e-Science e-Business
e-Government and their
Technologies
Introduction to Web Applications
Bryan Carpenter, Geoffrey Fox, Marlon Pierce
Pervasive Technology Laboratories
Indiana University Bloomington IN 47404
January 12 2004
[email protected]
[email protected]
[email protected]
http://www.grid2004.org/spring2004
1
Contents of this Lecture Set





Introduction: Web applications and interfaces.
The Tomcat Web server.
Java Servlets and HTTP.
JavaServer Pages
Deploying a JSP-based Web application.
2
Web Applications

We consider a Web Application to be a set of related programs
and resources embedded in, or accessed through, a Web server.
• The whole application may be made available as a single archive file,
ready for immediate deployment in a Web server.

One can identify two broad kinds of Web application:
• Presentation oriented, built around dynamic HTML pages.
• Service oriented, in which a Web Service is made available to other
applications through XML and HTTP protocols.

In this section we will be most interested in presentation oriented
Web Applications.
• Web services will be discussed at length in later sections. Note they
commonly make use of some of the same technologies we discuss here (e.g.
Servlets).
3
Web Interfaces

As a matter of common experience, today’s Web is
much more than a collection of static HTML
documents.
• We are all accustomed to Web sites that react in complex
ways to requests and queries submitted through browsers.

Many technologies have been introduced to facilitate
this interactivity.

One basic idea has been around since the earliest
days of the Web, and has come through relatively
unscathed, is dynamic generation of HTML.
4
Requesting a Static HTML Document
Server
HTTP Request
GET /index.html HTTP/1.0
User-Agent: Mozilla/4.51 [en]...
Host: sirah.csit.fsu.edu:8080
Accept: image/gif, ..., */*
...
HTTP/1.1 200 OK
Content-type: text/html
<html>
<head>...</head>
<body>
...
<html>
<head>…</head>
<body>
…
</html>
Static HTML files
</html>
Client
HTTP Response
5
Dynamic Generation of HTML
Server
HTTP Request
GET /hello.pl?who=bryan HTTP/1.0
User-Agent: Mozilla/4.51 [en]...
Host: sirah.csit.fsu.edu:8080
Accept: image/gif, ..., */*
...
#!/usr/bin/perl
$name = param(“who”) ;
print “Content-type: text/html\n\n” ;
print “<html><body>Hello $name!</body></html>\n” ;
HTTP/1.1 200 OK
Content-type: text/html
<html>
<body>
Hello bryan!
</body></html>
Client
Script, or method
HTTP Response
6
Server-side Frameworks

First briefly describe four general frameworks for
dynamic generation of HTML:
•
•
•
•

1. CGI
2. Java Servlets
3. Microsoft Active Server Pages (ASP)
4. JavaServer Pages (JSP)
All these technologies require some browser-side
mechanisms to get responses from the client
• Typically HTML forms, and/or Javascript.
But use of HTTP and HTML means that the browserside mechanisms are quite well-separated from the
server-side issues.
7
1. CGI

The earliest technique for responding dynamically to
browser input was CGI: Common Gateway Interface.
• The FORM element of some HTML document contains
input fields.
• Inputted data is read by browser, forwarded to server in a
GET or POST request.
• The URL in the action attribute of the HTML form
identifies an executable file somewhere in the Web Server’s
document hierarchy.

On the server side, the Web server is configured to
execute this program when it receives the HTTP
request created by submitting the form.
8
Action of a CGI Script

The executable file may be written in any language.
Here we assume it is written in Perl.
• The Web Server program will invoke the CGI script, and
pass it the form data, either through environment variables
or by piping data to standard input of the script.
• In modern Perl you can use the CGI module to hide many
of these details—especially extracting form parameters.

The CGI script generates a response to the form, on
its standard output. This is piped to the Web server,
which returns it to the browser.
• The response will be the text of a dynamically generated
HTML document, preceded by some HTTP headers.
9
CGI example

The form element in the initial HTML document is as
follows:
<form action=“http://server-name/cgi-bin/hello.pl”>
Name: <input type=text name=who size=32> <p>
<input type=submit>
</form>

The CGI script hello.pl is:
#!/usr/bin/perl
use CGI qw( :standard) ;
$name = param(“who”) ;
print “Content-type: text/html\n\n” ;
print “<html><body><h1>Hello
$name!</h1></body></html>\n” ;
10
2. Java Servlets

In conventional CGI, a Web site developer writes an
executable program that processes a form’s input.
The program (or script) must be executed every time
a form is submitted.

Servlets provide a more modern, Java-centric
approach.

The server incorporates a Java Virtual Machine,
which is running continuously.

Execution of a CGI script is replaced invocation of a
method on a servlet object.
• This is typically much cheaper than starting a whole new
program.
11
A “Hello” Servlet
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class Hello extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType(“text/html”) ;
PrintWriter out = response.getWriter() ;
out.println(“<html><body>”) ;
out.println(“Hello ” + request.getParameter(“who”) + “!”) ;
out.println(“</html></body>”) ;
}
}
12
Remarks


HttpServlet is the base class for servlets running in
HTTP servers.
The URL in the action attribute of the form might be
something like:
http://server-name/examples/servlet/Hello



The doGet() method is called in response to an
HTTP GET request directed at the servlet.
As the names suggest, the request object describes
the browser’s request, and the response object
describes and the servlet’s response.
Servlet code is very flexible, but verbose.
13
3. Microsoft ASP



Microsoft independently addressed the performance
issues of CGI, through ISAPI (Internet Server
Application Programming Interface).
Comparable to Java Servlets; an HTTP request for a
dynamic page triggers a DLL call within the running
server process, rather than execution of a program.
One application of ISAPI that has become important
is the library ASP.DLL—Active Server Pages.
• An ASP page looks basically like an HTML document, but
includes inserts of CGI-like code.
• Effectively turns CGI “inside out”.
• Scripting language for the inserts typically Visual Basic,
though other languages are possible.
14
ASP Example
<%@ LANGUAGE=“VBSCRIPT” %>
<HTML>
<HEAD><TITLE>Sample ASP</TITLE></HEAD>
<BODY>
Welcome. It is now approximately <%= Time() %>.<BR>
Some simple text formatting using server-side code:
<%
For intCounter = 1 to 5
%>
<FONT SIZE = <%= intCounter %>> Hello </FONT><BR>
<%
Next
%>
</BODY>
</HTML>
15
Remarks

The basic structure of the document looks like a
static HTML documents, with some special inserts.
• It includes processing directives (e.g. LANGUAGE) in
<[email protected]%> brackets.
• Server-side VB inserts appear in <%…%> brackets.
• <%=expression%> is shorthand for:
Response.Write expression
comparable with out.println(expression) in Servlets.
• Response is one of a number of predefined objects
available in “scripting” inserts.

