Submitting a user's file to an applet.

Question

Hi,
I am fairly new to Java, but I have spent a large part of this year learning about and using Java to write a program for biochemists, to allow them to study a particular class of enzymes. I have used Java because I wanted to enable to program to be used by people via the internet, and I wanted there to be a good graphical interface. Java Applets have enabled me to do this. Furthermore, some of the computation has to be done via the server, because the Applet interfaces to a Prolog program, and again this was easy with Java, because I was able to do this using (HTTP interface) Servlets.
There is one major drawback to this solution. The users will often develop 'models' of their enzyme, which they will want to save to a file, and reload at a later stage. However, Applets cannot access the user's file system, which means I had to find another way to achieve this. At the moment, loading (and saving) the models is done in a two-step process. A form, embedded in the web page below the Applet, is used to submit the file to a Servlet, which saves the contents of the file to a session object. Then, the user clicks on a button in the Applet, which connects to another Servlet, and the file is read from the session object, and transmitted back to the Applet. Saving a file is just the reverse of this process.
Quite apart from the fact that it is a nuisance to do this transaction in two steps, it causes problems when the user doesn't have cookies turned on. I can always alert people to the fact that they need to turn cookies on (or use URL encoding), but it still means that there are two steps to the process.
Does anyone have any suggestions how I might be able to write this sequence so that the user can load or save a file in only one step, instead of two? Someone suggested that I use sockets, but from the reading that I have done, it seems that Servlets are simply a convenient way to use sockets. So, it seems using sockets would just be a more difficult and time-consuming way to achieve the same end.
Any suggestions on this topic would be gratefully appreciated.
Regards,
Sarah

Answer 1

Thanks so much for your prompt reply.
Every time the user performs an action in the applet that connects to one of the servlets, I extend the lifetime of their session object. That way, when they submit a file to the server, it is available for them to access it for a couple of hours, before the session object will time out (unless the user accesses the server in the meantime, which resets the lifetime of the session to 2hrs).
The problem isn't actually passing the file from the servlet to the applet (or vice versa) - I already use IO streams to do this. The problem is can I get the file from the HTML form to the applet in one step?
As far as I can see, if the connection to the servlet comes from the html form, then the response goes back to the browser. Similarly, if the request comes from the applet, the response goes back to the applet. What I am wondering is, is there a way to force communication from a servlet to the applet when the initial connection to the servlet has come from the browser (i.e. from the HTML form)?