DT2283 Web Development

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DT228/3 Web Development
JSP Actions elements and JSTL
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JSP Action Elements
JSP pre-defined tags (standard actions elements)
Done
External Tag library -------? Custom
Done -------? Java Standard tag library
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JSP Action elements External tag libraries JSTL

• JSP 1.2 introduced supported for a special tag
  librarycalled the JSP Standard Tag Library
  (JSTL)
• Version 1.0 released in June 2002Version 1.1
  released in June 2004
• Specification Evaluation 1.2 Maintenance Release
  2
• released in Aug 2006
• The JSTL saves programmers from having to develop
  custom tag libraries for a range of common
  tasks, suchas database access, conditional loops
  etc
• Enabled developers to produce more maintainable,
  simpler JSP code
• Important development for JSP technology

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JSTL

• The JSP Standard Tag Library groups actions into
  four libraries as follows

more may be added in later releases
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JSTL

• To use any of these libraries in a JSP, need to
  declare using the taglib directive in the JSP
  page, specifying the URI and the Prefix

Example of declaring use of core library lt_at_
taglib prefix c uri http//java.sun.com/js
p/jstl/core gt
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JSTL Example
Example JSP page using JSTL that outputs 1 to 10
on a webpageusing the ltcforEachgt and ltcoutgt
tags of the core library
lt_at_ taglib uri"http//java.sun.com/jsp/jstl/core"
prefix"c" gt lthtmlgt ltheadgt lttitlegtCount to
10 Example (using JSTL)lt/titlegt lt/headgt
ltbodygt ltcforEach var"i" begin"1" end"10"
step"1"gt ltcout value"i" /gt ltbr /gt
lt/cforEachgt lt/bodygt lt/htmlgt
A taglib directive declare use of core library
JSTL tag examples
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JSTL Example ltcforeachgt
Looking more closely at ltcforEach taggt ..
ltcforEach var"i" begin"1" end"10" step"1"gt
ltcout value"i" /gt ltbr /gt
lt/cforEachgt lt/bodygt lt/htmlgt
The ltforEachgt tag enables loop logic. In this
case, will lookthrough 10 times. Equivalent to
java for loop Closing tag ltcforEachgt placed
after the end of body of loop
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JSTL Example ltcforeachgt
All JSTL tags have a set of attributes (similar
to HTML tags..) e.g. ltcforeachgt tag has 6
attributes var, items, varStatus,
begin, end, step The full details for each
attribute is in the JSTL specification document
(on distrib). See p 65 for ltcforeachgt tag
definition Willl need to use this document to
verify WHICH tag should be used and HOW is
should be used
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JSTL Example ltcoutgt
ltcoutgt .. outputs a value to webpage.
Usually uses just one attribute
value Examples ltcout value"i" /gt ltcout
valueThe result of 1 2 is 12 /gt ltcout
valueparam.userName" /gt
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JSTL Example ltcifgt
ltcifgt .. evaluates a condition. Uses an
attribute test to hold the condition Example
lt-- Simple if conditions --gt ltcif
test'param.p "someValue"'gt Generate
this template text if p equals someValue lt/cifgt
Example 2 ltcif test'param.p'gt Generate
this template text if p equals "true lt/cifgt
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JSTL Multiple if conditions
An if/else action requires the use of the
ltcchoosegt tag Syntax ltcchoosegt body
content (ltwhengt and ltotherwisegt
subtags) lt/cchoosegt
See page 52 of JSTL specification doc.
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JSTL Multiple if conditions
Uses ltcchoosegt, ltcwhengt and ltcotherwisegt Examp
le ltcchoosegt ltcwhen test'param.p
"0"'gt ltcout value zero recorded/gt
lt/cwhengt ltcwhen test'param.p "1"'gt
Generate this ltcout value single value/gt
lt/cwhengt ltcotherwisegt ltcout value Set
to param.p/gt lt/cotherwisegt lt/cchoosegt
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JSTL Other core ltc..gt actions
Other examples (NOT a complete list!)
ltcsetgt .sets the value of a variable
ltcremovegt .removes a scoped
variable ltccatchgt .catches an exception ltcurlgt
.. encodes a URL ltcimportgt imports the content
of a resource ltcredirectgt.. redirects to another
URL ltcparamgt.. adds a request parameter to other
actions
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JSTL ltfmt..gt example
JSTL contains a set of actions in the Formatting
library these tags are useful for formatting
numbers, times and dates e.g. ltfmtparseDategt
(on page 140 of JSTL specification)
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JSTL ltfmtparseDategt example
lt_at_ taglib uri"http//java.sun.com/jsp/jstl/fmt"
prefixfmt" gt lthtmlgt etc etc ltfmtparseDate
value param.empDate var parsedEmpDate
type date pattern yyyy-MM-dd /gt etc
etc lt/htmlgt
The fmtparseDate action takes the date or time
string specified by the value attribute (e.g.
2001-09-28) , interprets it according to the
pattern defined by the pattern attribute and
saves it in a variable called parsedEmpDate
more later
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JSTL other ltfmtgt actions
Other examples
ltfmtformatNumbergt - formats a numeric value
e.g. number of digits, currency, decimal
place e.g. ltfmtformatNumber value"12.3"
pattern".000"/gt will output 12.300 ltfmtforma
tDategt --formats a date and time
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JSTL Expression language

