Web Development

1
Scripting elements

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

2
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

3
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
4
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
5
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

6
Scripting elements Scriptlets
Example 2 jsp page that outputs Hello! The
time is now Wed Sep 03 131758 BST 2006 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
7
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
8
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
---?
9
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!")
10
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
11
Scripting elements

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

12
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
13
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
14
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
15
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.

16
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

17
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

18
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