
//toServlet.close();
/****get object df from servlet*********/
ObjectInputStream fromServlet=new ObjectInputStream(conn.getInputStream());
datafrom=(dataFromservlet)fromServlet.readObject();
}catch(Exception e){appleterrormessage=e.toString();appleterror=true;}