20jsp

1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
JavaServer Pages
JSP

JSP2 www.corewebprogramming.com
Agenda
• Introducing JavaServer PagesTM (JSPTM)
• JSP scripting elements
• The JSP page Directive: Structuring
Generated ServletsTM
• Including Files in JSP Documents
• Using JavaBeans™ components with JSP
• Creating custom JSP tag libraries
• Integrating servlets and JSP with the MVC
architecture

The Need for JSP
• With servlets, it is easy to
– Read form data
– Read HTTP request headers
– Set HTTP status codes and response headers
– Use cookies and session tracking
– Share data among servlets
– Remember data between requests
– Get fun, high-paying jobs
• But, it sure is a pain to
– Use those println statements to generate HTML
– Maintain that HTML

The JSP Framework
• Idea:
– Use regular HTML for most of page
– Mark servlet code with special tags
– Entire JSP page gets translated into a servlet (once), and
servlet is what actually gets invoked (for each request)
• Example:
– JSP
• Thanks for ordering
<I><%= request.getParameter("title") %></I>
– URL
• http://host/OrderConfirmation.jsp?title=Core+Web+Programming
– Result
• Thanks for ordering Core Web Programming

Benefits of JSP
• Although JSP technically can't do anything
servlets can't do, JSP makes it easier to:
– Write HTML
– Read and maintain the HTML
• JSP makes it possible to:
– Use standard HTML tools such as HomeSite or
UltraDev
– Have different members of your team do the HTML
layout and the programming
• JSP encourages you to
– Separate the (Java) code that creates the content from the
(HTML) code that presents it

Advantages of JSP Over
Competing Technologies
• Versus ASP or ColdFusion
– Better language for dynamic part
– Portable to multiple servers and operating systems
• Versus PHP
– Better language for dynamic part
– Better tool support
• Versus WebMacro or Velocity
– Standard
• Versus pure servlets
– More convenient to create HTML
– Can use standard tools (e.g., HomeSite)
– Divide and conquer
– JSP developers still need to know servlet programming

Setting Up Your Environment
• Set your CLASSPATH. Not.
• Compile your code. Not.
• Use packages to avoid name conflicts. Not.
• Put JSP page in special directory. Not.
– tomcat_install_dir/webapps/ROOT
– jrun_install_dir/servers/default/default-app
• Use special URL to invoke JSP page. Not.
• Caveats
– Previous rules about CLASSPATH, install dirs, etc., still
apply to regular classes used by JSP

Example
<!DOCTYPE HTML PUBLIC
"-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>JSP Expressions</TITLE>
<META NAME="author" CONTENT="Marty Hall">
<META NAME="keywords"
CONTENT="JSP,expressions,JavaServer,Pages,servlets">
<META NAME="description"
CONTENT="A quick example of JSP expressions.">
<LINK REL=STYLESHEET
HREF="JSP-Styles.css"
TYPE="text/css">
</HEAD>

Example (Continued)
<BODY>
<H2>JSP Expressions</H2>
<UL>
<LI>Current time: <%= new java.util.Date() %>
<LI>Your hostname: <%= request.getRemoteHost() %>
<LI>Your session ID: <%= session.getId() %>
<LI>The <CODE>testParam</CODE> form parameter:
<%= request.getParameter("testParam") %>
</UL>
</BODY>
</HTML>

Example Result
• With default setup, if location was
– C:jakarta-tomcatwebappsROOTExpressions.jsp
– C:Program FilesAllaireJRunserversdefault
default-appExpressions.jsp
• URL would be
– http://localhost/Expressions.jsp

Most Common Misunderstanding:
Forgetting JSP is Server-Side Technology
• Very common question
– I can’t do such and such with HTML.
Will JSP let me do it?
• Why doesn't this question make sense?
– JSP runs entirely on server
– It doesn’t change content the browser can handle
• Similar questions
– How do I put an applet in a JSP page?
Answer: send an <APPLET…> tag to the client
– How do I put an image in a JSP page?
Answer: send an <IMG …> tag to the client
– How do I use JavaScript/Acrobat/Shockwave/Etc?
Answer: send the appropriate HTML tags

2nd Most Common Misunderstanding:
Translation/Request Time Confusion
• What happens at page translation time?
– JSP constructs get translated into servlet code
• What happens at request time?
– Servlet code gets executed. No interpretation of JSP
occurs at request time. The original JSP page is ignored at
request time; only the servlet that resulted from it is used
• When does page translation occur?
– Typically, the first time JSP page is accessed after it is
modified. This should never happen to real user
(developers should test all JSP pages they install).
– Page translation does not occur for each request

JSP/Servlets in the Real World
• ofoto.com:
print and
manage
digital and
conventional
photos.

