This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
there is a portlet bridge and other technologies that allow you to write vanilla web applications and then make them portlets and deploy them, and there are several frameworks like Struts, Spring MVC and Vaardin that look almost the same between their web and portlet behaviours.
But the real answer is no, there is no single accepted tool for creating portlets.