Lecturer: Prof. Piero Fraternali, piero.fraternali@polimi.itTeaching Assistant: Alessandro Bozzon, bozzon@elet.polimi.it

Advanced Web Technologies:

Struts– Exercise Session

SummaryDeployment EnvironmentSome recap on Java Servlets Web Applications MVC2 StrutsExercises

Deployment Environment

Required Tools

Java JDK 1.5 ~ 1.6Eclipse IDE for Java EE Developers http://www.eclipse.org/downloads/

Tomcat 5.5 ~ 6.0.X http://tomcat.apache.org/download-55.cgi

Struts 1.3 http://struts.apache.org/download.cgi#struts1310

A database (e.g., Postgres) - optional

Recap

Servlet: analogous to CGI in the Java world

Servlet container: a Java program providing an execution environment for servlets It provides Web server features as Java objects

HTTPRequest

HTTPResponse

Browser

Servlet container

Applications(servlets)

JVMparametri

risposta

Java Servlet

A Web application is a collection of servlets, documents and resources

A Web application is sub-folder of the ervlet container’s webapps folder

The name of the sub-folder is the name of the web applications http://localhost:8080/APP_NAME

The Web.xml file is a deployment descriptor, as it configures the container Web applications

It has a standard structure Locations: /web-app-name/WEB-INF/web.xml

Libraries are stored under the /WEB-INF/lib folderCompiled Java classes are gathered under the /WEB-INF/classes folder

All the resources/files contained in WEB-INF are not accessible from the Web browser

Web application

Role of the presentation tier in Web based architectures

In Web applications, the middle tier is the most complex software componentIncludes several functionalities Receives requests and send responses back Creates the user interface Implements the business logic and the interactions towards the data level

database

latigid

SQL request

SQL response

HTTP request

Page templates

Business logic

MVC and Web architectures

In its classical version, MVC assumes The possibility of keeping track of the state of the application in the model

The possibility to refresh the user interface when a change to the model state is notified

Conversely, the HTTP protocol is stateless does not enable to automatically update a page displayed by the browser

MVC needs to be adapted to the three-tier architecture employed for Web based applications MVC 2

MVC2 for the Web based on Java

MVC components are implemented exploting the Web architecture and Java J2EE API

The client is a browser, requests and responses are HTTP

The controller is a servlet, that processes all the requests

The model is a set of Java classes (action and state objects)

The view is a JSP page template

View(page template)

responseHTTP

requestHTTP

webserver

Servlet container

Controller(servlet)

Model

View(page template)

Actions

State objects

Client(Browser)

invokes

createsinvoca

reads

Struts 1 architecture

Struts is an sophisticated implementation of the controllerIt is independent from the state implementation (state objects) and the page templatesThe link between the controller and the model is limited to specific Java classes called action classes

View(page template)

Servlet container

Struts controller

Model

View(page template)

Actionclasses

State objects

perform()

creates

redirect/forward

reads

Action servlet

Requestprocessor

struts-config.xml process()

Controller: componentsAction servlet (provided by the framework): Base class: org.apache.struts.action.ActionServlet Implements the main functionalities of the controller

Struts-config.xml (provided by the developer) XML configuration file of the controller:

Describes what action class shall be invoked for each request type

Specifies what view shall be invoked after an action is executed

Separates the control logic from the controller source code

Enables to modify the control flow by changing the configuration file

Request Processor (provided by the framework): Available in Struts 1.1+. It is a helper class that provides methods needed by the ActionServlet

Action classesInterface class between the controller and the modelIn some cases considered as part of the controller, in others part of the modelGoals: Hide details about the instantiation and invocation of business services

Provide to the controller (actionServlet) a unique interface to be invoked (method execute() in Struts 1.1+, perform() in Struts 1.0)

Receive the execution result from the business service and communicate it to the controller in a standard way

Communication controller-AC

The controller invokes a unique method:public ActionForward execute(ActionMapping mapping, ActionForm

form,HttpServletRequest request, HttpServletResponse response) throws Exception

An ActionClass, when invoked, receives: The pair HTTP request/response HTTP (AC does not use the response !!)

An ActionForm object that contains the user input (if existing) provided by means of a form

An ActionMapping that enables communicating to the Controller the result of the ActionClass execution

An ActionClass returns an ActionForward object that tells the Controller what to do

Controller configuration

The controller makes two decisions: What action to be invoked for a given user request

What view to be invoked for a given action termination

The controller decides what to do based on mappings specified in the struts-config.xml fileHTTP requests must be directed to the ActionServlet, defining in the web.xml file the correct servlet mapping

ModelThe Model contains business services needed to execute the applicationStruts is completely independent from the implementation of such servicesModel objects perform two functionalities Represent the application state (state objects)

Implement the application logic (business objects)

State objects

The application state can be represente by means of standard interface objects:

JavaBeans Enterprise JavaBeans

State can be volatile (i.e. Shopping cart) or durable (i.e.: received orders)Durability of the state requires a data-tier including one or more databasesDatabase access can be implemented with different Java2EE technologies:

JDBC Entity Enterprise Java Beans Java Data Objects

View

Typically the View is composed by one or more JSP page templates that produce the pages displayed by the client browserPage templates publish the content of the model state objectsThe best practice to organize page templates is to use tag librariesMost used tag libraries

Struts Tag Library Java Standard Tag Library (JSTL)

Whenever possible, it is better to use Java standard library: JSTL

Typical components of a tag library

JSP tag libraries contain four types of elements: Bean/Hierarchy tag: to access the content of JavaBeans or JavaBean hierarchies (i.e.: lists of lists)

HTML tag: to increase the flexibility of HTML tags (i.e.: forms with memory and error checking)

Logic tag: to avoid the use of scripting to express conditions (=, !=, strings comparison, ..) and iterations

Template tag: to dynamically fetch blocks of content and include them in the page

User’s input management

Issue: in HTML/HTTP there is an element dedicated to data entry (form), but:

User’s input is included in the HTTP request and must be extracted and processed by means of ad hoc code

There is no way to express input validation in a standard way and enforce error checking

There is no easy way to recall previous input when a form is refreshed (i.e. back button)

Struts enables an advanced input management that eases the creation of applications

Struts input management principles

Struts offers two functions to handle user input

Objectification services: data inserted by the user are used to populate a Java object (called FormBean)

Validation services: there is a framework to organize functions devoted to checking the correctness of input values and handle errors

NOTE: In general, in MVC2 validation can be performed in three ways, not mutually exclusive: At the client side (e.g., using

JavaScript/FLASH/RIA…) Using specific action classes (FormAction) Using business services

Exercises