• First USA Bank: largest credit card issuer in
the world; most on-line banking customers

• Delta Airlines: entire Web site, including
real-time schedule info

• wine.com: the Internet's leading wine
retailer

• American Century Investments: more than
70 mutual funds, $90 billion under
management, two million investors

• Excite: one of the top five Internet portals;
one of the ten busiest sites on the Web
– AltaVista also uses servlets/JSP on part of their site

core
programming
JSP Scripting
Elements

Uses of JSP Constructs
• Scripting elements calling
servlet code directly
servlet code indirectly (by
means of utility classes)
• Beans
• Custom tags
• Servlet/JSP combo
(MVC architecture)
Simple
Application
Complex
Application

Types of Scripting Elements
• Expressions
– Format: <%= expression %>
– Evaluated and inserted into the servlet’s output. I.e.,
results in something like out.println(expression)
• Scriptlets
– Format: <% code %>
– Inserted verbatim into the servlet’s _jspService method
(called by service)
• Declarations
– Format: <%! code %>
– Inserted verbatim into the body of the servlet class,
outside of any existing methods

JSP Expressions
• Format
– <%= Java Expression %>
• Result
– Expression evaluated, converted to String, and placed
into HTML page at the place it occurred in JSP page
– That is, expression placed in _jspService inside out.print
• Examples
– Current time: <%= new java.util.Date() %>
– Your hostname: <%= request.getRemoteHost() %>
• XML-compatible syntax
– <jsp:expression>Java Expression</jsp:expression>
– XML version not supported by Tomcat 3. Until JSP 1.2,
servers are not required to support it.

JSP/Servlet Correspondence
• Original JSP
<H1>A Random Number</H1>
<%= Math.random() %>
• Possible resulting servlet code
public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
request.setContentType("text/html");
HttpSession session = request.getSession(true);
JspWriter out = response.getWriter();
out.println("<H1>A Random Number</H1>");
out.println(Math.random());
...
}

Example Using JSP
Expressions
<BODY>
<H2>JSP Expressions</H2>
<UL>
<LI>Your hostname: <%= request.getRemoteHost() %>
<LI>Your session ID: <%= session.getId() %>
<LI>The <CODE>testParam</CODE> form parameter:
<%= request.getParameter("testParam") %>
</UL>
</BODY>

Predefined Variables
• request
– The HttpServletRequest (1st arg to doGet)
• response
– The HttpServletResponse (2nd arg to doGet)
• session
– The HttpSession associated with the request (unless
disabled with the session attribute of the page directive)
• out
– The stream (of type JspWriter) used to send output to the
client
• application
– The ServletContext (for sharing data) as obtained via
getServletConfig().getContext().

JSP Scriptlets
• Format
– <% Java Code %>
• Result
– Code is inserted verbatim into servlet's _jspService
• Example
– <%
String queryData = request.getQueryString();
out.println("Attached GET data: " + queryData);
%>
– <% response.setContentType("text/plain"); %>
– <jsp:scriptlet>Java Code</jsp:scriptlet>

• Original JSP
<%= foo() %>
<% bar(); %>
out.println(foo());
bar();
...
}

Example Using JSP Scriptlets
<HTML>
<HEAD>
<TITLE>Color Testing</TITLE>
</HEAD>
<%
String bgColor = request.getParameter("bgColor");
boolean hasExplicitColor;
if (bgColor != null) {
hasExplicitColor = true;
} else {
hasExplicitColor = false;
bgColor = "WHITE";
}
%>
<BODY BGCOLOR="<%= bgColor %>">

JSP Scriptlets: Results

JSP Declarations
• Format
– <%! Java Code %>
• Result
– Code is inserted verbatim into servlet's class definition,
outside of any existing methods
• Examples
– <%! private int someField = 5; %>
– <%! private void someMethod(...) {...} %>
– <jsp:declaration>Java Code</jsp:declaration>

• Original JSP
<H1>Some Heading</H1>
<%!
private String randomHeading() {
return("<H2>" + Math.random() + "</H2>");
}
%>
<%= randomHeading() %>

public class xxxx implements HttpJspPage {
private String randomHeading() {
return("<H2>" + Math.random() + "</H2>");
}
out.println("<H1>Some Heading</H1>");
out.println(randomHeading());
...
}

