1 Chapter 2 The Web Tier  Web Applications and Web ContainersWeb Applications and Web Containers  Dynamic Content CreationDynamic Content Creation  Application DesignsApplication Designs  Servlets TechnologyServlets Technology  JSP TechnologyJSP Technology

2 Web Applications and Web Containers  Web application A collection of HTML/XML documents, Web components and other resources Provides dynamic and interactive content to browser-based clients.  Web container A runtime environment for a Web application Provides Web components with a naming context and life cycle management.

3 Dynamic Content Creation  Common Gateway Interface (CGI) scripts  Java servlets technology  Java Server Pages technology

4 Servlets  Portable platform- and Web server-independent  Extensible  Perform better than CGI scripts

5 JavaServer Pages Technology  Providing a declarative, presentation-centric method  Content and display logic are separated  Reusing code through a component-based architecture

6 Application Designs

7 Applications with Basic JSP Pages and Servlets

8 JSP pages with JavaBeans components

9 EJB-Centric Applications  Model - Represents the data on which an application is based  View - Presents the data represented by the model in a way that's targeted at a specific type of client  Controller -A Central point of control. Maintains the data in the model and ensures data and view consistency.

10 Controller Conversion of HTTP Request to Model Change Event

11 Servlet Technology What Is a Servlet? Servlet API and Servlet life cycle HTTP Support Security Issues

12 What Is a Servlet? A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed via a request- response programming model.

13  A servlet is a Java component that can be plugged into a Java-enabled web server to provide custom services: New features Runtime changes to content Runtime changes to presentation New standard protocols (such as FTP) New custom protocols  Servlets are designed to work within a request/response processing model. What Is a Servlet?

14 Servlet API Defining the interface between servlets and servers: Package javax.servlet Package javax.servlet.http

15 Servlet API Functions  Servlet life cycle management  Access to servlet context  Utility classes  HTTP-specific support classes

16 Servlet Life Cycle  three main methods: init() service() destroy()  two ancillary methods: getServletConfig() getServletInfo()

17 Servlet life cycle  If an instance of the servlet does not exist, the Web container Loads the servlet class. Creates an instance of the servlet class. Initializes the servlet instance by calling the init method. Invokes the service method, passing a request and response object.  If the container needs to remove the servlet, it finalizes the servlet by calling the servlet's destroy method.

18 Writing Service Methods The general pattern for a service method is to extract information from the request, access external resources, and then populate the response based on that information. Getting Information from Requests Constructing Responses

19 Getting Information from Requests All requests implement the ServletRequest interface. This interface defines methods for accessing parameters, etal. An input stream can be retrieved from the request for manually parsing the data: –getInputStream( ) –getReader( ) HTTP servlets are passed an HTTP request object which contains the request URL, HTTP headers, query string, and so on.

20 Constructing Responses All responses implement the ServletResponse interface: Retrieve an output stream to use to send data to the client. –getWriter(), return PrintWriter –getOutputStream( ), return ServletOutputStream Indicate the content type (for example, text/html), being returned by the response. Indicate whether to buffer output Set localization information.

21 Sample Servlet-1 import java.io.*; import javax.servlet.*; public class SampleServlet implements Servlet { private ServletConfig config; public void init (ServletConfig config) throws ServletException { this.config = config; } public void destroy() {} // do nothing public ServletConfig getServletConfig() { return config; } public String getServletInfo() { return "A Simple Servlet"; }

22 public void service (ServletRequest req, ServletResponse res ) throws ServletException, IOException { res.setContentType( "text/html" ); PrintWriter out = res.getWriter(); out.println( " " ); out.println( " ); out.println( " A Sample Servlet " ); out.println( " " ); out.println( " A Sample Servlet " ); out.println( " " ); out.close(); } Sample Servlet-2

23 Servlet Context  Servlet Initialization Information  Server Context Information  Servlet Context During a Service Request

24 Utility Classes  Interface javax.servlet.SingleThreadModel for writing simple servlets  Two exception classes javax.servlet.ServletException javax.servlet.UnavailableException

25 HTTP Protocal HTTP defines a set of text-based request messages called HTTP methods. The HTTP methods include: –GET : Retrieves the resource identified by the request URL –HEAD : Returns the headers identified by the request URL –POST : Sends data of unlimited length to the Web server –PUT : Stores a resource under the request URL –DELETE : Removes the resource identified by the request URL –OPTIONS : Returns the HTTP methods the server supports –TRACE : Returns the header fields sent with the TRACE request

26 HTTP Support Javax.servlet.http.HttpServlet implements the javax.servlet.Servlet interface. HttpServlet methods: –Service(): Dispatches the HTTP message to one of severial special methods. –Others:Called by the server (via the service method) to allow a servlet to handle a GET, Post … request. doGet(): doDelete() doOptions() doPost() doTrace() To write an HTTP servlet, you can extend HttpServlet and add your own custom processing.

