JSP Expression Language

1
JSP Expression Language
Celsina Bignoli bignolic_at_smccd.net
2
Expression Language

• Introduced as part of standard JSP in version 2.0
• simple expression language for accessing
  variables
• starts with the delimiter and ends with
• anExpression
• Case sensitive. All keywords are lowercase

3
Using Expressions

• in static text
• The value of the expression is computed and
  inserted into the current output
• In any standard or custom tag attribute that can
  accept an expression
• ltsometag value"expr"/gt
• The expression is evaluated and the result is
  coerced to the attribute's expected type
• ltsometag value"someexprexprtextexpr"
  /gt
• The expressions are evaluated from left to right.
  Each expression is coerced to a String and then
  concatenated with any intervening text. The
  resulting String is then coerced to the
  attribute's expected type.

4
Variables

• The web container evaluates a variable that
  appears in an expression by looking up its value
  according to the behavior of PageContext.findAttri
  bute(String).
• For example, when evaluating the expression
  product, the container will look for product
  in the following scopes
• page
• request
• session
• application
• If product is not found, null is returned.

5
Variables

• Properties of variables are accessed using the .
  operator and can be nested arbitrarily
• For example
• pageContext.session
• myBean.attribute1.attribute2

6
Implicit Variables

• pageContext The context for the JSP page.
  Provides access to various other objects
  including
• session The session object for the client.
• request The request triggering the execution of
  the JSP page.
• response The response returned by the JSP page.

7
Implicit Variables

• param Maps a request parameter name to a single
  value
• ltcout valueparam.userName /gt
• paramValues Maps a request parameter name to an
  array of values
• header Maps a request header name to a single
  value
• headerValues Maps a request header name to an
  array of values
• cookie Maps a cookie name to a single cookie
• initParam Maps a context initialization
  parameter name to a single value

8
Implicit Variables

• pageScope Maps page-scoped variable names to
  their values
• requestScope Maps request-scoped variable names
  to their values
• sessionScope Maps session-scoped variable names
  to their values
• applicationScope Maps application-scoped
  variable names to their values

9
Literals

• The JSP expression language defines the following
  literals
• Boolean true and false
• Integer as in Java
• Floating point as in Java
• String with single and double quotes " is
  escaped as \", ' is escaped as \', and \ is
  escaped as \\.
• Null null

10
Literals

• the JSP expression language provides the
  following operators
• Arithmetic , - (binary), , / and div, and
  mod, - (unary)
• Logical and, , or, , not, !
• Relational , eq, !, ne, lt, lt, gt, gt, lt, ge,
  gt, le. Comparisons can be made against other
  values, or against boolean, string, integer, or
  floating point literals.
• Empty The empty operator is a prefix operation
  that can be used to determine whether a value is
  null or empty.
• Conditional A ? B C. Evaluate B or C,
  depending on the result of the evaluation of A.

11
Reserved Words

• The following words are reserved for the JSP
  expression language and should not be used as
  identifiers
• and   eq   gt   true   instanceof
  or    ne   le   false  empty not   lt   ge   null 
    div   mod

12
Example

• 4.0 gt 3 true
• empty param.add true if parameter add in the
  request is null or empty string
• pageContext.request.contextPath The context
  path
• sessionScope.cart.numberOfItems The value of
  the numberOfItems property of the session-scoped
  attribute named cart