For my project, I created a Java Servlet to receive the multi-part MIME POST request from Plupload.  My solution uses the streaming API from Apache Commons FileUpload.  The nice thing about the streaming API is that your Servlet can receive files that are too large to fit into memory.

Here is a link to (a simplified version of) my implementation of a Java Servlet backend for Plupload.  The ZIP file contains the source code for the Servlet and a JSP dump page that fulfills the same function as the dump.php PHP page that comes with the Plupload distribution.  The Servlet I am sharing is at the demonstration level of quality; it is not the same implementation I would actually use for a real application.  But it is a good demonstration of how to handle the POST from Plupload.  Some of my thoughts on using the Servlet include:

1. By default, the Servlet saves the uploaded files to the web application server’s temp directory.
2. Change the package for the Servlet class.  It is currently set to “shadhome”.
3. Add an entry in your web.xml configuration for the servlet.  The URL that you configure in your web.xml will be the same one you should configure for the page that is using Plupload.
4. For my project I have limited use for implementing the JSON responses that Plupload can accept, so that part is not very well represented.
5. Again, this is a demonstration level implementation.  Logging, exception handling, etc. are not well represented.

One final thought: technically this demonstration Servlet would handle saving files from any multi-part MIME POST request (not just from Plupload).

Drop me a note if you end up using this Servlet, or if you have any comments / questions.

UPDATE ON 2011-04-19: Lars has shared his example of a JSP that can receive the HTTP POST request from Plupload.

My question was, will the remove() method from an Iterator over a Set returned by Map.keySet() un-map the item from the underlying Map?

The answer is yes!

The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll retainAll, and clear operations. It does not support the add or addAll operations.

That is some fine documentation, folks!  As developers on our own systems, we know that our peers can always look at the code and try to figure out what is going on.  But, having documentation that allows the clients of your class to easily answer questions is vastly superior to people regularly resorting to looking at your code.