A really interesting article about running your own servlets on domino server can be found here: http://www.ibm.com/developerworks/cn/lotus/xpage-servlet/index.html
It’s chinese, but you can translate f.e. with Google’s Translator.
With 8.5.3 I had have some problems because the required interface IServletFactory could not be resolved in DDE.
To get the example running, you have to add a JAR to your build path. Open Project properties, select „Java Build Path“ and click „Add External JARs„:
Now you have to add the file „lwpd.domino.adapter.jar„. This can be found in the following folder:
The <VERSION> string depends on your current installation and Server-Version.
After adding this JAR to the build path, the compilation problems should be resolved.