Expression Language

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Expression Language

• Lec - 42

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Generating Dynamic Contents

• Technologies available
• Servlets
• JSP
• JavaBeans
• Custom Tags
• Expression Language (EL)
• JSTL
• JavaServer Faces (JSF)

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• Mike
• Hard code developer
• Handles all business logic
• and backend matters
• Expert in Java,
• Database, XML etc.

• Ernie
• Jack of all trades
• Not an expert in anything,
• but will eventually get the job done.

• Philippe
• Web site designer
• Knows how to make a
• Website look really cool !
• HTML / JavaScript expert

Credit Pierre Delisle (spec lead)
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EL Purpose (ni)

• Java as the scripting language in JSP scares many
  people (e.g. Philippe)
• Can we simplify ?
• Expression Language (EL)
• A language adapted for the Web Developer

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EL Benefits
Credit Pierre Delisle (spec lead)
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EL overview (ni)

• Not a programming or scripting language
• Major goal Simplicity (and really is)
• Inspiration from
• JavaScript
• XML path language (XPath)
• Geared towards looking up objects their
• properties and performing simple operation on
• them

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JSP Before EL
2. Must Know Type
1. Must Declare

• lt
• Person p (Person) request.getAttribute(
  person)
• gt
• .
• Person Name lt p.getName() gt
• lt if (p.getAddress( ).equals(defence) )
  gt
• .
• lt gt

4. Knowledge of Scripting Language required
even for simple manipulations
3. Awkward Syntax
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JSP After EL
2. Easier syntax
1. Direct access

• Person Name p.name
• ltcif test p.address param.add gt
• p.name
• lt/cifgt

3. All app data easily accessible
4. Better adapted expression language
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Expression Languagenuggets
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EL nuggets

• Expressions identifiers
• Arithmetic, logical relational operators
• Automatic type conversion
• Access to beans, arrays, lists maps
• Access to set of implicit objects servlet
  properties

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EL Syntax

• Format
• validExpression
• Valid Expressions
• Literals
• Operators
• Variables (object references)
• Implicit call to function using property name

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EL literals
Literals Literal Values
Boolean true or false
Integer Similar to Java e.g. 243, -9642
Floating Point Similar to Java e.g. 54.67, 1.83
String Any string delimited by single or double quote e.g. hello , hello
Null null
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EL literals (cont.)

• Examples
• false lt-- evaluate to false --gt
• 83 lt-- evaluate to 24 --gt

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EL Operators
Type Operator
Arithmetic - / (div) (mod)
Grouping ( )
Logical (and) (or) ! (not)
Relational (eq) ! (ne) lt (lt) gt (gt) lt (le) gt (ge)
Empty prefix operation to determine value is null or empty, returns boolean value
Conditional ?
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EL Operators (cont.)

• Examples
• (65) 5 lt-- evaluate to 35 --gt
• (x gt min) (x lt max)
• empty name
• Returns true if name is
• Empty string (),
• Null etc.

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EL Identifiers

• Identifiers
• Represents the name of the object
• Objects stored in JSP scopes (page, request,
  session, application) referred as scoped
  variables
• EL has 11 reserved identifiers, corresponding to
  11 implicit objects
• All other identifiers assumed to refer to scoped
  variables

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EL Identifiers (cont.)Implicit Objects 1
Category Implicit Object Description
JSP pageContext used to access JSP implicit objects
Scopes pageScope A Map associating names values of page scoped attributes
Scopes requestScope A Map associating names values of request scoped attributes
Scopes sessionScope A Map associating names values of session scoped attributes
Scopes applicationScope A Map associating names values of page scoped attributes
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EL Identifiers (cont.)Implicit Objects 2
Category Operator Description
Request Parameters Param Maps a request parameter name to a single String parameter value
Request Parameters paramValues Maps a request parameter name to an array of values
Request Headers header Maps a request header name to a single header value
Request Headers headerValues Maps a request header name to an array of values
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EL Identifiers (cont.)Implicit Objects 3
Category Operator Description
Cookies cookie A Map storing the cookies accompanying the request by name
Initialization parameters initParam A Map storing the context initialization parameters of the web application by name
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EL Identifiers (cont.)Implicit Objects

• Examples
• pageContext.response
• evaluates to response implicit object of JSP
• param.name
• Equivalent to request.getParameter(name)
• cookie.name.value
• Returns the value of the first cookie with the
  given name
• Equivalent to
• if (cookie.getName().equals(name)
• String val cookie.getValue()

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Example Code Summation of two numbers using EL

• netBeans project - elexample

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EL Identifiers (cont.)Scoped Variables

• Storing Scoped Varibales
• HttpSession ses request.getSession(true)
• Person p new Person()
• P.setName(ali)
• ses.setAttribute(person , p)
• Person p new Person()
• P.setName(ali)
• request.setAttribute(person , p)

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EL Identifiers (cont.)Scoped Variables

• Storing Scoped Varibales
• ltjspuseBean id class scope
• ltjspsetProperty namep propertyname
  valueali/gt

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EL Identifiers (cont.)(ni) Scoped Variables

• Retrieving Scoped Variables
• Any identifier (not an implicit object) is first
  checked against
• page scope,
• then request scope,
• then session scope and
• finally application scope
• If no such variable is located in four scopes,
  null is returned

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EL Identifiers (cont.)Scoped Variables

• Search for a scoped variable, E.g. p.name

page scope
page scope
request scope
request scope
Searching Order
session scope
session scope
name
Found, Calls getName()
p
application scope
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No slide insertion after
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EL Accessors

• The . operator let you access identifiers
  and their properties
• Dot (.) operator
• Typically used for accessing the properties of an
  object
• Bracket ( ) operator
• Typically used to retrieve elements of arrays
  collections

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EL Accessors (cont.)

• Dot (.) operator
• Consider the expression person.name
• The EL accesses object properties using the
  JavaBeans conventions (e.g. getName( ) must be
  defined)
• If property being accessed itself an object, the
  dot operator can be applied recursively.E.g.
• user.address.city

identifier
property
property of address
property object
identifier
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EL Accessors (cont.)

• Bracket ( ) operator
• For arrays collections implementing List
  interface e.g. ArrayList etc.
• Index of the element appears inside brackets
• For example, personList2 returns the 3rd
  element stored in it
• For collections implementing Map interface e.g.
  HashMap etc.
• key is specified inside brackets
• For example, myMapid returns the value
  associated with the id (key)

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EL Robust Features

• Multiple Expression can be combined mixed with
  static text
• For example,
• Hello user.firstName user.lastName
• Automatic type conversion
• EL can automatically wrap and unwrap primitives
  in their corresponding Java classes. E.g.
• begin student.marks

Integer
int
behind the scenes
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EL Robust Features (cont.)

• No NullPointerException ?
• If the object/identifier is null
• For example person.name, e.g. if person is
  null
• No exception would be thrown the result would
  also be null

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Using ExpressionLanguage
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Using EL Expressions

• Can be used in following situations
• As attribute values in standard custom
  actions. E.g.
• ltjspsetProperty id person value . /gt
• In template text the value of the expression is
  inserted into the current output. E.g.
• lth3gt . lt/h3gt
• With JSTL (discussed shortly)

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Example Code Address book using EL

• netBeans project - ExpressionLanguageExample