Example Using JSP
Declarations
<HTML><HEAD><TITLE>JSP Declarations</TITLE>
<LINK REL=STYLESHEET
HREF="JSP-Styles.css"
TYPE="text/css">
</HEAD>
<BODY>
<H1>JSP Declarations</H1>
<%! private int accessCount = 0; %>
<H2>Accesses to page since server reboot:
<%= ++accessCount %></H2>
</BODY>
</HTML>

JSP Declarations: Result
• After 15 total visits by an arbitrary number
of different clients

core
programming
The JSP page
Directive: Structuring
Generated Servlets

Purpose of the page Directive
• Give high-level information about the servlet
that will result from the JSP page
• Can control
– Which classes are imported
– What class the servlet extends
– What MIME type is generated
– How multithreading is handled
– If the servlet participates in sessions
– The size and behavior of the output buffer
– What page handles unexpected errors

The import Attribute
• Format
– <%@ page import="package.class" %>
– <%@ page import="package.class1,...,package.classN" %>
• Purpose
– Generate import statements at top of servlet
• Notes
– Although JSP pages can be almost anywhere on server,
classes used by JSP pages must be in normal servlet dirs
– For Tomcat, this is
install_dir/webapps/ROOT/WEB-INF/classes or
…/ROOT/WEB-INF/classes/directoryMatchingPackage
– For JRun, this is
install_dir/servers/default/default-app/WEB-INF/classes
or …/WEB-INF/classes/directoryMatchingPackage

Example of import Attribute
...
<BODY>
<H2>The import Attribute</H2>
<%-- JSP page directive --%>
<%@ page import="java.util.*,cwp.*" %>
<%-- JSP Declaration --%>
<%!
private String randomID() {
int num = (int)(Math.random()*10000000.0);
return("id" + num);
}
private final String NO_VALUE = "<I>No Value</I>";
%>

Example of import Attribute
(Continued)
<%
Cookie[] cookies = request.getCookies();
String oldID =
ServletUtilities.getCookieValue(cookies, "userID",
NO_VALUE);
String newID;
if (oldID.equals(NO_VALUE)) {
newID = randomID();
} else {
newID = oldID;
}
LongLivedCookie cookie =
new LongLivedCookie("userID", newID);
response.addCookie(cookie);
%>
<%-- JSP Expressions --%>
This page was accessed at <%= new Date() %> with a userID
cookie of <%= oldID %>.
</BODY></HTML>

Example of import Attribute:
Result
• First access
• Subsequent
accesses

The contentType Attribute
• Format
– <%@ page contentType="MIME-Type" %>
– <%@ page contentType="MIME-Type;
charset=Character-Set"%>
• Purpose
– Specify the MIME type of the page generated by the
servlet that results from the JSP page

Generating Excel Spreadsheets
First Last Email Address
Marty Hall hall@corewebprogramming.com
Larry Brown brown@corewebprogramming.com
Bill Gates gates@sun.com
Larry Ellison ellison@microsoft.com
<%@ page contentType="application/vnd.ms-excel" %>
<%-- There are tabs, not spaces, between columns. --%>

The isThreadSafe Attribute
• Format
– <%@ page isThreadSafe="true" %> <%-- Default --%>
– <%@ page isThreadSafe="false" %>
• Purpose
– To tell the system when your code is not threadsafe, so
that the system can prevent concurrent access
• Instructs servlet to implement SingleThreadModel
• Notes
– Default is true -- system assumes you have synchronized
updates to fields & other shared data
– Supplying a value of false can degrade performance

Example of Non-Threadsafe
Code (IDs Must Be Unique)
• What's wrong with this code?
<%! private int idNum = 0; %>
<%
String userID = "userID" + idNum;
out.println("Your ID is " + userID + ".");
idNum = idNum + 1;
%>

Is isThreadSafe Needed Here?
• No
<%! private int idNum = 0; %>
<%
synchronized(this) {
String userID = "userID" + idNum;
out.println("Your ID is " + userID + ".");
idNum = idNum + 1;
}
%>
• Totally safe, better performance in
high-traffic environments

Other Attributes of the page
Directive
• session
– Lets you choose not to participate in sessions
• buffer
– Changes min size of buffer used by JspWriter
• autoflush
– Requires developer to explicitly flush buffer
• extends
– Changes parent class of generated servlet
• errorPage
– Designates a page to handle unplanned errors
• isErrorPage
– Stipulates that page can be used as error page

core
programming
Including Files in
JSP Documents

Including Pages at Request
Time
• Format
– <jsp:include page="Relative URL" flush="true" />
• Purpose
– To reuse JSP, HTML, or plain text content
– JSP content cannot affect main page:
only output of included JSP page is used
– To permit updates to the included content without
changing the main JSP page(s)

