MVC Frameworks

1
MVC Frameworks

• Alternatives to coding the MVC pattern

2
The problem with MVC

• If you want to implement the MVC pattern
  correctly, there will be a lot administrative
  code
• Request processing
• Error handling
• View mapping
• Hard to maintain

3
The solution MVC Frameworks

• There are several frameworks that handles the MVC
  pattern for us
• The frameworks handles things like
• The frontcontroller
• Request mappings
• What should happened with different requests
• Error handling
• View handling
• Choosing the correct view
• Tag libraries to help us code the view

4
Different frameworks

• There are a lot of different frameworks to choose
  from
• Struts (from Jakarta)
• Java Server Faces (a new emerging Java standard)
• Vendor specific frameworks (like Oracles BC4J)
• We will talk about Struts

5
Struts

• A Open source framework from the Apache group
• Very widely used
• Great flexibility for the developer
• Easy to change the behavior after the application
  have been deployed
• All mapping between requests and views is done in
  a XML-file, struts-config.xml
• A very rich set of Custom tags (overlapping JSTL
  a bit)
• Great support for localization and
  internationalization

6
The main parts of Struts

• ActionServlet
• RequestProcessor
• ActionMapping
• ActionForm
• Action
• ActionForward
• ActionError

7
Struts architecture in short

• Requests are mapped to Action in
  struts-config.xml
• One Action per request
• Determines where to go next and tells the system
  by using ActionForward
• ActionForm is mapped to a Action and is used to
  capture all data sent to the system (from a form)
• Performs validation

8
ActionServlet

• The ActionServlet is the front controller in
  Struts, and its already implemented
• All incoming requests are mapped to the
  ActionServlet in web.xml
• ltservletgt
• ltservlet-namegtactionlt/servletgt
• ltservlet-classgt
• org.struts.action.ActionServlet
• lt/servlet-classgt
• lt/servletgt
• ltservlet-mappinggt
• ltservlet-namegtactionlt/servlet-namegt
• lturl-patterngt.dolt/url-patterngt
• lt/servlet-mappinggt

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RequestProcessor

• Acts as a dispatcher
• Instantiating a request handler (Action) and a
  corresponding form bean (ActionForm)
• ltaction-mappingsgt
• ltaction path"/editCustomerProfile"
• type"packageName.EditCustomerProfileAction"
• name"customerProfileForm"
• scope"request"/gt
• lt/action-mappingsgt
• ltform-bean name"customerProfileForm"
• type"packageName.customerProfileForm"/gt

10
RequestProcessor

• The path in ltaction-mappinggt identifies a
  url-pattern that should be handled by this action
• patheditCustomerProfile will be triggerd by
  editCustomerProfile.do

11
Navigation with ActionForward

• ActionForward is used to determine where to go
• ltaction-mappingsgt
• ltaction path"/editCustomerProfile"
• type"packageName.EditCustomerProfileAction"
• name"customerProfileForm"
• scope"request
• inputfailuregt
• ltforward name"success" path"/customer.jsp"/gt
• ltforward name"failure" path"/error.jsp"/gt
• lt/actiongt
• lt/action-mappingsgt

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ActionForward

• Name is a unique identifier to be able to
  identify where to go with a simple name, like
  success

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Action

• The method ActionForward execute(ActionMapping
  mapping, ActionForm form, HttpServletRequest
  request, HttpServletResponse response) is
  executed for each mapped request
• execute() is the main entry point
• execute() returns the mapping to be used by the
  RequestProcessor using the simple name
• return mapping.findForward("success")

14
ActionForm

• The ActionForm is automatically popluated with
  data from the request
• Used as a bridge between the view and the model

15
ActionErrors and ActionError

• ActionErrors is an array of ActionError
• Simplifies error handling and validation in the
  system
• ActionForm hava a method called validate() thats
  called to validate the input
• ActionErrors errors new ActionErrors()
• if(name null name.length() lt1)
• errors.add("name", new ActionError("error.na
  me.required"))
• return errors
• Returned errors will trigger the ActionForward
  defined in input of the action-mapping, i.e.
  error.jsp

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Walk through of simple example