Without a doubt, Struts is the predominant framework used by the Liferay designers. In the extension environment, edit struts-ext.xml and tilesdef-ext.xml to add your own Struts actions and forwards. In the plugin environment, the same files are used but without the  “-ext” portion in the name. Finally, rest assured that Liferay will add your Struts [...]

Need to pass data between portlet instances?  Its inelegant and not the best way, but you can try setting session attributes.  To set portlet attrbutes, do the following: request.setAttributes(String name, Object value); Just be sure to uniquely name the attributes so only the intended portlets can inspect them. And don’t forget to reset these attributes [...]

In your portlet.xml descriptor, you’re only required to indicate the View mode.  Accordingl to the Portlet Development guide, the other modes, EDIT and HELP, are optional.