Including Pages: Example Code
...
<BODY>
<TABLE BORDER=5 ALIGN="CENTER">
<TR><TH CLASS="TITLE">
What's New at JspNews.com</TABLE>
<P>
Here is a summary of our three most recent news stories:
<OL>
<LI><jsp:include page="news/Item1.html" flush="true" />
</OL>
</BODY>
</HTML>

Including Pages:
Example Result

Including Files at Page
Translation Time
• Format
– <%@ include file="Relative URL" %>
• Purpose
– To reuse JSP content in multiple pages,
where JSP content affects main page
• Notes
– Servers are not required to detect changes to the included
file, and in practice many don't
– Thus, you need to change the JSP files whenever the
included file changes
– You can use OS-specific mechanisms such as the Unix
"touch" command, or
• <%-- Navbar.jsp modified 3/1/02 --%>
<%@ include file="Navbar.jsp" %>

Reusable JSP Content:
ContactSection.jsp
<%@ page import="java.util.Date" %>
<%-- The following become fields in each servlet that
results from a JSP page that includes this file. --%>
<%!
private int accessCount = 0;
private Date accessDate = new Date();
private String accessHost = "<I>No previous access</I>";
%>
<P>
<HR>
This page © 2000
<A HREF="http//www.my-company.com/">my-company.com</A>.
This page has been accessed <%= ++accessCount %>
times since server reboot. It was last accessed from
<%= accessHost %> at <%= accessDate %>.
<% accessHost = request.getRemoteHost(); %>
<% accessDate = new Date(); %>

Using the JSP Content
…
<BODY>
<TR><TH CLASS="TITLE">
Some Random Page</TABLE>
<P>
Information about our products and services.
<P>
Blah, blah, blah.
<P>
Yadda, yadda, yadda.
<%@ include file="ContactSection.jsp" %>
</BODY>
</HTML>

Using the JSP Content: Result

core
programming
Using JavaBeans
Components with JSP

• Beans
• Custom tags
(MVC architecture)
Simple
Application
Complex
Application

Background: What Are Beans?
• Classes that follow certain conventions
– Must have a zero-argument (empty) constructor
– Should have no public instance variables (fields)
– Persistent values should be accessed through methods
called getXxx and setXxx
• If class has method getTitle that returns a String,
class is said to have a String property named title
• Boolean properties use isXxx instead of getXxx
• For more on beans, see
http://java.sun.com/beans/docs/

Basic Bean Use in JSP
• Format
– <jsp:useBean id="name" class="package.Class" />
• Purpose
– Allow instantiation of classes without explicit Java syntax
• Notes
– Simple interpretation: JSP action
<jsp:useBean id="book1" class="cwp.Book" />
can be thought of as equivalent to the scriptlet
<% cwp.Book book1 = new cwp.Book(); %>
– But useBean has two additional features
• Simplifies setting fields based on incoming request
params
• Makes it easier to share beans

Accessing Bean Properties
• Format
– <jsp:getProperty name="name" property="property" />
• Purpose
– Allow access to bean properties (i.e., calls to getXxx
methods) without explicit Java programming language
code
• Notes
– <jsp:getProperty name="book1" property="title" />
is equivalent to the following JSP expression
<%= book1.getTitle() %>

Setting Bean Properties:
Simple Case
• Format
– <jsp:setProperty name="name"
property="property"
value="value" />
• Purpose
– Allow setting of bean properties (i.e., calls to setXxx
methods) without explicit Java code
• Notes
– <jsp:setProperty name="book1"
property="title"
value="Core Servlets and JSP" />
is equivalent to the following scriptlet
<% book1.setTitle("Core Servlets and JSP"); %>

Example: StringBean
package cwp;
public class StringBean {
private String message = "No message specified";
public String getMessage() {
return(message);
}
public void setMessage(String message) {
this.message = message;
}
}
• Installed in normal servlet directory

JSP Page That Uses StringBean
<jsp:useBean id="stringBean" class="cwp.StringBean" />
<OL>
<LI>Initial value (getProperty):
<I><jsp:getProperty name="stringBean"
property="message" /></I>
<LI>Initial value (JSP expression):
<I><%= stringBean.getMessage() %></I>
<LI><jsp:setProperty name="stringBean"
property="message"
value="Best string bean: Fortex" />
Value after setting property with setProperty:
<I><jsp:getProperty name="stringBean"
property="message" /></I>
<LI>
<% stringBean.setMessage("My favorite: Kentucky Wonder"); %>
Value after setting property with scriptlet:
<I><%= stringBean.getMessage() %></I>
</OL>

