FT2283 Web Development

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FT228/3 Web Development
JSP Directives and Scripting elements
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JSP techniques
JSP provides variety of techniques to enable
dynamic processing

• Directive elements

In this topic

• Scripting elements (java)

• Action elements and Java Standard Tags Library

• Java Beans

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Development JSP Pages

• Q Do you need to use Java to develop JSP pages?
• A You can (using scripting elements), but you
  dont have to and there are good reasons not to.
  Can use other techniques provided as part JSP
  technology, in particular The Java Standard
  Tag Library (JSLT) provides a set tags which
  provide the equivalent functionalty of writing
  java code in JSP pages.

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Development JSP Pages Template Text
Template text all text within the JSP page that
is NOT JSP code For web sites, usually template
text HTML but Template text can be anything
WML for mobile devices, javascript, HTML, PDF
etc HTML (or whatever) is just "passed through"
to the client by the servlet created to handle
the page. The HTML can be created by whatever
tools you already are using for building Web
pages. e.g. FrontPage
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Development JSP Pages Template Text example
ltHTMLgt
ltBODYgt
ltTABLE BORDER2gt
lt
    for ( int i 0 i lt 10 i )
        gt
        ltTRgt
        ltTDgtNumberlt/TDgt
        ltTDgtlt i1 gtlt/TDgt
        lt/TRgt
        lt
   
gt
lt/TABLEgt
lt/BODYgt
lt/HTMLgt
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Development JSP Pages Comments
A comment in a JSP page looks like lt--
calculate the sum of x and z here
--gt Everything between the lt-- and --gt is
ignored by the JSP container The comments are
JSP code elements so are NOT send to the browser
(so not visible in View source) -? Hidden
comments
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JSP Pages Directive Elements

• Directive Elements - Used to provide information
  about the general set-up of the page. e.g.
  inclusion of header files, name of page to report
  errors, whether session tracking is required etc
• Always enclosed between lt_at_ gt
• Syntax lt_at_ directivename attribute
  value, attribute value . gt

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JSP Pages Directive Elements

• There are Three directives available to use
• lt_at_ page .. gt
• lt_at_ include gt
• lt_at_ taglib .. gt

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Directive Elements

• Each directive has a set of associated attributes
  (similar to some tags in HTML)
• Usually placed at top of JSP file but doesnt
  have to be
• Example
• lt_at_ page import"java.util., java.lang." gt
• Full list of attributes available at
  http//java.sun.com/products/jsp/syntax/1.1/syntax
  ref11.html

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Directive Elements Page
Page directive - defines attributes that apply to
an entire JSP page. Examples lt_at_page
contentType text/html gt lt_at_page language
java gt lt_at_ page errorPage"error.jsp" gt
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Directive Elements Page
List of attributes includes lt_at_ page
language"java" import"package.class
package., ..." session"truefalse"
isThreadSafe"truefalse" multiple threads
allowed or not info"text" gives info about
the page to administration errorPage"relativeUR
L" contentType"mimeType charsetcharacterS
et " "text/html charsetISO-8859-1"
isErrorPage"truefalse" Specifies whether
exception object available or notgt
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Directive Elements Page

• Details for all Page directive attribute can be
  found at http//java.sun.com/products/jsp/syntax/1
  .1/syntaxref11.html
• Home page for this is located in Web
  development on distrib/ft228-3
• E.g. extract for page directive

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Directive Elements Include
Include directive - Includes a static file in a
JSP file attranslation time Syntax lt_at_ include
file"relativeURL" gt The included file can be
an HTML file, a JSP file, a text file, or a code
file written in the Java programming
language Useful for including repetitive HTML
content across a web site headers, navigation
bars, footers etc Useful for common logic e.g.
date displays
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Directive Elements Include
Example jsp page name includexample.jsp
lthtmlgtltheadgtlttitlegtAn Include Testlt/titlegtlt/headgt
ltbody bgcolor"white"gtltfont color"blue"gtThe
current date and time islt_at_ include
file"date.jsp" gtlt/fontgtlt/bodygtlt/htmlgt
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Directive Elements Include
Example (continued) jsp page name date.jsp
lt_at_ page import"java.util." gtlt (new
java.util.Date() ).toLocaleString() gt
Included into includexample.jsp from previous page
When includexample.jsp is run, displays as The
current date and time areAug 30, 1999 23840
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Directive Elements Taglib
Taglib directive - Defines a tag library and
prefix for the custom tags used in the JSP page.
Syntax lt_at_ taglib uri"URIToTagLibrary
prefix"tagPrefix" gt Example lt_at_ taglib
urihttp//java.sun.com/jstl/core prefixc"
gt More on taglib directives later when we use
Java Standard Tag Library (JSTL)
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Directive Elements summary