We will get very familiar with this syntax later,
because JavaServer Pages steals it, almost
verbatim!
16
4. JavaServer Pages

JavaServer Pages (JSP) allow special tags and Java code to be
embedded in HTML files. These tags and code are processed
by the Web server to dynamically produce standard HTML.
• Produce dynamic Web pages on the server side (like Servlets), but
separate application logic from the appearance of the page.
• The tags allow previously compiled Java components, in the form of
JavaBeans, to be used.
• One can also define custom tag libraries.
• May produce XML documents, instead of HTML.

JavaServer Pages were developed as a response to ASP.
• Built on top of Java Servlets, and use many of the same utility classes.
17
JSP elements

A JSP page looks like standard HTML or XML, with
extra elements processed by the JSP container.
• Typically, these elements create text that is inserted into the
resulting document.

JSP elements include:
• Scriptlet enclosed in <% and %> markers: a small script in
Java to perform arbitrary functions.
• Expression: anything between <%= and %> markers is
evaluated by the JSP engine as a Java expression.
• JSP directive enclosed in <%@ and %> markers—passes
information to the JSP engine (guide “compilation”).
• JSP actions or tags are a set of customizable, XML-style
tags for particular actions, e.g. predefine jsp:useBean
instantiates a Java Bean class on the server.
18
3 Tier Architecture
URL
JSP page
request
HTTP request
JSP container
compiles to
a servlet
HTTP response
response
HTTP page
Browser
JavaBean
properties,
call methods Library
DB
Web server
19
“Hello” Servlet Revisited
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class Hello extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType(“text/html”) ;
PrintWriter out = response.getWriter() ;
out.println(“<html><body>”) ;
out.println(“Hello ” + request.getParameter(“who”) + “!”) ;
out.println(“</html></body>”) ;
}
}
20
An Equivalent JSP Page
<html> <body>
Hello <%= request.getParameter(“who”) %> !
</body> </html>
21
Remarks

A suitable form element to front-end this page might
be:
<form action=“hello.jsp”>
Name: <input type=text name=who size=32> <p>
<input type=submit>
</form>


The JSP version is much more compact and easier to
understand!
We can expect this to be generally the case when the
logic behind a dynamic page is relatively simple, and
the bulk of the content is static HTML.
22
References

Web Development with JavaServer Pages, Duane K. Fields
and Mark A. Kolb, Manning Publications, 2000.
• Thorough, insightful.

Core JSP, Damon Houghland and Aaron Tavistock, Prentice
Hall, 2001.
• Some imaginative examples. Good section on JSP and XML.


JavaServer Pages, 3rd ed, Hans Bergsten, O’Reilly, 2004.
Core Servlets and JavaServer Pages, Marty Hall, Prentice
Hall, 2000.
• Useful reference for Servlets, especially.

JavaServer Pages, v2, and other documents, at:
http://java.sun.com/products/jsp/
23
Getting Started with Tomcat
24
Server Software


Standard Web servers typically need some additional
software to allow them to run Servlets and JSP—a socalled Servlet Container.
There are commercial products e.g.:
• New Atlanta ServletExec (www.newatlanta.com)
• Macromedia Jrun (www.macromedia.jrun/software/jrun)

For this course you will be expected to use:
• Apache Tomcat (http://jakarta.apache.org/tomcat)
 The official reference implementation for the Servlet 2.4
and JSP 2 specifications.
 Can be used as a small, standalone Web server, or as a
servlet container for Apache, IIS, iPlanet, etc servers—
look under JK in the Tomcat documentation.
25
Tomcat



Tomcat is an open source Web Server, implemented
entirely in Java.
It can be used to serve ordinary static Web content
(HTML, etc), but its main claim to fame is as the
reference implementation of the Servlet and JSP
specifications.
For purposes of coursework you should download
and install Tomcat on a computer available to you.
• Any PC running Windows (for example) should suffice.
• For debugging class projects, you can access your server
through localhost URLs, so you don’t need a registered
domain name (or even an Internet connection).
26
Downloading Tomcat 5

I will give instructions for installing Tomcat 5 under Windows
(details for Windows XP).
• You can also easily install on Linux, or other UNIX flavor. If you choose
to work on a shared UNIX machine, be aware that you may have adjust
your port number to avoid clashing with servers run by other users.
• I assume Java is already installed on your computer (preferably Sun J2SE
1.4 or later).



Go to http://jakarta.apache.org/tomcat/
Follow the “Binaries” link for download, and scroll down to
Tomcat 5 archives and installers.
Get the installer for Windows: at the time of this writing the file
was called “jakarta-tomcat-5.0.18.exe”.
• Size of the file was about 10MB.
27
Installation


Run the installer. On the installation wizard, click through,
accepting the Apache License agreement.
In the instructions that follow I assume you accept the default
installation options, so the install location will probably be:
C:\Program Files\Apache Software Foundation\Tomcat 5.0
• You will need about 40MB of free disk space.

I also assume that you accept the default port number for
Tomcat, which is 8080.
• You may choose 80, if there is no other Web server running on the
computer, but I will assume the 8080 default.


You may take the opportunity to set a good admin password for
the Web server.
You will also be prompted for the name of the folder where Java
is installed, but the wizard usually makes a good guess.
28
Running the Server



By default on Windows the server will start automatically when
installation completes. You should see a progress window like:
When startup completes this window disappears, and you will
probably see an icon like this in your taskbar:
If things don’t happen exactly like this, don’t panic! There are
several different ways to start and stop the server, discussed
shortly.
29
Check Your Server is “Up”

If you are running Tomcat on your local PC, point your Web
browser at:
http://localhost:8080

If you are running your web server on a remote host called
hostname, point your browser at:
http://hostname:8080

You should see the default Tomcat home page, which contains
some useful documentation:
30
Restarting the Server

This is a seemingly trivial skill that needs to be
properly mastered!
• You will find that restarting the server is a vital part of the
debugging cycle for Web applications: if you think you
restarted the server, but it didn’t really restart, you can go
down frustrating blind alleys of debugging.