JSP Page That Uses StringBean

Associating Bean Properties
with Request (Form) Parameters
• If property is a String, you can do
– <jsp:setProperty ... value='<%= request.getParameter("...") %>' />
• Scripting expressions let you convert types,
but you have to use Java syntax
• The param attribute indicates that:
– Value should come from specified request param
– Simple automatic type conversion performed
• Using "*" for the property attribute indicates
that:
– Value should come from request parameter whose name
matches property name
– Simple type conversion should be performed

Setting Bean Properties Case 1:
Explicit Conversion & Assignment
<!DOCTYPE ...>
...
<jsp:useBean id="entry"
class="cwp.SaleEntry" />
<%-- getItemID expects a String --%>
<jsp:setProperty
name="entry"
property="itemID"
value='<%= request.getParameter("itemID") %>' />

<%
int numItemsOrdered = 1;
try {
numItemsOrdered =
Integer.parseInt(request.getParameter("numItems"));
} catch(NumberFormatException nfe) {}
%>
<%-- getNumItems expects an int --%>
<jsp:setProperty
name="entry"
property="numItems"
value="<%= numItemsOrdered %>" />

<%
double discountCode = 1.0;
try {
String discountString =
request.getParameter("discountCode");
discountCode =
Double.valueOf(discountString).doubleValue();
} catch(NumberFormatException nfe) {}
%>
<%-- getDiscountCode expects a double --%>
<jsp:setProperty
name="entry"
property="discountCode"
value="<%= discountCode %>" />

Case 2: Associating Individual
Properties with Input Parameters
<jsp:setProperty
name="entry"
property="itemID"
param="itemID" />
<jsp:setProperty
name="entry"
property="numItems"
param="numItems" />
<jsp:setProperty
name="entry"
property="discountCode"
param="discountCode" />

Case 3: Associating All
Properties with Input Parameters
<jsp:setProperty name="entry" property="*" />

Sharing Beans
• You can use scope attribute to specify
where bean is stored
– <jsp:useBean id="…" class="…" scope="…" />
– Bean still also bound to local variable in _jspService
• Lets multiple servlets or JSP pages share
data
• Also permits conditional bean creation
– Create new object only if you can't find existing one

Values of the scope Attribute
• page
– Default value. Bean object should be placed in the
PageContext object for the duration of the current
request. Lets methods in same servlet access bean
• application
– Bean will be stored in ServletContext (available through
the application variable or by call to getServletContext()).
ServletContext is shared by all servlets in the same Web
application (or all servlets on server if no explicit Web
applications are defined).

Values of the scope Attribute
• session
– Bean will be stored in the HttpSession object associated
with the current request, where it can be accessed from
regular servlet code with getAttribute and setAttribute, as
with normal session objects.
• request
– Bean object should be placed in the ServletRequest object
for the duration of the current request, where it is
available by means of getAttribute

Conditional Bean Operations
• Bean conditionally created
– jsp:useBean results in new bean object only if no bean
with same id and scope can be found
– If a bean with same id and scope is found, the preexisting
bean is simply bound to variable referenced by id
• Bean properties conditionally set
– <jsp:useBean ... />
replaced by
<jsp:useBean ...>statements</jsp:useBean>
– The statements (jsp:setProperty elements) are executed
only if a new bean is created, not if an existing bean is
found

Conditional Bean Creation:
AccessCountBean
public class AccessCountBean {
private String firstPage;
private int accessCount = 1;
public String getFirstPage() {
return(firstPage);
}
public void setFirstPage(String firstPage) {
this.firstPage = firstPage;
}
public int getAccessCount() {
return(accessCount);
}
public void setAccessCountIncrement(int increment) {
accessCount = accessCount + increment;
}
}

Conditional Bean Creation:
SharedCounts1.jsp
<jsp:useBean id="counter"
class="coreservlets.AccessCountBean"
scope="application">
<jsp:setProperty name="counter"
property="firstPage"
value="SharedCounts1.jsp" />
</jsp:useBean>
Of SharedCounts1.jsp (this page),
<A HREF="SharedCounts2.jsp">SharedCounts2.jsp</A>, and
<A HREF="SharedCounts3.jsp">SharedCounts3.jsp</A>,
<jsp:getProperty name="counter" property="firstPage" />
was the first page accessed.
<P>
Collectively, the three pages have been accessed
<jsp:getProperty name="counter" property="accessCount" />
times.
<jsp:setProperty name="counter"
property="accessCountIncrement"
value="1" />

