What are the Portlet Filters ?

A portlet filter is same as a servlet filter. The only difference is that a servlet has only one request handling method, service() and therefore there is only one type of the servlet filter. A portlet on the other hand has four types of request handling methods and therefore there are four different types of portlet filters.

The portlet API defines the following interfaces for creating portlet filters:
javax.portlet.filter.ActionFilter - For processAction method
javax.portlet.filter.EventFilter - For processEvent method
javax.portlet.filter.RenderFilter - For render method
javax.portlet.filter.ResourceFilter - For serveResource method

Each of the above filter interface contains a single doFilter(Request, Response, FilterChain chain) method which differs in the type of request and response object. For example, the doFilter() method of ActionFilter takes instances of ActionRequest and ActionResponse objects while the doFilter() method of RenderFilter takes instances of the RenderRequest and RenderResponse.

Each filter interface extends a common base interface called javax.portlet.filter.PortletFilter. This common base interface contains two methods:
init(javax.portlet.filter.FilterConfig filterConfig) and destroy(). The init() method makes sure that every filter has access to a FilterConfig object from which it can obtain its initialization parameters, a reference to the PortletContext which it can use, for example, to load resources needed for filtering tasks. The destroy() method signifies the end of service of the filter. The init() and destroy() methods of a portlet filter are called only once during their lifetime.

A single filter class can provide filter functionality for more than one life cycle method. Also, a single filter can provide filter functionality for more than one portlet. Multiple filters can be associated with one life cycle method of a portlet. The doFilter() method of a portlet filter might create customized request and response objects by using the RequestWrapper and ResponseWrapper classes and by passing these wrappers to the doFilter() method of the FilterChain object.

Note: A portlet filter is a Java technology-based component that can be used to modify the content of the portlet request and portlet response before or after any life cycle method of the portlet.


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