There are at least 3 mechanisms for starting and
stopping the server under Windows.
31
Stopping and Starting the Server

Three ways:
1. To Start: use the shortcut you find under:
start→All Programs→Apache Tomcat 5.0
To Stop: right-mouse-click on Tomcat icon that appears on
the taskbar, and select “Shutdown”.
2. Use the Windows “Services” tool that you may find through
the Windows Management console or perhaps through
start→Control Panel→Administrative Tools→Services
This gives a convenient way to start, stop, or restart services,
including Tomcat.
3. From a Windows Command Prompt, run the batch files
startup.bat, shutdown.bat which you will find in bin\ under
the installation folder, typically:
C:\Program Files\Apache Software Foundation\Tomcat 5.0\bin\
32
Tips for Restarting


Choose one of the approaches on the previous slide: mixing one
approach to starting with another for stopping isn’t likely to
work properly!
Approach 1 is perhaps the most “natural” under Windows, but
for me it seemed error-prone.
•

Approach 2 seems OK. Approach 3 is robust because it displays a
console window while the server is running, so you know for sure
whether the server is up or down.
Until you are confident you have this mastered, regularly check
whether or not the server is running by:
1.
2.
3.
clearing your browser cache (Tools→Internet Options→Delete Files
under IE 6), then
(for added assurance) restarting your browser, then
trying to visit the URL http://localhost:8080
33
Using Linux


Tomcat runs very well under Linux. Follow the
instructions in the Tomcat documentation for
installation.
To start or stop the server, use the scripts
startup.sh, shutdown.sh in the Tomcat installation
bin/ directory.
34
A First Web Application


As a toy example, we will deploy the trivial “Hello
World” of dynamic HTML.
We create a Web Application that consists of one
static HTML document with a form prompting for
the visitor’s name, and a dynamic JSP document
that displays a customized greeting for that visitor.
35
Preparing a folder


Under Tomcat, each web application should be created
in a dedicated folder, normally created under the folder
called webapps\ in the server installation directory.
First I make a folder:
C:\Program Files\Apache Software Foundation\Tomcat 5.0\webapps\grid04hello\

Within the new folder I must create a nested folder
called “WEB-INF\”.
• For now the subfolder can be empty. But without a
grid04hello\WEB-INF\, Tomcat 5 will not automatically
recognize grid04hello\ as a web application folder!
36
A “Hello Web” Application

In the grid04hello/ folder I create the two files index.html:
<html>
<body>
<form method=“get” action=“hello.jsp”>
Name: <input type=“text” name=“who” size=“32”/> <p/>
<input type=“submit”/>
</form>
</body>
</html>
and hello.jsp:
<html>
<body>
Hello <%= request.getParameter(“who”) %> !
</body>
</html>
37
Files and Folders
Tomcat 5.0\
webapps\
grid04hello\
hello.html
hello.jsp
WEB-INF\
[currently empty]
…other Web applications …
…Tomcat server code, configuration, logs, etc…
38
Starting the Web Application

To “start” the Web application (i.e. to get it loaded
into your server) you just need to restart your Tomcat
server.
• If you are developing a simple Web application like the one
here, using only HTML and JSP files, you don’t usually
need to restart more than once. A running server will
recognize when deployed JSP files are edited, and
recompile them automatically. But you do have to restart
the server to get the new application recognized initially
(later we describe an alternative way to deploy the
application).
39
Using the Web Application

Visiting http://localhost:8080/grid04hello/hello.html, I
should see something like:

On entering my name, and hitting submit, I see:
40
The Rest of this Section




We discuss Java Servlets in some detail. For
presentation-oriented Web Applications JSP is usually
more convenient. But JSP and other Java Web
technologies are built on top of Servlets, and you will
need some knowledge of the Servlet classes to use JSP
effectively.
We cover writing Web Applications with JSP.
We will also say something about managing Tomcat.
General organization of Servlet-based Web
applications.
41
Java Servlets
42
Servlets

At its most abstract, a Java Servlet is a Java class that can be
embedded in a some general-purpose server program. A subset
of requests arriving at the server—presumably those matching
some selection criterion—will be redirected to the servlet for
special handling.

Although servlets are supposedly deployable in various kinds
of servers (HTTP, Databases, FTP, …?), we will only be
interested in HTTP servlets, embedded in Web servers.

Most commonly a servlet is responsible for handling HTTP
GET or POST requests, directed at a particular URL.
43
Contents of this Section

Introduction
• Review of HTTP features.
• Simple servlet.
• Servlet deployment issues.

Servlet programming.
• Form processing with Servlets.
• Servlet Life Cycle.
• Generating responses.

Cookies and session-tracking.
44
References

Core Servlets and JavaServer Pages, Marty Hall,
Prentice Hall, 2000.
• Good coverage, with some discussion of the Tomcat
server.
• There is a 2001 sequel called “More Servlets and
JavaServerPages”.

Java Servlet Programming, 2nd Edition, Jason
Hunter and William Grawford, O’Reilly, 2001.
• Haven’t seen second edition, but 1st ed (1998) was good,
with some good examples.

Java Servlet Specification, v2.4, and other
documents, at:
http://java.sun.com/products/servlet/
45
The HTTP Protocol



We begin by reviewing some essential features of Hyper
Text Transfer Protocol.
HTTP is a textual, client-server protocol, consisting of
requests and responses.
It is commonly implemented over TCP, though other
underlying protocols are possible.
• e.g. some handheld devices implement the most important
parts of HTTP, but not general IP protocols.

Like many W3C documents, the detailed specification
is bizarrely complex, but the most important ideas are
very simple.
46
A GET Request

When one points Internet Explorer 6 (say) at a URL like
http://www.grid2004.org/index.html,
the HTTP request sent to the server may look something like:
GET /index.html HTTP/1.1
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg,
application/vnd.ms-excel, application/vnd.ms-powerpoint,
application/msword, */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
Host: www.grid2004.org
Connection: Keep-Alive
• Exercise: adapt the “Simple Server” program, outlined early in the
section on Java sockets, to print out the HTTP request it receives.
Confirm the format above by pointing a browser at your “server”.
47
Anatomy of an HTTP Request

If you are not very familiar with the HTTP protocol,
study the format of this request. The individual lines
here are called “headers”. Even without prior
knowledge you may be able to guess the meaning of
many of the headers.
• After the headers, there is a blank line—not obvious on the
previous slide, but important to a well-formed request.
48
POST Requests

Another common HTTP request is the POST request.
• In ordinary Web browsing this is commonly used to send
some data to the server, submitted using an HTML form.
• In this context the data might include values of fields typed
directly into the browser window, and uploaded contents of
local files, selected through a form.

The format is very similar to GET, except that the
first word in the method header is “POST”, and the
blank line terminating the headers is followed by the
posted data.
• The following example was generated by submitting a
form. Note two extra headers: Content-length and Cachecontrol. The posted data is simply: “who=Bryan”.
49
An Example POST Request
POST /handleform HTTP/1.1
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg,
application/vnd.ms-excel, application/vnd.ms-powerpoint,
application/msword, */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
Host: www.grid2004.org
Content-Length: 9
Connection: Keep-Alive
Cache-Control: no-cache
who=Bryan
50
The HTTP Response

A typical HTTP response has a format similar to the
HTTP post request: a series of headers, a blank line,
then the returned data.
• The returned data may for example be an HTML page.