• Up to now, could only use Java expressions to
  assign dynamic values ? syntax errors common
• JSTL now provides an expression language (EL) to
  support the tags ? simpler syntax, less errors
• The EL is now part of the JSP specification (as
  of versions JSP2.0) can be used in JSTL tags or
  directly in JSP pages.

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JSTL Expression language

• All EL expressions are evaluated at runtime
• The EL usually handles data type conversion and
  null values --? easy to use
• An EL expression always starts with a and ends
  with a

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JSTL Expression language

• The expression can include
• - literals ( 1, 100 etc)
• - variables
• - implicit variables (more later)
• Examples
• ltcout value 123 /gt
• ltcif test param.Address D6 /gt

expression
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JSTL Expression language - operators
! lt
- / or div
gt lt gt

• Logical operators consist of (or and), (or
  or), and ! (or not).
• The empty operator is a prefix operator that can
  used to determine if a value is null or empty.
  For example
• ltcif testempty param.namegt
• Please specify your name.
• lt/cifgt

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JSP Implicit objects

• In JSP, need to be able to access information
  about the environment in which the page is
  running e.g. the parameters passed in a request
  for a form, the browser type of the user etc.
• Implicit objects are a set of Java objects that
  the JSP Container makes available to developers
  in each page. These objects may be accessed as
  built-in variables via scripting elements

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JSTL implicit variables

• The JSTL EL allows these objects to be accessed
  as Implicit Variables
• Implicit variable are just pre-agreed fixed
  variable names that can be used in JSTL
  Expressions
• --? Think of as variables that are
  automatically available to your JSP page..

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JSTL Expression language Implicit Objects

• Full set of implicit variables on page 211 of
  JSTL documentation (includes header,
  pageScope..) or on page 74 of OReillys Java
  Server Pages book
• Very common implicit object is param
• param refers to parameter passed in a request
  message (e.g. information entered into a form by
  a user).
• e.g. ltcout value param.userName/gt
• Further Examples of using param in next topic

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Comparing JSTL vs Scriptlets
JSTL removes complexity by using tags instead of
java code(abstraction) JSP pages using JSTL
usually easier to maintain JSTL allows HTML
tag developers to program JSTL often more
difficult to debug Note Using JSTL does not
eliminate scriplets entirely.. may still need
them sometimes for more complex logic
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Info on JSTL
1. Java Server Pages by Hans Bergsten full
reference on all JSTL tags in the Appendix. 2.
Suns JSTL 1.0 tag specification on distrib.
Good for definition of each tag. Poor on
examples. 3. Specification Evaluation 1.2
Maintenance Release 2 on distrib.
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Setting up JSTL in a web application

• JSTL is provided as part of Tomcat 5
• Ensure the two JSTL .jar files are copied into
  the WEB-INF\lib directory of your web application
• Ensure the taglib directive(s) is in your JSP page