Accessing SharedCounts1,
SharedCounts2, SharedCounts3
• SharedCounts2.jsp was accessed first.
• Pages have been accessed twelve previous
times by an arbitrary number of clients

core
programming
Creating Custom
JSP Tag Libraries

• Beans
• Custom tags
(MVC architecture)
Simple
Application
Complex
Application

Components That Make Up a
Tag Library
• The Tag Handler Class
– Must implement javax.servlet.jsp.tagext.Tag
– Usually extends TagSupport or BodyTagSupport
– Goes in same directories as servlet class files and beans
• The Tag Library Descriptor File
– XML file describing tag name, attributes, and
implementing tag handler class
– Goes with JSP file or at arbitrary URL
• The JSP File
– Imports a tag library (referencing descriptor file)
– Defines tag prefix
– Uses tags

Defining a Simple Tag Handler
Class
• Extend the TagSupport class
• Import needed packages
– import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
import java.io.*;
• Override doStartTag
– Obtain the JspWriter by means of pageContext.getOut()
– Use the JspWriter to generate JSP content
– Return SKIP_BODY
– Translated into servlet code at page-translation time
– Code gets called at request time

Defining a Simple Tag Handler
Class: Example
package cwp.tags;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
import java.io.*;
import java.math.*;
import cwp.*;
public class SimplePrimeTag extends TagSupport {
protected int len = 50;
public int doStartTag() {
try {
JspWriter out = pageContext.getOut();
BigInteger prime =
Primes.nextPrime(Primes.random(len));
out.print(prime);
} catch(IOException ioe) {
System.out.println("Error generating prime: " + ioe);
}
return(SKIP_BODY);
}}

Defining a Simple Tag Library
Descriptor
• Start with XML header and DOCTYPE
• Top-level element is taglib
• Each tag defined by tag element containing:
– name, whose body defines the base tag name.
In this case, I use <name>simplePrime</name>
– tagclass, which gives the fully qualified class name of the
tag handler. In this case, I use
<tagclass>cwp.tags.SimplePrimeTag</tagclass>
– bodycontent, which gives hints to development
environments. Optional.
– info, which gives a short description. Here, I use
<info>Outputs a random 50-digit prime.</info>

TLD File for SimplePrimeTag
<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE taglib ...>
<taglib>
<tlibversion>1.0</tlibversion>
<jspversion>1.1</jspversion>
<shortname>cwp</shortname>
<info>
A tag library from Core Web Programming 2nd Edition,
http://www.corewebprogramming.com/.
</info>
<tag>
<name>simplePrime</name>
<tagclass>cwp.tags.SimplePrimeTag</tagclass>
<info>Outputs a random 50-digit prime.</info>
</tag>
</taglib>

Accessing Custom Tags From
JSP Files
• Import the tag library
– Specify location of TLD file
<%@ taglib uri= "cwp-taglib.tld" prefix= "cwp" %>
– Define a tag prefix (namespace)
<%@ taglib uri="cwp-taglib.tld" prefix= "cwp" %>
• Use the tags
– <prefix:tagName />
• Tag name comes from TLD file
• Prefix comes from taglib directive
– E.g., <cwp:simplePrime />

Using simplePrime Tag
…
<H1>Some 50-Digit Primes</H1>
<%@ taglib uri="cwp-taglib.tld" prefix="cwp" %>
<UL>
<LI><cwp:simplePrime />
</UL>

Using simplePrime Tag:
Result

Intermediate and Advanced
Custom Tags
• Tags with attributes
• Tags that include their body content
• Tags that optionally include their body
• Tags that manipulate their body
• Tags that manipulating their body multiple
times (looping tags)
• Nested tags
• See book for details (related chapter online
in PDF at Java Developer’s Connection)
– http://developer.java.sun.com/developer/Books/cservletsjsp/

Tags with Attributes:
prime Tag
…
<H1>Some N-Digit Primes</H1>
<UL>
<LI>20-digit: <cwp:prime length="20" />
<LI>Default (50-digit): <cwp:prime />
</UL>

Using prime Tag: Result

Including Body Content:
heading Tag
…
<cwp:heading bgColor="#C0C0C0">
Default Heading
</cwp:heading>
<P>
<cwp:heading bgColor="BLACK" color="WHITE">
White on Black Heading
</cwp:heading>
<P>
<cwp:heading bgColor="#EF8429"
fontSize="60" border="5">
Large Bordered Heading
</cwp:heading>
…

Using heading Tag: Result

Optionally Including Tag Body:
debug Tag
Top of regular page. Blah, blah, blah.
<P>
<cwp:debug>
<B>Debug:</B>
<UL>
<LI>Requesting hostname: <%= request.getRemoteHost()%>
<LI>Session ID: <%= session.getId() %>
</UL>
</cwp:debug>
<P>
Bottom of regular page. Blah, blah, blah.

