Middleware

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Middleware

3/29/2001 Kang, Seungwoo Lee, Jinwon
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Description of Topics

• 1. CGI, Servlets, JSPs
• 2. Sessions/Cookies
• 3. Database Connection(JDBC, Connection pooling)

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The Static Web
HTTP Server
Browser
HTTP Request
HTML
Operating System
Operating System
HTML Files
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The Dynamic Web
HTTP Server
Browser
HTTP Request
Servlet Engine
HTML
JVM
Operating System
Operating System
.java files
Data Files
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The Dynamic Web

• To evolve from the static to the dynamic web
  requires
• Generating HTML with scripts/programs
• CGI, Servlet
• Tools techniques to make it manageable
• JSP
• Accessing and Updating data
• JDBC

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CGI

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CGI

• CGI is not a language. It's a simple protocol
  that can be used to communicate between Web
  server and CGI program.
• A CGI script can be written in any language
• (Virtually any Programming language C, Perl,
  shell script)

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CGI

• include ltstdio.hgt
• void main()
• / Print the CGI response header, required for
  all HTML output. /
• printf(Content-typetext/html\n\n)
• / Print the HTML response page to STDOUT. /
• printf(lthtmlgtltheadgtlttitlegtHello,CGIlt/titlegtlt/head
  gt\n)
• printf(ltbodygtlth1gtHello CGIlt/h1gtlt/bodygtlt/htmlgt)

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CGI
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CGI Pros and Cons

• Pros
• Simplicity
• Cons
• Performance problem
• Security problem

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Servlet

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Servlet Basics

• What is Servlet?
• A program written in Java that runs on the
  server, as opposed to the browser (applets).
• Invoked with servlet keyword in URL
• E.g. http//domain-name/servlet/MyServlet

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Servlets vs. CGI

• Servlets have several advantages over CGI
• A Servlet does not run in a separate process.
• A Servlet stays in memory between requests. A CGI
  program needs to be loaded and started for each
  CGI request.
• There is only a single instance which answers all
  requests concurrently. This saves memory
• A Servlet can be run in a Sandbox, which allows
  secure use of untrusted and potentially harmful
  Servlets.

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Servlet Container (engine)

• Take and convert clients requests to an object
  understood by a servlet
• Provide an object to be used for response
• Manage the servlet lifecycle
• Make a servlet instance
• Call the instances init()
• Call the instances service() upon request
• Call the instances destroy() before destroying
  the instance
• Mark the instance for garbage collection

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• Add-on Servlet Engines
• Plug into popular web servers
• Stand-alone Servlet Engines
• Supports Web Server function
• JSDK(Java Servlet Development Kit)2.1
• Apache JServ 1.1/Tomcat 3.2.1

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Servlet Flow
http//nclab.kaist.ac.kr/servlet/MyServlet
response (HTML page)
MyServlet
HTTP request
Servlet engine
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Servlet Flow

• A client Web browser submits HTTP request
  specifying servlet, e.g., http//nclab.kaist.ac.kr
  /servlet/MyServlet
• The HTTP server receives the request from the
  client. The server parses the request and
  forwards it to the servlet.

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Servlet Flow

• An instance of the servlet is created
• (or activated) to process the request.
• The servlet processes any form input data
  generates an HTML response.
• This response is relayed back through the HTTP
  server to the client browser, which displays the
  page.

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Temporary Servlets

• loaded on demand
• offer a good way to conserve resources in the
  server for less-used functions.

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Permanent Servlets

• Loaded when the server is started, and live until
  the server is shutdown
• Servlets are installed as permanent extensions
  to a server when
• their start-up costs are very high (such as
  establishing a connection with a DBMS),
• they offer permanent server-side functionality
  (such as an RMI service)
• they must respond as fast as possible to client
  requests.

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Key Servlet Packages

• Java Servlet API
• Defines the interface between servlets and
  servers
• javax.servlet
• javax.servlet.http

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javax.servlet.Servlet Interface

• init(ServletConfig)
• Initializes the servlet.
• service(ServletRequest, ServletResponse)
• Carries out a single request from the client.
• destroy()
• Called by the servlet container to indicate to a
  servlet that the servlet is being taken out of
  service.
• allow servlet to clean up any resources (such as
  open files or database connections) before the
  servlet is unloaded

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• getServletConfig()
• Returns a ServletConfig object, which contains
  any initialization parameters and startup
  configuration for this servlet.
• getServletInfo()
• Returns a string containing information about the
  servlet

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HTTP Support Classes

• javax.servlet.http.HttpServlet provides an
  implementation of the javax.servlet.Servlet
  interface
• The easiest way to write an HTTP servlet is to
  extend HttpServlet and add your own processing.
• The class HttpServlet provides a service() method
  that dispatches the HTTP messages to one of
  several methods
• doGet(), doPost(), doHead(), doDelete(), doPut()
  
• corresponding directly with the HTTP protocol
  methods.

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• A servlet's doGet() method should
• Read request data, such as input parameters
• http//localhost/servlets/LoanCalculator?principal
  200interest0.05
• Set response headers (length, type, and encoding)
  
• Write the response data

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HelloServlet Example

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class HelloServlet extends HttpServlet
• public void doGet (HttpServletRequest request,
  HttpServletResponse response) throws
  ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• out.println("lthtmlgt")
• out.println("ltbodygtltheadgtlttitlegtHello
  ???lt/titlegtlt/headgt")
• out.println("lth1gt??? ??? ???. lt/h1gt")
• out.println("lt/bodygtlt/htmlgt")

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Servlet - features

• Platform-independent
• Performance
• Easily extended due to class/package/bean
  concepts in Java

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JSP
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JSP

• Why do we need JSP technology if we already have
  servlets?

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JSP

• Combine static HTML and dynamic Java in one
  programming concept
• Simplified, fast creation of web pages that
  display dynamically-generated content.
• Separation of web presentation from web content
• Microsofts similar concept
• Active Server Pages

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JSP

• JSP code is compiled into Java servlet class,
  then executed
• Remains in memory
• Only when any change in source file, repeat the
  first step

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JSP tags

• Comment
• lt!-- comment --gt HTML comment
• lt-- comment --gt Hidden comment
• Declaration
• lt! declaration gt
• Expression
• lt expression gt

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JSP tags

• Scriptlet
• lt code fragment gt
• Include Directive
• lt_at_ include filerelative URL gt
• Page Directive
• lt_at_ page languagejava gt
• lt_at_ page importpackage. gt
• lt_at_ page contentTypetext/html gt
• lt_at_ sessiontruefalse gt

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Simple JSP Code

• lt for (int i0 i lt 5 i)
• out.println("ltligt" i)
• gt
• lt/ulgt
• lt str gt
• lt/centergt
• lt/bodygt
• lt/htmlgt

• lthtmlgt
• ltheadgt
• lttitlegt ??? jsp ??? lt/titlegt
• lt/headgt
• ltbody bgcolorivorygt
• ltcentergtlth1gtfirst jsp pagelt/h1gt
• lt! String strNC Lab." gt
• ltulgt

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More informations

• http//developer.java.sun.com/developer/onlineTrai
  ning/Servlets/Fundamentals/servlets.html
• http//java.sun.com/products/servlet/2.3/javadoc
• Servlet packages
• http//java.sun.com/j2ee/docs.html
• J2EE Documents
• http//java.sun.com/products/jdk/1.2/docs/api
• J2SE v1.2.2 API Specification
• http//www.javanuri.com
• http//www.javastudy.co.kr