Q: What is RequestProcessor and RequestDispatcher?
Answer:
  The controller is responsible for intercepting and translating user input into actions to be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations. The Controller receives the request from the browser, invoke a business operation and coordinating the view to return to the client.

The controller is implemented by a java servlet, this servlet is centralized point of control for the web application. In struts framework the controller responsibilities are implemented by several different components like
The ActionServlet Class
The RequestProcessor Class
The Action Class

The ActionServlet extends the javax.servlet.http.httpServlet class. The ActionServlet class is not abstract and therefore can be used as a concrete controller by your application.
The controller is implemented by the ActionServlet class. All incoming requests are mapped to the central controller in the deployment descriptor as follows.
<servlet>
<servlet-name>action</servlet-name>
<servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
</servlet>

All request URIs with the pattern *.do are mapped to this servlet in the deployment descriptor as follows.

<servlet-mapping>
<servlet-name>action</servlet-name>
<url-pattern>*.do</url-pattern>
<url-pattern>*.do</url-pattern>
A request URI that matches this pattern will have the following form.
http://www.my_site_name.com/mycontext/actionName.do

The preceding mapping is called extension mapping, however, you can also specify path mapping where a pattern ends with /* as shown below.
<servlet-mapping>
<servlet-name>action</servlet-name>
<url-pattern>/do/*</url-pattern>
<url-pattern>*.do</url-pattern>
A request URI that matches this pattern will have the following form.
http://www.my_site_name.com/mycontext/do/action_Name
The class org.apache.struts.action.requestProcessor process the request from the controller. You can sublass the RequestProcessor with your own version and modify how the request is processed.

Once the controller receives a client request, it delegates the handling of the request to a helper class. This helper knows how to execute the business operation associated with the requested action. In the Struts framework this helper class is descended of org.apache.struts.action.Action class. It acts as a bridge between a client-side user action and business operation. The Action class decouples the client request from the business model. This decoupling allows for more than one-to-one mapping between the user request and an action. The Action class also can perform other functions such as authorization, logging before invoking business operation. the Struts Action class contains several methods, but most important method is the execute() method.
public ActionForward execute(ActionMapping mapping,
ActionForm form, HttpServletRequest request, HttpServletResponse response)    throws Exception;
The execute() method is called by the controller when a request is received from a client. The controller creates an instance of the Action class if one doesn?t already exist. The strut framework will create only a single instance of each Action class in your application.

Action are mapped in the struts configuration file and this configuration is loaded into memory at startup and made available to the framework at runtime. Each Action element is represented in memory by an instance of the org.apache.struts.action.ActionMapping class . The ActionMapping object contains a path attribute that is matched against a portion of the URI of the incoming request.
<action>
path= “/somerequest”
type=”com.somepackage.someAction”
scope=”request”
name=”someForm”
validate=”true”
input=”somejsp.jsp”
<forward name=”Success” path=”/action/xys” redirect=”true”/>
<forward name=”Failure” path=”/somejsp.jsp” redirect=”true”/>
</action>
Once this is done the controller should determine which view to return to the client. The execute method signature in Action class has a return type org.apache.struts.action.ActionForward class. The ActionForward class represents a destination to which the controller may send control once an action has completed. Instead of specifying an actual JSP page in the code, you can declaratively associate as action forward through out the application. The action forward are specified in the configuration file.
<action>
path= “/somerequest”
type=”com.somepackage.someAction”
scope=”request”
name=”someForm”
validate=”true”
input=”somejsp.jsp”
<forward name=”Success” path=”/action/xys” redirect=”true”/>
<forward name=”Failure” path=”/somejsp.jsp” redirect=”true”/>
</action>
The action forward mappings also can be specified in a global section, independent of any specific action mapping.
<global-forwards>
<forward name=”Success” path=”/action/somejsp.jsp” />
<forward name=”Failure” path=”/someotherjsp.jsp” />
</global-forwards>

public interface RequestDispatcher

Defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name.
This interface is intended to wrap servlets, but a servlet container can create RequestDispatcher objects to wrap any type of resource.

getRequestDispatcher

public RequestDispatcher getRequestDispatcher(java.lang.String path)

Returns a RequestDispatcher object that acts as a wrapper for the resource located at the given path. A RequestDispatcher object can be used to forward a request to the resource or to include the resource in a response. The resource can be dynamic or static.
The pathname must begin with a “/” and is interpreted as relative to the current context root. Use getContext to obtain a RequestDispatcher for resources in foreign contexts. This method returns null if the ServletContext cannot return a RequestDispatcher.

Parameters:
path – a String specifying the pathname to the resource
Returns:
a RequestDispatcher object that acts as a wrapper for the resource at the specified path
See Also:
RequestDispatcher, getContext(java.lang.String)

getNamedDispatcher

public RequestDispatcher getNamedDispatcher(java.lang.String name)

Returns a RequestDispatcher object that acts as a wrapper for the named servlet.
Servlets (and JSP pages also) may be given names via server administration or via a web application deployment descriptor. A servlet instance can determine its name using ServletConfig.getServletName().
This method returns null if the ServletContext cannot return a RequestDispatcher for any reason.

Parameters:
name – a String specifying the name of a servlet to wrap
Returns:
a RequestDispatcher object that acts as a wrapper for the named servlet
See Also:
RequestDispatcher, getContext(java.lang.String), ServletConfig.getServletName()

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