Web Programming with Java pptx

1
Web Programming with Java
Servlets
Huynh Huu Viet
University of Information Technology
Department of Information Systems
Email:
2008 © Department of Information Systems - University of Information Technology
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Outline
Introduction
Overview of Servlet technology
Servlet basics
The Servlet Lifecycle
Retrieving and Sending HTML
Servlet Sessions
2008 © Department of Information Systems - University of Information Technology
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Introduction
Java networking capabilities:
 Socket-based and packet-based
communications
• Package java.net
 Remote Method Invocation (RMI)
• Package java.rmi
 Servlets and JavaServer Pages (JSP)
• Request-response model
• Packages javax.servlet
– javax.servlet.http
– javax.servlet.jsp
 the Web tier of J2EE

2008 © Department of Information Systems - University of Information Technology
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Introduction (cont.)
Servlets
 Thin clients
 Request/response mechanism
 Session-tracking capabilities
 redirection/forwarding control
Servlets: small Java programs that
run on a web server
 Just as applets run on browser
Provide web-based applications
Extends functionality of web server
2008 © Department of Information Systems - University of Information Technology
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Outline
Introduction
Overview of Servlet technology
Servlet basics
The Servlet Lifecycle
Retrieving and Sending HTML
Servlet Sessions
2008 © Department of Information Systems - University of Information Technology
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A Servlet's Job
 Read explicit data sent by client (form
data)
 Read implicit data sent by client (request
 headers)
 Generate the results

 Send the explicit data back to client
(HTML or binary)
 Send the implicit data back to client
(status codes and response headers)
2008 © Department of Information Systems - University of Information Technology
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Why Build Web Pages Dynamically?
The web page is based on data given
by the user
 results from search engines and order
confirmation pages at on-line stores
The web page is derived from data
that changes frequently
 weather report or news headlines page
The web page uses information from
databases or other server-side
sources
 e-commerce site can list current price and
availability
2008 © Department of Information Systems - University of Information Technology
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Outline
Introduction
Overview of Servlet technology
Servlet basics
The Servlet Lifecycle
Retrieving and Sending HTML
Servlet Sessions
2008 © Department of Information Systems - University of Information Technology
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Servlet Basics
Servlet container (or Servlet engine)
 Server that executes a Servlet
Web servers and application servers
 Netscape iPlanet Application Server
 Microsoft’s Internet Information Server (IIS)
 Apache HTTP Server
 BEA’s WebLogic Application Server
 IBM’s WebSphere Application Server
 JBoss Application Server
2008 © Department of Information Systems - University of Information Technology
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Servlet Architecture
2008 © Department of Information Systems - University of Information Technology
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Creating a Servlet
 Need to get Servlet libraries on CLASSPATH
 Part of J2EE application server
 Also come with Tomcat reference implementation
 For a general Servlet you must subclass
javax.servlet.GenericServlet
 Seldom do this
 To process HTTP requests you subclass
javax.servlet.http.HttpServlet
 Override doGet() or doPost() to handle GET and
 POST requests from browser
2008 © Department of Information Systems - University of Information Technology
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Processing Requests
 Typical steps:

 Read values from request
• HttpServletRequest req
 Process and log as required
 Write response to client
• HttpServletResponse res
 HttpServletResponse has getWriter()
method
 Returns a PrintWriter to write data to client
 Data starts with content type
 Use setContentType() before writing any data
 E.g. res.setContentType("text/html");
2008 © Department of Information Systems - University of Information Technology
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Example
2008 © Department of Information Systems - University of Information Technology
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Outline
Introduction
Overview of Servlet technology
Servlet basics
The Servlet Lifecycle
Retrieving and Sending HTML
Servlet Sessions
2008 © Department of Information Systems - University of Information Technology
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The Servlet Lifecycle
 Init
 Executed once when the servlet is first loaded.
 Not called for each request.
 Service

 Called in a new thread by server for each request.
 Dispatches to doGet, doPost, etc.
 Do not override this method!
 doGet, doPost, doXxx
 Handles GET, POST, etc. requests.
 Override these to provide desired behavior.
 Destroy
 Called when server deletes servlet instance.
 Not called after each request.
2008 © Department of Information Systems - University of Information Technology
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Why You Should Not Override service
The service method does other things
besides just calling doGet
 You can add support for other services
later by adding doPut, doTrace, etc.
 You can add support for modification
dates by adding a getLastModified method
 The service method gives you automatic
support for:
• HEAD requests
• OPTIONS requests
• TRACE requests
 Alternative: have doPost call doGet
2008 © Department of Information Systems - University of Information Technology
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Outline
 Introduction
 Overview of Servlet technology
 Servlet basics