• Three directives page, include, taglib
• Used to define general set-up information about
  the JSP page
• By themselves, dont do anything
• At least one used in most JSP pages

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JSP dynamic processing
Done

• Directive elements

• Scripting elements

• Action elements and JSTL

• Java Beans

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Scripting elements
Developers in JSP can insert java code directly
into a JSP pages using scripting elements The
code is executed when the page is requested

• Should be used with extreme care
• Too much code ? difficult to maintain
• Difficult for HTML programmers
• More suitable for simple applications with small
  development team

Q How do you specify that the language being
used in page by scripting elements is java?
A Page directive language attribute
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Scripting elements

• Three types of scripting elements
• Expressions The expression syntax lt ... gt
  defines a scripting language expression ..
  result
• Scriptlets The scriptlet syntax lt ... gt can
  handle declarations, expressions, or any other
  type of code fragment valid in the page scripting
  language.
• When you write a scriptlet, end the scriptlet
  with gt before you switch to HTML, text, or
  another JSP tag
• 3. Declarations The declaration syntax lt! ...
  gt declares variables or methods.

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Scripting elements Expressions

• Expressions
• Contains any valid java expression in the JSP
  page
• Used to output dynamic values directly onto web
  page (e.g. result of a calculation, dates)
• Enclosed between lt and gt
• output as a string
• Syntax lt expression gt e.g. lt 11gt

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Expressions - examples

• E.gs any valid java expression
• lt Math.sqrt(2) gtlt itemsi gtlt a b
  c gtlt new java.util.Date() gt

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Scripting elements Expressions
Example lthtmlgt ltbodygt Current time is lt
new java.util.Date() gt lt/bodygt lt/htmlgt
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Scripting elements Expressions

• Note
• Evaluated at run time. Result is added to the
  response body and output directly to web page
• Can use an expression within a line of text,
  whether or not it is tagged with HTML
• Must be a valid java expression
• No required at end of expression (unlike
  scriptlets)

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Scripting elements

• Three types of scripting elements
• Expressions
• Scriptlets
• Declarations

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Scripting elements Scriptlets

• Scriplets are java code fragments with a JSP page
• Enables more complex functionality than
  expressions
• Java code is placed between lt and gt characters
  (just like expressions, but without the sign at
  the start of the sequence.)
• Can have any number of valid java code statements
• Scriptlet contains Java code that is executed
  every time the JSP is invoked
• Syntax lt code gt

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Scripting elements Scriptlets
Example jsp page that outputs numbers 1 to 10
onto a web page
htmlgt ltheadgt lttitlegtCount to 10 in JSP
scriptletlt/titlegt lt/headgt ltbodygt lt for(int
i1ilt10i) gt ltigtltbr/gt lt gt
lt/bodygt lt/htmlgt
Scriptlets
Expression used to output to page. Cant put
HTML Tags into the scriptlet. Can onlycontain
valid java code
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Scripting elements Scriptlets
Example jsp page that outputs numbers 1 to 10
onto a web page Output in browser will be..
Count to 10 in JSP 1 2 3 4 5 6 7 8 9 10
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Scriplets mixing scriplets with HTML and other
tags

• When you mingle scripting elements with HTML and
  JSP tags, you must always end a scripting element
  before you start using tags and then reopen the
  scripting element afterwards, like this
• lt else gt lt!-- closing the scriptlet
  before the tags start --gt... tags follow
  ...lt gt lt!-- reopening the scriptlet to
  close the language block --gt
• At first, this may look a bit strange, but it
  ensures that the scripting elements are
  transformed correctly when the JSP source file is
  compiled

