Understanding Front Controller Design Pattern

The front controller design pattern means that all requests that come for a resource in an application will be handled by a single handler and then dispatched to the appropriate handler for that type of request. The front controller may use other helpers to achieve the dispatching mechanism.

While reading the brief overview you may have thought about ActionServlet in Struts 1 or Dispatchers in Struts 2. Yes both of them are implementations of Front controller design pattern. But most of the developers that I have interacted to (specially the beginners) end their thought process of Front Controller at Struts only.

The Front Controller design pattern is applicable and useful for all kind of applications be it web or desktop applications and is not limited to any single programming language or framework. Before the rollout of MVC frameworks in the market, the design pattern was still in use.

The only downside was that not all application could make use of it because of the effort involved. Only the applications which had custom framework in place were using the design pattern.

So for example in case of a Java based web application, which is not using any MVC framework, all the requests would be mapped to a single servlet in web.xml file. This we know can be done by using the servlet mapping (same as we do for ActionServlet in case of Struts 1).

<web-app>    <servlet>        <servlet-name> FrontServlet</servlet-name>        <servlet-class> com.example.test.FrontServlet</servlet-class>    </servlet>    <servlet-mapping>        <servlet-name>FrontServlet<servlet-name>        <url-pattern>/*</url-pattern>    <servlet-mapping></servlet-mapping></servlet-name></servlet-name></servlet-mapping></web-app>

Once the request reached the FrontServlet, it uses some configuration files to decide the appropriate dispatcher for the request and thus forwards the request using forward mechanism.

From http://extreme-java.blogspot.com/2011/04/understanding-front-controller-design.html