The first header now gives an error status for the
request: a status code of 200 indicates success.
The following response could be generated by the
server when the course home page is requested.
• Exercise: compile and run the TrivialBrowser class, given early in the
section on Java sockets. You should see something similar to the
contents of the next slide.
51
Example HTTP Response
HTTP/1.1 200 OK
Date: Fri, 06 Feb 2004 01:34:56 GMT
Server: Apache/1.3.29 (Unix) PHP/4.3.4
Last-Modified: Thu, 29 Jan 2004 22:27:25 GMT
ETag: "832b65-2cf-401988cd"
Accept-Ranges: bytes
Content-Length: 719
Content-Type: text/html
<!DOCTYPE HTML PUBLIC …>
<HTML>
<HEAD>
<TITLE>e-Science e-Business e-Government and their Technologies</TITLE>
…
</HTML>
52
The HttpServlet class

Having reviewed some essential features of HTTP, we are
ready to start describing servlets.

Every HTTP servlet will extend the base class HTTPServlet,
which is defined in the package javax.servlet.http.

This class defines a series of request-handling methods:
doGet()
doPost()
doPut()
doDelete()
doOptions()
doTrace()

Handle HTTP GET request.
Handle HTTP POST request.
Handle HTTP PUT request.
Handle HTTP DELETE request.
Handle HTTP OPTIONS request.
Handle HTTP TRACE request.
When you define a new servlet class, you normally override one
or more of these methods.
53
A “Hello World” Servlet
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class Hello extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType(“text/html”) ;
PrintWriter out = response.getWriter() ;
out.println(“<html><body>”) ;
out.println(“<h1>Hello World!</h1>”) ;
out.println(“</html></body>”) ;
}
}
54
A Request Handling Method

All HttpServlet request-handling methods (doGet(), doPost(),
etc) have a similar interface.

They are passed parameters of type HttpServletRequest,
HttpServletResponse
• Needless to say, these are Java objects representing the HTTP request and
response.

The request and response objects have methods for accessing the
headers, the data “payload”, and other properties of the HTTP
messages.
• In the example, the first line of doGet() sets the Content-Type header of
the response, and the remaining lines write the data payload—a short
HTML document—to the response.
• The servlet container can complete most headers in the response, but
setting the Content-Type header is mandatory.
55
Deploying Servlets





At the end of the section introducing Tomcat, we
deployed a trivial Web app using a JSP page.
We nested a folder in webapps/ for our applications,
and also created an empty subfolder called WEB-INF/.
In general the WEB-INF/ folder of an application holds
associated classes and XML descriptors.
In particular servlet class files and the deployment
descriptor (“web.xml” file) go in the WEB-INF/ folder.
In the following example we recycle the grid04hello/
folder created earlier.
56
Files and Folders
Tomcat 5.0\
webapps\
grid04hello\
…HTML and JSP files …
WEB-INF\
web.xml
classes\
Hello.class
…other Web applications …
…Tomcat server code, configuration, logs, etc…
57
Compiling the Servlet

The servlet source should be contained in a file
Hello.java.
• You can conveniently put the source itself in the classes\
subfolder of WEB-INF\, and compile it “in situ”.
• Or compile the source wherever you like, and just copy the
class file to classes\.
58
Setting the Class Path

If you are using javac from a command prompt, you
must add the servlet classes to the class path, so the
compiler knows where to find them.
• e.g. click to
start→Control Panel→System→Advanced→Environment Variables
and define new system variables:
TOMCAT_HOME
c:\Program Files\Apache Software Foundation\Tomcat 5.0
CLASSPATH
.;%TOMCAT_HOME%\common\lib\servlet-api.jar

If you are using an IDE like JBuilder, there will
probably be built-in support for developing servlets.
You may not have to worry about these details.
59
The Deployment Descriptor



Tomcat 5 apparently requires a deployment descriptor
mentioning every servlet class in the Web application.
Create a file web.xml in the WEB-INF\ folder—a minimal
descriptor for our servlet is given on the next slide.
Don’t worry if this doesn’t make much sense to you—we
will cover XML shortly. You may copy the example
web.xml file from:
Tomcat 5/servlets-examples/WEB-INF/web.xml
and edit it appropriately.
• The important parts are the <servlet> and <servlet-mapping>
elements, which define the servlet class and the URL it is found at.
60
Minimal web.xml File
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app
PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>
<display-name>grid2004 Hello examples</display-name>
<description>grid2004 Hello examples.</description>
<servlet>
<servlet-name>Hello</servlet-name>
<servlet-class>Hello</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Hello</servlet-name>
<url-pattern>/servlet/Hello</url-pattern>
</servlet-mapping>
</web-app>
61
Viewing the Servlet

Restart your Tomcat server.

Visit the URL:
http://localhost:8080/grid04hello/servlet/Hello

You should see something like:
62
Servlets vs JSP




Compared with our earlier “Hello Web” JSP
application, you will notice that writing and deploying
a Java servlet is much more work.
This isn’t surprising, because JSP is a higher level
application layered on servlets.
If you are building an application around dynamic
HTML, it will nearly always be preferable to use JSP.
But it is important to know something about raw
servlets, because eventually you will want to
implement HTTP applications (e.g. Web services) that
do something other than generate dynamic HTML.
For these JSP won’t help.
63
Servlet Programming
64
Form processing with Servlets

Assume we have a form in an HTML document like
<form method=“get”
action=“http://hostname:8080/grid04hello/servlet/Hello”>
Name: <input type=text name=who size=32> <p>
<input type=submit>
</form>
This supposes Tomcat is running on port 8080 on the
host hostname.
65
Getting Form Parameters

A suitable goGet() method in the Hello servlet class would be:
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType(“text/html”) ;
PrintWriter out = response.getWriter() ;
out.println(“<html><body>”) ;
out.println(“Hello ” + request.getParameter(“who”) + “!”) ;
out.println(“</html></body>”) ;
}

The method getParameter() of HttpServletRequest will parse
the HTTP request and extract the named form parameter
(“who” in this example).
• If a single form parameter has multiple values (allowed in HTML), can
instead use request.getParameterValues(), which returns an array.
66
GET vs POST: Idempotence

If the method attribute on the form is GET, the form data is
appended to the path component of the action URL, and sent
in an HTTP GET request.
• Be aware browsers may silently retry GET requests that apparently
failed. Use the GET method only for actions that are idempotent—
actions where accidental repeats don’t have observable effects:
 A Google search for a particular topic is effectively idempotent.
 Transferring $1000 from your bank account is not idempotent.

If the method is POST, the URL is not extended: the form
data is sent in the data payload of an HTTP POST request.
• Browsers are supposed to prompt before retrying a POST request.

To handle a POST request with a servlet, define the doPost()
method instead of the doGet() method: other details are
unchanged.
67
URL Encoding

URL encoding is a method of wrapping up form-data in a way
that will make a legal URL for a GET request.
• The encoded data consists of a sequence of name=value pairs, separated
by &.
• Spaces in values are replaced by +.
• Non-alphanumeric characters are converted to the form %XX, where XX
is a two digit hexadecimal code.
• In particular, line breaks in multi-line form data (e.g. addresses) become
%0D%0A—the hex ASCII codes for a carriage-return, new-line
sequence.


