The Portlet interface is the main abstraction of the Portlet API. All portlets implement this interface either directly or, more commonly, by extending a class that implements the interface.
The Portlet API includes a GenericPortlet class that implements the Portlet, EventPortlet and ResourceServingPortlet interface and provides default functionality. Developers should typically extend, directly or indirectly, the GenericPortlet class to implement their portlets.
The Portlet provides the additional optional lifecycle interfaces EventPortlet and ResourceServingPortlet that the portlet can implement.
Basic four lifecycle methods
The init(PortletConfig config) is called once, immediately after a new portlet instance is created. It can be used to perform startup tasks and is akin to a servlets init method. PortletConfig represents read-only configuration data, specified in a portlet's descriptor file, portlet.xml(more on this file later). For example, PortletConfig provides access to initialization parameters.
The processAction(ActionRequest request, ActionResponse response) method is called in response to a user action such as clicking a hyperlink or submitting a form. In this method, a portlet may invoke business logic components, such as JavaBeans, to accomplish its goal. The ActionRequest and ActionResponse Interfaces are subinterfaces of PortletRequest and PortalRequest. In processAction, a portlet may modify its own state as well as persistent information about a portlet.
The render(RenderRequest request, RenderResponse response) method follows processAction in the chain of lifecycle methods. Render generates the markup that will be made accessible to the portal user. RenderRequest and RenderResponse methods, also subinterfaces of PortletRequest and PortletResponse, are available during the rendering of a portlet. The way in which the render method generates output may depend on the portlet's current state.
The destroy() method is the last lifecycle method, called just before a portlet is garbage collected and provides a last chance to free up portlet resources.
Additional lifecycle operation using EventPortlet
An event is a life cycle operation that occurs before the rendering phase. Events can be described as a loosely coupled, brokered means of communication between portlets. Events allow portlets to respond on actions or state changes not directly related to an interaction of the user with the portlet.
A portlet can declare events in its deployment descriptor by using the event-definition element in the portlet application section. In the portlet section, each portlet specifies the events it would like to publish through the supported-publishing-event element and the events it would like to process through the supported-processing-event element.
The supported-publishing-event and supported-processing-event elements must reference the event name defined in the portlet application section in a event-definition element.
The portlet creates events using the setEvent() method during action processing. This will be processed by the portlet container after the action processing has finished. Portlets can also create events during event phase by calling setEvent() method on EventResponse.
To receive events, the portlet must implement the javax.Portlet.EventPortlet interface. The portlet container calls the processEvent() method for each event targeted to the portlet with an EventRequest and EventResponse object. The portlet can access the event that triggered the current process event call by using the EventRequest.getEvent() method. This method returns an object of type Event encapsulating the current event name and value.
Additional lifecycle operation using ResourceServingPortlet
The serveResource method can be used to implement Ajax use cases by invoking the resource URL through the XMLHttpRequest(or XMLPortletRequest) in client-side JavaScript code.
