There isn't anyway to send info from Java to server without postback. Java is client side and asp.net C# or vb
code is server side. Like someone posted real quick, if you want to eliminate postback you will have to do AJAX. I have wrote a few pages that would take variables from java and store them in my ASP.NET 2.0 session through using the xmlHttp object.
You can find sources out there to help you. If this interests you, Wrox has written a good AJAX beginning book I have read through.