java programming: advanced topics 1 building web applications chapter 13

Java Programming: Advanced Topics

1

Building Web Applications

Chapter 13


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Objectives• Review the way the Web works and the

non-Java technology that participates in Web applications

• Learn how J2EE packages Web applications

• Program dynamic Web content in servlets• Become familiar with the Servlet API• Provide continuity as the user navigates

through your Web application


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Objectives (Cont.)• Learn how to generate dynamic Web

content in JavaServer Pages• Design Web applications based on

servlets and JavaServer Pages• Use the JavaServer Page tags and

Servlet API• Apply design patterns and frameworks to

Web applications• Discuss design issues related to Web

applications


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The Technology of the Web• Web apps are composed of Web and Java

technologies• A Web app requires a Web server and an

application server on the server side and a Web browser on the client side.

• Servlets and JavaServer Pages (JSPs) are the components that form the bridge between Web pages and Java application code


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Web Relationships


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The HTTP and HTTPS Protocols• The browser communicates with a Web server

HyperText Transfer Protocol (HTTP) or HyperText Transfer Protocol Secure (HTTPS)

• In a Web application, the content of HTML or XHTML pages can be dynamically created at runtime by Java components that run on the server side

• A Web container provides the context within which servlets and JSPs run


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HTTP Request Methods


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J2EE Web Application Packaging• To be deployed on an application server, a J2EE

Web app must be packaged in a Web archive (war file) that packaged in an enterprise application archive (ear file)

• The ear and war files contain deployment descriptors– Deployment descriptors: XML files that describe your

Web app for the application server and request run-time services from the application server


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Servlets• A servlet is a server-side Java program that runs

in response to an HTTP request• The role of a servlet is to accept requests from

the client, invoke the appropriate application logic to fulfill the request, and return the results to the client

• Each servlet is an entry point into a Web app or an enterprise application running a J2EE-compliant application server


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How a Web App Processes HTTP Requests


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Named and Anonymous Servlets• Named servlet: when a servlet is assigned a

name in the deployment descriptor• Anonymous servlet: a servlet not listed in the

Web deployment descriptor • To protect your servlets from unauthorized use,

make all servlets named servlets, disable anonymous servlets, and apply security settings to your Web app


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The Servlet API

• The Servlet API is included in two packages:– javax.servlet– javax.servlet.http

• Servlet classes extend javax.servlet.http.HttpServlet


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Common Types in the Servlet API


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Common Types in the Servlet API (Cont.)


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A Simple Servlet


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The User Experience: Building a Web App with Continuity

• The HTTP protocol is stateless, so every request and response is a complete, independent transaction

• The Servlet API provides a simple mechanism to save information on the server side or on the client side to provide some continuity between browser sessions


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Storing Data in HTTP Sessions • A session is a place to store state information

for a specific client, so that the information is available to different servlets

• You can access or create a session for a client by calling the HttpServletRequest.getSession method

• An HttpSession object contains a collection of name-value pairs


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Places to Store State Data on the Server Side

• A servlet can store data in and retrieve it from four different scopes:– Data stored in the session scope is specific to one

client.– Data stored in the application scope is global to the

Web app.– The lifetime of data stored in the request scope is the

duration of one HTTP request-response cycle.– Data stored in the page scope is accessible only in

the current page


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Providing Continuity with Cookies• You can also use Cookie objects to store data

on the client side from one browser session to the next

• Cookies: data objects stored on the client side of a Web app and passed back and forth between the Web browser and the server

• URL rewriting appends the cookie data to the URL


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JavaServer Pages• A JSP is Web resource with embedded

Java code• A JSP takes the form of an HML or

XHMTL document that includes ordinary HTML tags and some additional JSP-specific tags

• The application server converts the entire page to HTML and resolves all dynamic content before sending the page to the client browser


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Implementing MVC with Servlets and JSPs

• The MVC design pattern:– The view layer consists of static HTML pages and

JSP documents– The controller layer consists of servlets that receive

HTTP requests– The model layer consists of classes and other

components that perform the core functionality of the application


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Implementing MVC with Servlets and JSPs


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JSP Tags and API • JSPs that conform to HTML or XHTML syntax

have four types of tags: – Java directives are used to import packages and set

other parameters of the page as a whole– Declarations define methods and variables used by

Java code in the JSP– Java expressions are evaluated, and its string

representations of the value are inserted into the resulting HTML

– Scriptlets contain one or more complete Java statements or blocks of code


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HTML-Based JSP Tags


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HTML-Based JSP Tags (Cont.)


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A Simple JSP


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An XML JSP


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How the Server Processes JSPs • The Web container converts a JSP into a servlet

using the page compile process• The Web container generates a service method

from the body of the JSP source • By default, a JSP is compiled when it is first

called• If you change the .jsp file between calls, the

server reads the source page and compiles, loads, and runs it again


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Java Coding in JSPs • In a servlet, the doGet, doPost, and service

methods receive the HTTP request and response objects as arguments and access the servlet context by calling methods on those arguments

• The JSP can call the getAttribute method on the variables application, session, or request to access objects stored there


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Implicit Objects in JSPs


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How to Retrieve Data


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Custom Tags in JSPs

• Custom tags can help remove all Java code from JSPs

• Custom tags can have attributes • Custom tags are grouped into libraries, and

each library has a unique prefix• For every tag library, there is a descriptor file

identified by the uri attribute of the taglib directive


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JSP Tags for JavaBeans


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JSP Tags for JavaBeans (Cont.)


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Frameworks for Building Web Applications

• Frameworks are productivity aids for creating resources that are common requirements in Web apps

• Frameworks can incorporate best practices for Web app design so that the Web apps they produce are extensible, easy to maintain, and lend themselves to being made secure


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Building Robust Web Apps• Servlets should be designed for multithreaded

use• Turn off caching for all pages and responses

that include dynamic content• Build precondition checks for servlets or JSPs• Use JavaScript to disable the submit button after

the first click or program the servlet to record the start and stop of processing as session state data


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Summary• In a Web application, the content of HTML or

XHTML pages can be dynamically created at runtime by Java components that run on the server side

• A Web container provides the context within which servlets and JSPs run

• A J2EE Web app must be packaged in a Web archive (war file)

• A servlet is a server-side Java program that runs in response to an HTTP request

• The Servlet API is included in javax.servlet and javax.servlet.http packages


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Summary (Cont.)• HTTP requests and responses are stateless• You can use Cookie objects to store data on the

client or session objects on the server• A servlet can store data in and retrieve it from

global, session, request, and page scopes• JSPs are HTML, XHTML, or XML documents

that contain snippets of Java code• JSP use directives, declarations, expressions,

scriptlets, and custom tags• For Web apps use the MVC design pattern

java programming: advanced topics 1 building web applications chapter 13

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