Asynchronous file upload (AJAX file upload) using jsp and javascript [duplicate]
The two common approaches are to submit the form to an invisible iframe, or to use a Flash control such as YUI Uploader. You could also use Java instead of Flash, but this has a narrower install base.
(Shame about the layout table in the first example)
The latest dwr (http://directwebremoting.org/dwr/index.html) has ajax file uploads, complete with examples and nice stuff for users (like progress indicators and such).
It looks pretty nifty and dwr is fairly easy to use in general so this will be pretty good as well.
I don't believe AJAX can handle file uploads but this can be achieved with libraries that leverage flash. Another advantage of the flash implementation is the ability to do multiple files at once (like gmail).
SWFUpload is a good start : http://www.swfupload.org/documentation
jQuery and some of the other libraries have plugins that leverage SWFUpload. On my last project we used SWFUpload and Java without a problem.
Also helpful and worth looking into is Apache's FileUpload : http://commons.apache.org/fileupload/index.html