Filter is used as controller in Struts2 - servlets

Question

In struts2 why is a Filter is used as a controller instead of ActionServlet?
What is the advantage of using a Filter over ActionServlet?

Answer 1

As per Struts2 Budi Karnival struts2 book, There is one distinct advantage of using a filter over a servlet as a controller. With a filter you can conveniently choose to serve all the resources in your application, including static ones.
With a servlet, your controller only handles access to the dynamic part of the application. Note that the url-pattern element in the web.xml file in the previous application is
<servlet>
<servlet-name>Controller</servlet-name>
<servlet-class>...</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Controller</servlet-name>
<url-pattern>*.action</url-pattern>
</servlet-mapping>
With such a setting, requests for static resources are not handled by the servlet controller, but by the container. You wouldn't want to handle static resources in your servlet controller because that would mean extra work.
A filter is different. A filter can opt to let through requests for static contents. To pass on a request, call the filterChain.doFilter method in the filter's doFilter method.
Consequently, employing a filter as the controller allows you to block all requests to the application, including request for static contents. You will then have the following setting in your deployment descriptor:
<filter>
<filter-name>filterDispatcher</filter-name>
<filter-class>...</filter-class>
</filter>
<filter-mapping>
<filter-name>filterDispatcher</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
Advantage of this filter : One thing for sure, you can easily protect your static files from curious eyes.
The following code will send an error message if a user tries to view a JavaScript file:
public void doFilter(ServletRequest request, ServletResponse response,FilterChain filterChain) throws IOException, ServletException {
HttpServletRequest req = (HttpServletRequest) request;
HttpServletResponse res = (HttpServletResponse) response;
String uri = req.getRequestURI();
if (uri.indexOf("/css/") != -1 && req.getHeader("referer") == null) {
res.sendError(HttpServletResponse.SC_FORBIDDEN);
} else {
// handle this request
}
}
It will not protect your code from the most determined people, but users can no longer type in the URL of your static file to view it. By the same token, you can protect your images so that no one can link to them at your expense.
Another advantage :
The introduction of Interceptors in Struts2 framework.It not just reduce our coding effort,but helps us write any code which we would have used filters for coding and necessary change in the web.xml as opposed to Struts1.So now any code that fits better in Filter can now moved to interceptors( which is more controllable than filters), all configuration can be controlled in struts.xml file, no need to touch the web.xml file

Answer 2

We generally Use a Filter when we want to filter and/or modify requests based on specific conditions.
For S2 to work it needs to perform certain reprocessing and modification work in order for a successful execution of your request while on other hands we use Servlet when we want to control, preprocess and/or post-process requests.
For controlling request S2 use Servlet under the hood but being hidden away to make the overall application structure more clean and easy to use.
This is what we have for Filters in The Java EE 6 Tutorial.
A filter is an object that can transform the header and content (or both) of a request or response. Filters differ from web components in that filters usually do not themselves create a response. Instead, a filter provides functionality that can be “attached” to any kind of web resource. Consequently, a filter should not have any dependencies on a web resource for which it is acting as a filter; this way, it can be composed with more than one type of web resource.