URL encoding is somewhat redundant for the POST method,
but it is the default anyway.
If you are using servlets you usually don’t need to worry about
any of this, because getParameter() and related methods do the
decoding for you.
68
Reading Posted Data

Where posted data is not simply URL Encoded form parameters
(e.g. if a file is uploaded, producing to posted data in multipart
MIME format), the data may be read directly:
public void doPost(HttpServletRequest req,
HttpServletResponse resp) {
…
BufferedReader in = new BufferedReader(req.getReader()) ;
String line ;
while((line = in.readLine()) != null) {
…
}
…
}

Don’t try to combine reading data using getReader() with the
higher-level approach using getParameter()—choose one or the
other.
69
Information from Request Headers

There is a series of convenience methods that read information
from the request headers, including:
getMethod(), getRequestURI(), getProtocol()
Method header
getContentLength()
Content-Length header
getContentType()
Content-Type header
getAuthType(), getRemoteUser()
Authorization header
getCookies()
See later
getHeader(String name)
getHeaders(String name)
getHeaderNames()

Any HTTP request header
Enumeration of headers
Enumeration of header names
Exercise: Study the servlet RequestHeaderExample, which
comes in the “servlet examples” in the Tomcat release, for an
example inspecting request headers.
70
Servlet Life Cycle

By default, a Web server only creates one instance
of a servlet class for each <servlet> element in the
web.xml file.
• All requests to the same URL are handled by the same
servlet class instance.

This means that a request may see effects of
processing earlier requests through values of
instance variables.
• Or class variables.

By default, though, each request is handled in a
different Java thread.
• So mutual exclusion is an issue.
71
A Counter Servlet
public class Counter extends HttpServlet {
int count = 0 ;
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType(“text/html”) ;
PrintWriter out = response.getWriter() ;
out.println(“<html><head></head><body>”) ;
out.println(“This servlet instance has been accessed ” +
(count++) + “ times”) ;
out.println(“ </body></html>”) ;
}
}
72
Remarks

The first time I point my browser at this servlet I get a
response page containing the message:
This servlet has been accessed 0 times

Each time I reload the URL, the count increases.

Since count is an instance variable of the class, this
illustrates the case that only a single instance of
Counter is created.
73
Mutual Exclusion

The counter servlet is not completely reliable, because it is
possible to have concurrent requests handled in different
threads. The instance variable count is shared by threads.
This could lead to problems of interference.

For example, the increment of count could be done in a
synchronized statement:
int myCount ;
synchronized(this)
myCount = count++ ;
Subsequently the local variable myCount—which is
private to the thread—is printed in the response.
74
The init() Method

The init() method:
public void init() throws ServletException {. . .}
is called once when the servlet is created. You
override it to define initialization code for your
servlet instance.
75
Initialization Parameters

A new counter servlet, defining an init() method:
public class InitCounter extends HttpServlet {
int count ;
public void init() throws ServletException {
ServletConfig config = getServletConfig() ;
try {
count = Integer.parseInt(config.getInitParameter(“initial”)) ;
} catch (NumberFormatException e) {
count = 0 ;
}
}
… definition of doGet() method …
}
76
Defining Initialization Parameters

In web.xml, I add the element:
<servlet>
<servlet-name>Counter2</servlet-name>
<servlet-class>InitCounter</servlet-class>
<init-param>
<param-name>initial</param-name>
<param-value>50</param-value>
</init-param>
</servlet>

If I restart the server and point my browser at this
servlet I see the (misleading) message:
This servlet has been accessed 50 times
77
Generating Responses

A minimal server response to a client request
might be:
HTTP/1.1 200 OK
Content-Type: text/plain
Hello World!

We already saw how to set the content type
explicitly using setContentType().
• In general the content type can be any MIME type
recognized by the browser.

A servlet can explicitly set other values by using
the setStatus() method.
78
Explicitly Returning Status Codes

Status values are available as predefined
constants in the HttpServletResponse class, e.g:
final int SC_OK = 200 ;
final int SC_FOUND = 302 ;
final int SC_NOT_FOUND = 404 ;
Use like:
response.setStatus(HttpServletResponse.SC_OK) ;
79
Common Cases

There are a couple of convenience methods on
HttpServletResponse for dealing with common cases:
void sendError(int sc, String message)
• send specified status, with generated page containing message.
void sendRedirect(String location)
• send SC_TEMPORARY_REDIRECT status, and include
Location header.

By sending the SC_FOUND or
SC_TEMPORARY_REDIRECT status, together with a
dynamically generated URL, a servlet can cause a the
browser to go directly to a different page or site (without
the user manually clicking another link).
80
Cookies and Session
Tracking
81
The Problem

HTTP is said to be a stateless protocol. It consists of a
series of request/response transactions with no intrinsic
association between different transactions.
• Stateless protocols have reliability advantages: recovery from
failure, robustness and simplicity of protocol implementation.
• Modern HTTP isn’t completely stateless; by default it tries to
keep TCP connections open between transactions.



Very often a Web application needs a server to engage
in an extended dialog with a client, involving a series of
requests and responses.
So the problem is how to define and keep track of a
particular “session” between client and Web server.
This is called session tracking.
82
Example

Classic example of “session information” is the contents
of a customer’s shopping cart, at an online store.

Typically the shopping carts of all customers actively
browsing a site will be stored on the server side (e.g. in
data structures managed by a servlet).

But every HTTP request coming from a customer must
somehow be tagged with information correlating the
transaction with a particular session.
• When a customer chooses a new item, the server needs a
nugget of identifying information, to decide which shopping
cart the new item should go in.
83
Basic Approaches

Practical approaches to attaching session information to
transactions include:
• URL-Rewriting (“fat URLs”)
Assumes all pages associated with the session are dynamically
generated by the server. Session identification is directly appended to
any URLs in the generated pages that refer back to the same server.
• Cookies
An extension to HTTP that lets a Web server ask a browser to store
small amount of identifying information. The browser returns this
information in subsequent HTTP request headers, when it revisits the
same server.