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Scripting elements Scriptlets
Example 2 jsp page that outputs Hello! The
time is now Wed Sep 03 131758 BST 2003 Your
machine's address is 127.0.0.1 onto a web page
ltHTMLgt ltBODYgt lt java.util.Date date new
java.util.Date() gt Hello! The time is now lt
out.println( date ) out.println( "ltBRgtYour
machine's address is " ) out.println(
request.getRemoteHost()) gt lt/BODYgt lt/HTMLgt
Note could have put this line outside the
scriptlet as HTML
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Scripting elements Scriptlets
Note Any text, HTML tags, or JSP elements you
write must be outside the scriptlet Readability
Mixture of Tags and Java code can be
difficult to read especially for HTML
developers
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Scripting elements Scriptlets
lt if (Math.random() lt 0.5) gt Have a
ltBgtnicelt/Bgt day! lt else gt Have a
ltBgtlousylt/Bgt day! lt gt
In background, JSP container compiles the JSP
page into Java code.. converting the HTML
snippets into java code to out.print statements
---?
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Scripting elements Scriptlets
JSP code on previous page will be converted in
background to the following java code..
if (Math.random() lt 0.5)
out.println("Have a ltBgtnicelt/Bgt day!")
else out.println("Have a ltBgtlousylt/Bgt
day!")
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Scripting elements Scriptlets
Example 3 fragment of JSP page that generates a
table in HTML containing numbers 1 to n
ltTABLE BORDER2gt lt for ( int i 0 i lt n
i ) gt ltTRgt
ltTDgtNumberlt/TDgt ltTDgtlt i1 gtlt/TDgt
lt/TRgt lt gt lt/TABLEgt
HTML within for loop is output n times
Note would need to supply int variable n before
it will work
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Scripting elements

• Three types of scripting elements
• Expressions
• Scriptlets
• Declarations

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Scripting elements Declarations
Declaration Declares a variable or method that
can be used throughout the JSP page Examples lt
! int i 0 gtlt! int a, b, c gtlt! Circle a
new Circle(2.0) gt Syntax lt!
declarations gt
Note the
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Scripting elements Declarations
Declarations dont generate any output onto the
page by themselves -? usually used with
expressions and/or scriptlets Examples lt!
private int accessCount gt Accesses to page
since server reboot lt accessCount gt
Prints the number of times the current page has
been requested since the server was booted
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Scripting elements Declarations
Example Declares method getDate()
lt_at_ page import"java.util." gt ltHTMLgt ltBODYgt lt!
Date theDate new Date() Date
getDate() System.out.println( "In
getDate() method" ) return theDate
gt Hello! The time is now lt getDate()
gt lt/BODYgt lt/HTMLgt
Method getDate() returns a Date object
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Declaration

• Declarations (between lt! and gt tags) contain
  one or more variable or method declarations that
  end or are separated by semicolons
• lt! int i 0 gtlt! int a, b double c gtlt!
  Circle a new Circle(2.0) gt You must declare
  a variable or method in a JSP page before you use
  it in the page. The scope of a declaration is
  usually a JSP file, but if the JSP file includes
  other files with the include directive, the scope
  expands to cover the included files as well.

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Declarations

• Warnings!
• Declaring variables in a JSP page using
  declarations can cause synchronisation problems..
  Compiled servlets see these as variables which
  may be shared across all users using the JSP
  page.
• On previous date example, Date stays the same
  when the page is reloaded.. Because only a single
  instance of the page (and there of the variable
  theDate is available). Better to use local
  variables within declared methods or scriptlets

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Scripting elements - Summary

• Scripting elements enable java code to be
  embedded directly into the JSP page
• Be aware of code readability, complexity,
  maintainability
• Three forms of Scripting elements
• Expressions lt .. gt
• Scriptlets lt ..gt
• Declarations lt! . gt

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Topic summary
Page Include Taglib lt_at_ gt

• Directive elements
• Specify set-up info
• about the JSP page

Expressions lt gt Scriptlets lt code
gt Declarations lt! gt

• Scripting elements
• Enable java code to be embedded into the JSPpage