 The Servlet Lifecycle
 Retrieving and Sending HTML
 Form Data
 Request Headers
 Status Codes
 Response Headers
 Servlet Sessions
2008 © Department of Information Systems - University of Information Technology
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Creating Form Data: HTML Forms
<HTML>
<HEAD><TITLE>A Sample Form Using GET</TITLE></HEAD>
<BODY BGCOLOR="#FDF5E6">
<H2 ALIGN="CENTER">A Sample Form Using GET</H2>
You normally use a relative URL for the ACTION. This URL is just for testing because
<FORM ACTION="SomeProgram">
<CENTER>
First name:
I am running a test server that echoes the data it receives.
<INPUT TYPE="TEXT" NAME="firstName" VALUE="J. Random"><BR>
Last name:
<INPUT TYPE="TEXT" NAME="lastName" VALUE="Hacker"><P>
<INPUT TYPE="SUBMIT"> <! Press this to submit form >
</CENTER>
</FORM>
</BODY>
</HTML>
2008 © Department of Information Systems - University of Information Technology
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Reading Parameters

out.println(
"<HTML>\n" +
"<HEAD><TITLE>"+title + "</TITLE></HEAD>\n" +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<H1 ALIGN=\"CENTER\">" + title + "</H1>\n" +
"<UL>\n" +
" <LI><B>param1</B>: "
+ request.getParameter("firstName") + "\n" +
" <LI><B>param2</B>: "
+ request.getParameter(“lastName") + "\n" +
" <LI><B>param3</B>: "
+ request.getParameter("param3") + "\n" +
"</UL>\n" +
"</BODY></HTML>");
2008 © Department of Information Systems - University of Information Technology
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Reading Form Data
 request.getParameter("name")
 Returns URL-decoded value of first occurrence of
name in query string
 Works identically for GET and POST requests
 Returns null if no such parameter is in query data
 request.getParameterValues("name")
 Returns an array of the URL-decoded values of all
occurrences of name in query string
 Returns a one-element array if param not repeated
 Returns null if no such parameter is in query
 request.getParameterNames() or
request.getParameterMap()
 Returns Enumeration or Map of request params

 Usually reserved for debugging
2008 © Department of Information Systems - University of Information Technology
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Reading All Parameters
Enumeration paramNames = request.getParameterNames();
while(paramNames.hasMoreElements()) {
String paramName = (String)paramNames.nextElement();
out.print("<TR><TD>" + paramName + "\n<TD>");
String[] paramValues =request.getParameterValues(paramName);
if (paramValues.length == 1) {
String paramValue = paramValues[0];
if (paramValue.length() == 0)
out.println("<I>No Value</I>");
else
out.println(paramValue);
} else {
out.println("<UL>");
for(int i=0; i<paramValues.length; i++) {
out.println("<LI>" + paramValues[i]);
}
out.println("</UL>");
}
}
2008 © Department of Information Systems - University of Information Technology
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Checking for Missing and Malformed Data
 Missing
 Field missing in form
• getParameter returns null
 Field blank when form submitted

• getParameter returns an empty string (or possibly a
string with whitespace in it)
 Must check for null before checking for empty string
• String param = request.getParameter("someName");
• if ((param == null) || (param.trim().equals(""))) {
– doSomethingForMissingValues( );
•} else {
– doSomethingWithParameter(param);
•}
 Malformed
• Value is a nonempty string in the wrong format
2008 © Department of Information Systems - University of Information Technology
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Handling Missing and Malformed Data
 Use default values
 Replace missing values with application-specific
standard values
 Redisplay the form
 Show the form again, with missing values flagged
 Previously-entered values should be preserved
 Four options to implement this
• Have the same servlet present the form, process the data,
and present the results.
• Have one servlet present the form; have a second servlet
process the data and present the results.
• Have a JSP page “manually” present the form; have a servlet
or JSP page process the data and present the results.
• Have a JSP page present the form, automatically filling in the
fields with values obtained from a data object. Have a servlet
or JSP page process the data and present the results

2008 © Department of Information Systems - University of Information Technology
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Outline
 Introduction
 Overview of Servlet technology
 Servlet basics
 The Servlet Lifecycle
 Retrieving and Sending HTML
 Form Data
 Request Headers
 Status Codes
 Response Headers
 Servlet Sessions
2008 © Department of Information Systems - University of Information Technology
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A Typical HTTP Request
 GET /servlet/Search?keywords=servlets+jsp
HTTP/1.1
 Accept: image/gif, image/jpg, */*
 Accept-Encoding: gzip
 Connection: Keep-Alive
 Cookie: userID=id456578
 Host: www.somebookstore.com
 Referer: /> User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)