We will focus on use of cookies, because they provide the
most flexibility, and they are used almost universally today.
84
Cookies




If a browser receives an HTTP response including a
Set-Cookie header (and is willing to take the cookie) it
stores the information in the header.
This information can either be stored in the memory of
the browser process (“session cookie”) or saved to disk
(“persistent cookie”).
Whenever a browser constructs an HTTP request for a
server, it checks whether it is storing any cookies it
previously received from the server.
If so, it returns the cookie information in a Cookie
header attached to the new request.
85
Uses of Cookies
1.
Recognizing a regular customer
•
2.
Session Tracking
•


A persistent cookie can save some identification information for a
particular customer. The stored information could be actual name or
details, but more likely some opaque key into a database on the server.
Within a single dialog with a site, session cookies can be used as the
underlying mechanism for session tracking.
The first application is limited, because many people configure
their browsers to reject persistent cookies, regarding them as
an intrusion of privacy.
But nearly everybody will accept session cookies (you have to
work quite hard to make Internet Explorer reject session
cookies).
86
The Servlet Cookie API


A servlet creates a cookie by using a constructor
for the class Cookie.
Various attributes can be set for the cookie before
sending it to the client. They include:
• The name and value of cookie
• The domain to which the cookie should be returned.
By default the cookie will only be returned to the
server that sent it, but this default can be overridden.
• The URI path to which the cookie should be returned.
By default, the cookie is only returned to pages in the
same directory as the page that sent the cookie.
• The time when a persistent cookie expires.
87
Example

View the “Cookies” example in the Tomcat release. If
you are running Tomcat locally, go to
http://localhost:8080/servlets-examples/

In the example, the lines that add a set-cookie header to
a server response are:
Cookie c = new Cookie(name, value);
response.addCookie(c);

The resulting HTTP response will include a header like:
Set-Cookie: name=value
88
Browser Behavior

If the browser accepts a cookie, it returns a cookie
header like
Cookie: name=value
in HTTP requests, when revisiting the same server.

In servlet code you read the set of cookies returned
by the browser by, e.g.:
Cookie[] cookies = request.getCookies();
• Exercise: Set one or two cookies using the “Cookies”
example in the Tomcat release. After setting the cookies,
also visit the “Request Headers” example to see the
returned HTTP cookie headers themselves.
89
The Session Tracking API


You should be able to see in a general way how the
Servlet cookie API can be used for session tracking.
Actually you don’t need to explicitly manipulate
cookies to achieve this: servlets provide a higher-level
API that transparently uses cookies to keep track of
sessions.
• Using the higher-level API has the added benefit that it also
supports URL-rewriting as a tracking mechanism, in the
(fairly unlikely) event that a browser doesn’t support session
cookies.
90
The HttpSession class


A session is defined as an association—lasting for some
time—between a particular browser and a particular
Web application.
It is represented by an instance of the HttpSession
class.
• A servlet method retrieves the current session object by
applying the method getSession() to the HttpRequest.
• If no session object exists on a call to getSession(), a new one
will be created.

Objects of any type can be bound to sessions by storing
them as named attributes in the session, using the
setAttribute(), getAttribute() methods of HttpSession.
91
Shopping Cart Using a Session
public void doGet(HttpServletRequest req,
HttpServletResponse resp) throws … {
HttpSession session = request.getSession(true) ;
ArrayList shoppingCart =
(ArrayList) session.getAttribute(“cart”) ;
if(shoppingCart == null) {
// New session
shoppingCart = new ArrayList() ;
session.setAttribute(“cart”, shoppingCart) ;
}
}
… Add or remove items from cart …
92
Remarks


The true argument of getSession() means that a
new session object will be created if one does not
already exist—you should probably always use this
argument.
See the “Sessions” example in the Tomcat release
for a complete example using various session
tracking methods.
• It is instructive to revisit the “Cookies” example after
starting a session: you should find a cookie called
JSESSIONID has been established.
93
Session Attributes vs. Instance Variables



In Java programs, local variables are normally declared
inside methods to hold values computed and used within
the invocation.
Typically, instance variables are used to hold values that
need to be shared across multiple invocations.
In servlet programming—where several browser
sessions may be concurrently operating on a single
servlet instance—this role for instance variables is
naturally taken over by attributes of the session object.
• Think hard before declaring an instance variable in a servlet!
In many cases you should probably be using a session
attribute instead.
94
The Scope of a Session


A servlet context is a group of servlets (and possibly
other resources), collected together as a Web
application.
Several servlets may be involved in the same session,
hence share the same HttpSession object.
• This sharing is automatic if the servlets are in the same
context, and are interacting with the same browser.

Servlets from different contexts in the same server, or
interacting with different browsers, always have distinct
HttpSession objects.
95
Life-Time of a Session


In general a session expires after some interval.
The method:
public void setMaxInactiveInterval(int seconds)

on HttpSession can be used to request that the session
will be invalidated if there has been no transaction in
the specified interval.
The method:
public void invalidate()
on HttpSession can be used to forcibly end a session.
96
JSP: JavaServer Pages
97
What is a JSP Page?

According to the JavaServer Pages Specification:
• “A JSP page is a text-based document that describes how to
process a request to create a response. The description
intermixes template data with some dynamic actions.’’

JSP deliberately supports multiple paradigms for
authoring dynamic content.
• These include Scriptlets, JavaBeans and Tag Libraries.
98
JSP Features

Fixed template data—the parts of a JSP page that are
used verbatim in the response. In simple cases this data
will take the form of plain HTML. It may also be XML,
or plain text.

Standard directives guiding translation of a JSP page to
a servlet.

Scripting elements: scriptlets, expressions, and
declarations that include Java fragments to embed
dynamic content.

Standard actions which are predefined XML-like tags.

A tag extension mechanism, called tag libraries.
99
Translating and Executing JSP Pages

A JSP page is executed in a JSP container,
generally installed in a Web server.

The underlying semantic model is that of a
servlet.
• A JSP container will translate the JSP page to a Java
servlet.

By default, translation and compilation of a JSP
page is likely to occur the first time it is accessed.
• With Tomcat 5, you can find the generated Java and
the class files in a subdirectory under the folder Tomcat
5.0\work\ .
100
Directives vs Request-time Operations

Directives provide “global” information about the
behavior of a JSP page—independent of any
specific request.
• In practice, they guide the process of translation from
JSP to a Java Servlet. They are “compile-time”
instructions.