Using debug Tag: Results

Manipulating Tag Body:
the filter Tag
<TR CLASS="COLORED"><TH>Example<TH>Result
<TR>
<TD><PRE><cwp:filter>
<EM>Some emphasized text.</EM><BR>
<STRONG>Some strongly emphasized text.</STRONG><BR>
<CODE>Some code.</CODE><BR>
…
</cwp:filter></PRE>
<TD>
<EM>Some emphasized text.</EM><BR>
<STRONG>Some strongly emphasized text.</STRONG><BR>
<CODE>Some code.</CODE><BR>
…
</TABLE>

Using the filter Tag: Results

Manipulating the Body Multiple
Times: the repeat Tag
<OL>

<cwp:repeat
reps='<%= request.getParameter("repeats") %>'>
<LI><cwp:prime length="40" />
</cwp:repeat>
</OL>

Using the repeat Tag: Results

Nested Tags: the if Tag
<cwp:if>
<cwp:condition>true</cwp:condition>
<cwp:then>Condition is true</cwp:then>
<cwp:else>Condition is false</cwp:else>
</cwp:if>
…
Some coin tosses:<BR>
<cwp:repeat reps="10">
<cwp:if>
<cwp:condition>
<%= Math.random() < 0.5 %>
</cwp:condition>
<cwp:then><B>Heads</B><BR></cwp:then>
<cwp:else><B>Tails</B><BR></cwp:else>
</cwp:if>
</cwp:repeat>

Using the if Tag: Results

Open Source Tag Libraries
http://jakarta.apache.org/taglibs/
• Internationalization (I18N)
• Database access
• Sending email
• JNDITM
• Date/time
• Populating/validating form fields
• Perl regular expressions
• Extracting data from other Web pages
• XSL transformations
• Etc

core
programming
Integrating Servlets
and JSP
The MVC Architecture

• Beans
• Custom tags
(MVC architecture)
Simple
Application
Complex
Application

Why Combine Servlets & JSP?
• Typical picture: use JSP to make it easier to
develop and maintain the HTML content
– For simple dynamic code, call servlet code from scripting
expressions
– For moderately complex cases, use custom classes called
from scripting expressions
– For more complicated cases, use beans and custom tags
• But, that's not enough
– For complex processing, JSP is awkward
– Despite the convenience of separate classes, beans, and
custom tags, the assumption behind JSP is that a single
page gives a single basic look

Architecture
• Approach
– Original request is answered by a servlet
– Servlet processes request data, does database lookup,
accesses business logic, etc.
– Results are placed in beans
– Request is forwarded to a JSP page to format result
– Different JSP pages can be used to handle different types
of presentation
• Terminology
– Often called the “Model View Controller” architecture or
“Model 2” approach to JSP
– Formalized further with Apache “Struts” framework
• See http://jakarta.apache.org/struts/

Dispatching Requests
• First, call the getRequestDispatcher method
of ServletContext
– Supply a URL relative to the Web application root
– Example
• String url = "/presentations/presentation1.jsp";
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(url);
• Second
– Call forward to completely transfer control
to destination page. See following example
– Call include to insert output of destination page
and then continue on. See book.

Forwarding Requests:
Example Code
public void doGet(HttpServletRequest request,
String operation = request.getParameter("operation");
if (operation == null) {
operation = "unknown";
}
if (operation.equals("operation1")) {
gotoPage("/operations/presentation1.jsp",
request, response);
} else if (operation.equals("operation2")) {
gotoPage("/operations/presentation2.jsp",
request, response);
} else {
gotoPage("/operations/unknownRequestHandler.jsp",
request, response);
}
}

Forwarding Requests:
Example Code (Continued)
private void gotoPage(String address,
HttpServletRequest request,
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(address);
dispatcher.forward(request, response);
}

Reminder: JSP useBean
Scope Alternatives
• request
– <jsp:useBean id="..." class="..." scope="request" />
• session
– <jsp:useBean id="..." class="..." scope="session" />
• application
– <jsp:useBean id="..." class="..." scope="application" />
• page
– <jsp:useBean id="..." class="..." scope="page" />
or just
<jsp:useBean id="..." class="..." />
– This scope is not used in MVC architecture

