Transcript Progettazione di soluzioni verticali per il Web

Advanced Web Technologies:
Struts– Exercise Session
Lecturer: Prof. Piero Fraternali, [email protected]
Teaching Assistant: Alessandro Bozzon, [email protected]
Summary
Deployment Environment
Some recap on
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Java Servlets
Web Applications
MVC2
Struts
Exercises
Deployment
Environment
Required Tools
Java JDK 1.5 ~ 1.6
Eclipse IDE for Java EE Developers
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http://www.eclipse.org/downloads/
Tomcat 5.5 ~ 6.0.X
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http://tomcat.apache.org/download-55.cgi
Struts 1.3
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http://struts.apache.org/download.cgi#struts1310
A database (e.g., Postgres) - optional
Recap
Java Servlet
HTTP
Request
Browser
HTTP
Response
JVM
parametri
Servlet
container
risposta
Applications
(servlets)
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
Web application
A Web application is a collection of servlets, documents and resources
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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
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http://localhost:8080/APP_NAME
The Web.xml file is a deployment descriptor, as it configures the
container Web applications
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It has a standard structure
Locations: /web-app-name/WEB-INF/web.xml
Libraries are stored under the /WEB-INF/lib folder
Compiled Java classes are gathered under the /WEBINF/classes folder
All the resources/files contained in WEB-INF are not accessible from
the Web browser
Role of the presentation tier in
Web based architectures
SQL request
d i gi t a l
HTTP request
database
SQL response
Page
templates
Business
logic
In Web applications, the middle tier is the
most complex software component
Includes several functionalities
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Receives requests and send responses back
Creates the user interface
Implements the business logic and the interactions
towards the data level
MVC and Web architectures
In its classical version, MVC assumes
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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
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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
Servlet container
Controller
(servlet)
request
HTTP
Client
(Browser)
web
server
response
HTTP
Model
invokes
Actions
creates
invoca
View
View
(page
(page template)
template)
reads
State
objects
MVC components are implemented exploting the Web
architecture and Java J2EE API
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The
The
The
The
client is a browser, requests and responses are HTTP
controller is a servlet, that processes all the requests
model is a set of Java classes (action and state objects)
view is a JSP page template
Struts 1 architecture
Servlet container
Struts controller
Action
servlet
struts-config.xml
Model
perform()
Action
classes
Request
processor
process()
View
redirect/forward
(page
template)
View
reads
(page template)
creates
State
objects
Struts is an sophisticated implementation of the
controller
It is independent from the state implementation (state
objects) and the page templates
The link between the controller and the model is
limited to specific Java classes called action classes
Controller: components
Action servlet (provided by the framework):
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Base class: org.apache.struts.action.ActionServlet
Implements the main functionalities of the controller
Struts-config.xml (provided by the developer)
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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):
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Available in Struts 1.1+. It is a helper class that
provides methods needed by the ActionServlet
Action classes
Interface class between the controller and the
model
In some cases considered as part of the
controller, in others part of the model
Goals:
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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:
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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:
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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
file
HTTP requests must be directed to the
ActionServlet, defining in the web.xml file
the correct servlet mapping
Model
The Model contains business services needed
to execute the application
Struts is completely independent from the
implementation of such services
Model objects perform two functionalities
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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:
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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 databases
Database access can be implemented with different
Java2EE technologies:
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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 browser
Page templates publish the content of the model
state objects
The best practice to organize page templates is to
use tag libraries
Most used tag libraries
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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:
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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:
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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
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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
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Exercises