Does the JSR 286 spec require the presence of a web.xml file in WARs containing portlets? At first, I thought so but then I created a portlet without a web.xml, deployed it in Liferay and it worked flawlessly. So is it an extension (or a bug) of Liferay, or is it not necessary to have such a file?
As Olaf rightly said portlet is nothing but a web application. Liferay has a listener that gets triggered when the portlet auto deploys. It explodes the war and adds web.xml and the content that is necessary. You can check the logic if you have source. The class name is PortletDeployer and the method is getServletContent. After add the web.xml and stuffing it, they just touch it using FileUtil.touch.