Scripting elements and actions have some effect in
the context of a particular request.
• e.g. generating some output that depends on form
parameters, creating an object in the JSP container, or
reading or writing entries in a database.
101
Scripting Elements

The two most important kinds of scripting element are:
• Scriptlets: arbitrary fragments of Java that are executed in
the request handling method. Syntax is e.g.:
<% out.println(“Your lucky number is ” + Math.random()) ; %>
• Expressions: any Java expression, cast to a String. Evaluated
in the context of the request handling method. Syntax is e.g.:
<%= Math.sqrt(2) %>

Don’t confuse JSP scripting elements with JavaScript! JSP is
executed on the server side. JavaScript is usually executed in the
browser. You can freely use JSP to generate HTML documents
that include JavaScript inserts, which will be executed later in
the browser. But these are different from JSP scripting elements,
and have different syntax.
102
A JSP Page with a Scriplet
<html><head></head><body>
<% out.println(“Now is ” + new java.util.Date()) ; %>
</body></html>


This is the complete JSP page. Just save this text in,
e.g., “date.jsp”, and install it in a suitable document
directory.
Don’t forget the semicolon!
• The scriptlet must yield a legal Java program when it is
inserted into the automatically generated servlet code.
• Since compilation is done on the fly, syntax errors are
reported through the browser the first time you visit the
page. If you forget the semicolon you will get a cryptic
error message referring the generated servlet.
103
Viewing the date.jsp Page

If we visit date.jsp we see something like:
104
“Compound” Scriptlets


A compound statement doesn’t have to be confined to a
single scriptlet element—it can be assembled from several
scriptlet elements, surrounding blocks of template text.
Here is an example using a conditional around blocks of
template data:
<html><head></head><body>
<% if (Math.random() < 0.5) { %>
Dean for President!
<% } else { %>
Vote Kerry!
<% } %>
</body></html>

Earlier warnings about syntax errors apply—take special
care not to lose any braces!
105
Expressions




A JSP expression is very similar to a scriplet, except
that the enclosed Java code is required to be an
expression.
The expression is evaluated, then converted to a String.
The string is interpolated into the template text—
wherever the element appears—and sent to the browser
as part of the generated HTML.
The syntax of an expression element is:
<%= Java expression in here %>
• Distinguished from a scriptlet by the = sign.
106
Avoiding out.println()

We can use an expression to replace the first scriplet in
this lecture, viz:
<html><head></head><body>
<% out.println(“Now is” + new java.util.Date()) ; %>
</body></html>
with:
<html><head></head><body>
Now is <%= new java.util.Date() %>
</body></html>

In general, expression elements save us writing many
scriptlets that just include calls to out.println().
107
Predefined Variables in JSP


Some variables are predefined for use in JSP
scriptlets and expressions.
In earlier examples we used the variable out, for
example. This refers to the PrintWriter associated
with the servlet HttpResponse object.
108
Some Predefined Variables

request
The HttpServletRequest object describing the
current request.

response
The HttpServletResponse object describing the
response currently being prepared.

out
The PrintWriter object associated with response.

session
The HttpSession object associated with the request.
109
Directives

The general syntax is of a directive is:
<%@ name attr1=“value1” attr2=“value2” … %>

where name is the name of the directive, and attr1,
attr2, . . . are the names of various attributes
associated with this directive.
The currently allowed values for name are page,
include and taglib:
• The page directive defines various attributes associated
with a page.
• The include directive allows another file to be textually
included.
• The taglib directive imports a tag library.
110
The import attribute

For now the most important attribute that can be set
by the page directive is the import attribute.

This simply adds an import directive to the generated
Java code.

Syntax is, e.g.:
<%@ page import=“java.util.*” %>
111
Actions and Beans

Scripting elements of JSP are powerful.
• Give us essentially all the power of Java and servlets, but with
the code embedded directly in an HTML page.


Code fragments and HTML are simply separated by
<%, %> brackets.
This is both an advantage and a disadvantage:
• Allows a dynamic page to be developed and deployed quickly.
• Equally quickly, the JSP source can become difficult to read.

An HTML page containing many Java inserts can be
difficult to read and maintain.
• The same remarks may presumably be applied to
syntactically similar systems (e.g. ASP, PHP).
112
Actions



JSP actions are special elements that follow an XMLlike syntax.
They sit naturally within an HTML document (and
presumably are comfortable for Web designers).
It is possible to write JSP pages that use actions
exclusively—there need not be any Java source
embedded in the HTML page.
• One can still invoke Java code through JavaBeans.

More practically, one can use actions to instantiate and
set properties of beans, but still admit scripting
elements for invoking methods on beans, and for
adding conditional and looping control to the JSP page.
113
The Standard Actions

Predefined action tags include:
• jsp:useBean - declares the usage of an instance of a
JavaBeans component.
• jsp:setProperty - this tag can be used to set properties of
a bean.
• jsp:getProperty - gets the property of the Bean, converts
it to a String, and puts it into document.
• jsp:forward - forwards the request to another JSP page
or servlet.
• jsp:include - include another JSP page
114
Syntax

All actions follow an XML-like syntax: an element
either has the form:
< name attr1=“value1” attr2=“value2” . . . />
or the form:
< name attr1=“value1” attr2=“value2” . . . >
JSP body of element
</ name >

Here name is the name of the element, and attr1,
attr2, . . . are the names of various attributes
associated with this element.
115
JavaBeans


JavaBeans is a component architecture. It dictates a set
of rules that software developers follow to create
reusable components. A Bean is an instance of a class
that was coded following these rules.
For our purposes, a Bean is simply an instance of a selfcontained Java class used in an JSP page. JSP interacts
with Beans through “actions”, or simply by calling
methods on the bean, from scripting elements.
116
Elementary Use of Beans in JSP

The jsp:useBean action tells a page that a particular
Bean is to be used here.

Its minimal form names a Bean, and specifies its class.

If there is no Bean of the specified name currently “in
scope” the jsp:useBean method creates a new Bean
instance.
• A JavaBean must have a public no-argument constructor.
This allows a Bean container to instantiate the class without
previous knowledge of the class.
117
The “Use Bean” Element.

The basic form of the element is:
<jsp:useBean id=“bean-name” class=“class-name”
scope=“scope”/>
• The name bean-name should be a legal Java variable name.
Appearance of this tag causes a variable of this name, and
type class-name, to become available in any following
scripting elements.
• The class called class-name is typically defined in the classes\
folder of the Web application.
• The jsp:useBean element may also contain “conditional text”
that is only included if the Bean is freshly created.
• The scope attribute is discussed in the next slide.
118
Controlling a Bean’s Scope


The accessibility and lifetime of a Bean created by
a JSP page is controlled by the scope attribute of
the jsp:useBean element.
The most useful values are
session
The current session. The bean automatically becomes
an attribute of the HttpSession object. The bean is
typically created at the start of a client session, and
garbage-collected when the session ends.
application
Available to any request in the same Web application
(servlet context). Effectively a global variable that lives
as long as the Web server is running, and is shared
between client sessions.
119
“Session Beans”


By declaring beans with session scope we can
avoid having to deal explicitly with the
HttpSession object.
We may loosely refer to these as “session beans”,
but they should not be confused with the EJB
concept of a session bean.
• Enterprise JavaBeans is a much more specialized
architecture than “ordinary” JavaBeans.
120
JSP Tag Libraries




We saw that the JSP standard actions are a set of
tags for creating and manipulating beans, and a few
other purposes. They are useful but fairly limited.
JSP allows creating of custom tags through tag
libraries.
This is a powerful feature, that allows you to build
whole new XML-like vocabularies for creating
dynamic Web pages.
We don’t have time to do full justice to tag libraries
here, but you can download many standard tag
libraries from:
http://jakarta.apache.org/taglibs
121
A Simple Web Application
122
A Complete Web Application

We round off this section by giving the full source of a
tiny but self-contained Web application, which uses
multiple (two) JSP pages and a JavaBean, and does
non-trivial session tracking.