27 HTTP Support

28 Security Issues  The Servlet Sandbox  Access Control Lists (ACLs)

29 JSP Technology  IntroductionIntroduction  JSP ArchitectureJSP Architecture  JSP Access ModelsJSP Access Models  JSP Syntax BasicsJSP Syntax Basics

30 What Is a JSP Page? A JSP page is a text-based document that contains two types of text: static template data, which can be expressed in any text-based format, such as HTML, WML, and XML; and JSP elements, which construct dynamic content.

31 Components of JSP pages  Static HTML/XML components.  Special JSP tags  Snippets of code written in the Java programming language called "scriptlets."

32 JSP Advantages  Separation of static from dynamic content  Write Once Run Anywhere  Dynamic content can be served in a variety of formats  Recommended Web access layer for n-tier architecture  Completely leverages the Servlet API

33 JSP Architecture

34 JSP Architecture JSP pages are subject to a translation phase and a request processing phase.

35 …... public class _0005cjsp_0005cjsptest_0002ejspjsptest_jsp_0 extends HttpJspBase { … … public void _jspService(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { JspFactory _jspxFactory = null; … Date d = new Date(); String today = DateFormat.getDateInstance().format(d); // end // begin out.write("\r\nToday is: \r\n ");…... Translation

36 The Life Cycle of a JSP Page

37 JSP Access Models -1

38 JSP Access Models-2

39 JSP Syntax Basics  Directives  scripting elements  standard actions

40 Directives  tag.  two primary directives page include

41 scripting elements  Declarations, tag.  Expressions, tag  Scriptlets, tag  Comments, tag

42 Standard Actions  Using JavaBean Components <jsp:useBean id="user" class="com.jguru.Person" scope="session" />  Forwarding Requests  Including Requests

43 Java Bean JAVA BEANS components are Java classes that can be easily reused and composed together into applications. Any Java class that follows certain design conventions can be a JavaBeans component.

44 Java Bean Design Pattern The bean encapsulates its properties by declaring them private; –Read/write, read-only, or write-only –Simple, which means it contains a single value, or indexed, which means it represents an array of values The bean provides public accessor (getter/setter) methods for reading and modifying properties’ values. – For each readable property, the bean must have a method of the form PropertyClass getProperty() {... } – For each writable property, the bean must have a method of the form setProperty(PropertyClass pc) {... }

45 Java Bean Design Pattern In addition to the property methods, a JavaBeans component must define a constructor that takes no parameters.

46 An Example of Java Bean An Example of Java Bean public class Currency { private Locale locale; private double amount; public Currency() { locale = null; amount = 0.0; } public void setLocale(Locale l) { locale = l; } public void setAmount(double a) { amount = a; } public String getFormat() { NumberFormat nf = NumberFormat.getCurrencyInstance(locale); return nf.format(amount); }

47 Object Scope

48 Accessing Objects from a JSP Page

49 Servlets and JSP Pages  Can be used to solve identical problems.  Servlets are expressed in Java ; JSP pages are text-based documents  A servlet uses print statements to post HTML data Cannot preview the look and feel of an HTML page Difficult to maitain when data or its display format needs changes

50  JSP can use graphical development tools, use JavaBeans components and/or custom tags that handle complex dynamic formatting of HTML  Like servlets, JSP can work in a portable platform- or application-independent means and supports a reusable component model  A sophisticated Web application can consist solely of JSP pages, custom tags, and JavaBeans components  servlets are rarely necessary. Servlets and JSP Pages

51 When Servlet More Appropriate  Generating Binary Data  Extending a Web Server's Functionality  Best suited for low-level application functions not frequently modified  Communicating with Applets or applications