Storing Data for Later Use:
The Servlet Request
• Purpose
– Storing data that servlet looked up and that JSP page will
use only in this request.
• Servlet syntax to store data
SomeClass value = new SomeClass(…);
request.setAttribute("key", value);
// Use RequestDispatcher to forward to JSP page
• JSP syntax to retrieve data
<jsp:useBean
id="key"
class="SomeClass"
scope="request" />

The Session Object
• Purpose
use in this request and in later requests from same client.
session.setAttribute("key", value);
<jsp:useBean
id="key"
class="SomeClass"
scope="session" />

The Servlet Context
• Purpose
use in this request and in later requests from any client.
getServletContext().setAttribute("key", value);
<jsp:useBean
id="key"
class="SomeClass"
scope="application" />

An On-Line Travel Agent

core
programming
JSP Review

Review: JSP Introduction
• JSP makes it easier to create/maintain
HTML, while still providing full access to
servlet code
• JSP pages get translated into servlets
– It is the servlets that run at request time
– Client does not see anything JSP-related
• You still need to understand servlets
– Understanding how JSP really works
– Servlet code called from JSP
– Knowing when servlets are better than JSP
– Mixing servlets and JSP
• Other technologies use similar approach,
but aren’t as portable and don't let you use
Java for the “real code”

• Beans
• Custom tags
(MVC architecture)
Simple
Application
Complex
Application

Review: Calling Java Code
Directly: JSP Scripting Elements
• JSP Expressions
– Format: <%= expression %>
– Evaluated and inserted into the servlet’s output.
• JSP Scriptlets
– Format: <% code %>
– Inserted verbatim into the _jspService method
• JSP Declarations
– Format: <%! code %>
– Inserted verbatim into the body of the servlet class
• Predefined variables
– request, response, out, session, application
• Limit the Java code in page
– Use helper classes, beans, custom tags, servlet/JSP combo

Review: The JSP page Directive:
Structuring Generated Servlets
• The import attribute
– Changes the packages imported by the servlet that results
from the JSP page
• The contentType attribute
– Specifies MIME type of result
– Cannot be used conditionally
• The isThreadSafe attribute
– Turns off concurrent access
– Consider explicit synchronization instead

Review: Including Files in JSP
Documents
• <jsp:include page="Relative URL"
flush="true" />
– Output inserted into JSP page at request time
– Cannot contain JSP content that affects entire page
– Changes to included file do not necessitate changes to
pages that use it
• <%@ include file="Relative URL" %>
– File gets inserted into JSP page prior to page translation
– Thus, file can contain JSP content that affects entire page
(e.g., import statements, declarations)
– Changes to included file might require you to manually
update pages that use it

Review: Using JavaBeans
Components with JSP
• Benefits of jsp:useBean
– Hides the Java programming language syntax
– Makes it easier to associate request parameters with
objects (bean properties)
– Simplifies sharing objects among multiple requests or
servlets/JSPs
• jsp:useBean
– Creates or accesses a bean
• jsp:getProperty
– Puts bean property (i.e. getXxx call) into output
• jsp:setProperty
– Sets bean property (i.e. passes value to setXxx)

Review: Creating Custom
JSP Tag Libraries
• For each custom tag, you need
– A tag handler class (usually extending
TagSupport or BodyTagSupport)
– An entry in a Tag Library Descriptor file
– A JSP file that imports library, specifies prefix,
and uses tags
• Simple tags
– Generate output in doStartTag, return SKIP_BODY
• Attributes
– Define setAttributeName method. Update TLD file
• Body content
– doStartTag returns EVAL_BODY_INCLUDE
– Add doEndTag method

Review: Integrating Servlets
and JSP
• Use MVC (Model 2) approach when:
– One submission will result in multiple basic looks
– Several pages have substantial common processing
• Architecture
– A servlet answers the original request
– Servlet does the real processing & stores results in beans
• Beans stored in HttpServletRequest, HttpSession, or
ServletContext
– Servlet forwards to JSP page via forward method of
RequestDispatcher
– JSP page reads data from beans by means of jsp:useBean
with appropriate scope (request, session, or application)

More Information
• Core Servlets and JavaServer Pages
– http://www.coreservlets.com
– More detail on all topics presented here
• More Servlets and
JavaServer Pages
– http://www.moreservlets.com
– New features in servlets 2.3 and JSP 1.2 (filters and
listeners), Web applications, security, standard JSP tag
library
• Servlet home page
– http://java.sun.com/products/servlet/
• JavaServer Pages home page
– http://java.sun.com/products/jsp/

core
programming
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