We mentioned earlier that the classic example of
“session information” is the contents of a customer’s
shopping cart at an online store.

In the interests of fitting code in slides, we scale this
down and deal with selections from a virtual snackvending machine.
• “. . . Think of clocks and counters and telephones and board
games and vending machines.” C.A.R Hoare, Communicating
Sequential Processes, 1985.
123
Snack-Vending Machine





The first page of our Web application is the selection
page, implemented by a JSP called vendingmachine.jsp.
The generated page contains a series of small HTML
forms, each with a single button for selecting snack
items.
When we click on a snack, we return to the selection
page, but our selection is now appended to a list
maintained by the server.
In our implementation, this list is abstracted in a session
bean of class vending.VendingBean.
This bean also maintains the fixed list of all snacks
available from the vending machine.
124
vendingmachine.jsp
<html><head></head><body>
<jsp:useBean id="vendingBean" class="vending.VendingBean"
scope="session"/>
<%
String snack = request.getParameter("selection") ;
if(snack != null) vendingBean.addSelection(snack) ;
%>
<%
for (int i = 0 ; i < vendingBean.getNumSnacks() ; i++) {
%>
<form action="vendingmachine.jsp">
<input type="submit" name="selection"
value="<%= vendingBean.getSnacks(i) %>" />
</form>
<%
}
%>
<a href="checkout.jsp">Proceed to checkout</a>
</body></html>
125
Generated HTML


If we view the generated HTML of the initial page,
(View→Source in Internet Explorer) it includes a
series of forms—one per snack.
The submit button for each form sets a value for
the parameter called selection, value is name of the
snack:
<html><head></head><body>
<form action="vendingmachine.jsp">
<input type="submit" name="selection" value="Chips"/>
</form>
<form action="vendingmachine.jsp">
<input type="submit" name="selection" value="Popcorn"/>
</form>
...
<a href="checkout.jsp">Proceed to checkout</a>
</body></html>
126
The Initial Page

Visiting the vendingmachine.jsp page, I see something like:
127
Selecting a Snack



On the first visit to the vendingmachine.jsp, the form
parameter selection will presumably be null, so the
first scriptlet in the page does nothing.
Clicking on a snack button sets the selection
parameter, and returns to the same page.
But this time selection is non-null, so the
addSelection() method of the bean is called.
128
The VendingBean class
package vending ;
import java.util.ArrayList;
public class VendingBean {
final String [] snacks = {"Chips", "Popcorn", "Peanuts", "Snickers bar",
"Twix", "Pop Tarts", "Chocolate Donut"} ;
public int getNumSnacks() { return snacks.length ; }
public String getSnacks(int i) { return snacks [i] ; }
ArrayList selections = new ArrayList() ;
public int getNumSelections() {
return selections.size() ;
}
public String getSelections(int i) {
return (String) selections.get(i) ;
}
public void addSelection(String selection) {
selections.add(selection) ;
}
}
129
Viewing the Selections




After selecting a few snacks, click on the “Proceed to
checkout” link.
This takes us to the second JSP, checkout.jsp.
This accesses the same Bean, and prints out the current
selections it contains.
Finally it terminates the current session, so the next
visit to vendingmachine.jsp will start with a fresh
session bean.
130
checkout.jsp
<html><head></head><body>
<h3>Your selections were:</h3>
<jsp:useBean id="vendingBean" class="vending.VendingBean“
scope="session"/>
<ul>
<%
for (int j = 0 ; j < vendingBean.getNumSelections() ; j++) {
%>
<li> <%= vendingBean.getSelections(j) %> </li>
<%
}
%>
</ul>
<p>Enjoy!</p>
<%
session.invalidate() ;
%>
<p> <a href="vendingmachine.jsp">Back</a> </p>
</body></html>
131
The Checkout Page

If I click a few selections on the main page, then follow
the link to the checkout, I may see something like:
132
A JSP Programming Style

JSP deliberately supports multiple programming styles, but the
style we have illustrated here is a reasonable one:
• Capture non-trivial logic of the application (“business logic”) in one or
more JavaBean classes.
 The interface of the bean class should be natural for the application
logic—decoupled from presentation.
 Avoiding generating HTML fragments and minimize use of servlet
support classes inside the bean classes.
• Simple logic associated with presentation—e.g. loops or conditionals
associated with generating HTML elements—is better handled in
scriptlets.
 Or specialized tag libraries, but we haven’t had time to discuss tag
libraries in detail.
• It is possible to use <jsp:setProperty>, etc actions to manipulate the Java
beans using tags alone, but it is often more natural to call Java methods on
the beans directly, from scriptlets.
133
Web Application Directory Hierarchy
Tomcat 5.0\
webapps\
grid04vending\
vendingmachine.jsp
checkout.jsp
WEB-INF\
web.xml
classes\
vending\
VendingBean.class
134
Deployment Issues


The folders for the vending machine application are
illustrated on the previous slide.
The web.xml file is probably optional here, because
we have no servlets.
• But it is useful, if only to give a “display-name” for the
application.

Bean classes, like servlet classes, normally go in the
WEB-INF\classes\ folder.
• We defined our bean class to belong to the package vending,
so there is a folder vending\ nested inside classes\.
135
The Web Archive File

It is convenient to bundle all the files associated with an
application into a Web Archive, for easy deployment.
• This can include ordinary, static Web content, servlet classes,
JSP pages, bean classes, …

Assuming the components of your application are
organized in folders as shown above, cd to the
grid04vending\ folder (the top-level folder of your app)
and execute the command:
jar cvf grid04vending.war *
• This assumes you are using a command prompt, and you have
J2SE or equivalent Java software installed.

This should create an archive file called
grid04vending.war.
136
Deploying Archive Files




Once you have a Web archive, deployment in a new
Tomcat server (or other servlet container) is very easy.
You can manually deploy the application simply by
copying the .war file to the Tomcat 5.0\webapps\
folder.
When you restart the Web server, it notices the archive
file, automatically expands it, and loads the application.
Alternatively you can use the Tomcat Application
Manager interface.
137
Tomcat Management Interface

Starting from the home page of your Web server (e.g.
http://localhost:8080) follow the link “Tomcat Management”.
• You may have to modify account information in
Tomcat 5.0\conf\tomcat-users.xml, first.

You use this page to start/stop/reload running applications.

To deploy a .war file in
a running server, copy
it to the webapps\
folder, then use this
form (right):

Alternatively you can upload a .war file through your
browser using this form (below):
138
Descargar

Document