JSP - Expressión Language

Expression Language  2.0

Lenguaje de expresión utilizado principalmente en los JSP para acceder a información de los Java Beans, que pueden estar en algún Ambiente(Aplicación, Sesión, Petición o Página),

Se utilizan como atributos de Etiquetas JSP.

<c:if test="${sessionScope.cart.numberOfItems > 0}"> 
  ...
</c:if> 

<c:if test="${a.nombre == 'Juan'}" >
...
</c:if> 



Variables

Cualquier nombre de Atributo en algún Scope (Contexto).



Objetos Implícitos

The JSP expression language defines a set of implicit objects:




pageContext: The context for the JSP page. Provides access to various objects including:





servletContext: The context for the JSP page's servlet and any web components contained in the same application. See Accessing the Web Context.




session: The session object for the client. See Maintaining Client State.




request: The request triggering the execution of the JSP page. See Getting Information from Requests.




response: The response returned by the JSP page. See Constructing Responses.

In addition, several implicit objects are available that allow easy access to the following objects:




param: Maps a request parameter name to a single value




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




Finally, there are objects that allow access to the various scoped variables described in Using Scope Objects.




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









Literales



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



Operators



In addition to the . and [] operators discussed in Variables, the JSP expression language provides the following operators:




Arithmetic: +, - (binary), *, / and div, % and mod, - (unary)




Logical: and, &&, or, ||, not, !




Relational: ==, eq, !=, ne, <, lt, >, gt, <=, ge, >=, 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.







The precedence of operators highest to lowest, left to right is as follows:




[] .




() - Used to change the precedence of operators.




- (unary) not ! empty




* / div % mod




+ - (binary)




< > <= >= lt gt le ge




== != eq ne




&& and




|| or




? :







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 



EL Expression



Result




${1 > (4/2)}



false




${4.0 >= 3}



true




${100.0 == 100}



true




${(10*10) ne 100}



false




${'a' < 'b'}



true




${'hip' gt 'hit'}



false




${4 > 3}



true




${1.2E4 + 1.4}



12001.4




${3 div 4}



0.75




${10 mod 4}



2




${empty param.Add}



True if the request parameter named Add is null or an empty string




${pageContext.request.contextPath}



The context path




${sessionScope.cart.numberOfItems}



The value of the numberOfItems property of the session-scoped attribute named cart




${param['mycom.productId']}



The value of the request parameter named mycom.productId




${header["host"]}



The host




${departments[deptName]}



The value of the entry named deptName in the departments map




${requestScope['javax.servlet.
forward.servlet_path']}



The value of the request-scoped attribute named javax.servlet.
forward.servlet_path