JSP Expression Language

Author：Eve Cole Update Time：2024-12-30 16:00:01

JSP Expression Language (EL) makes accessing data stored in JavaBeans very simple. JSP EL can be used to create both arithmetic and logical expressions. Integers, floating point numbers, strings, constants true, false, and null can be used in JSP EL expressions.

a simple syntax

Typically, when you need to specify an attribute value in a JSP tag, you simply use a string:

 <jsp:setProperty name="box" property="perimeter" value="100"/>

JSP EL allows you to specify an expression to represent a property value. A simple expression syntax is as follows:

 ${expr}

Among them,

 expr

refers to an expression. The common operators in JSP EL are

"."

and

 "[]"

. These two operators allow you to access a variety of

 JavaBeans

Properties.

For example, the above

 <jsp:setProperty>

Tags can be rewritten using the expression language as follows:

 <jsp:setProperty name="box" property="perimeter" value="${2*box.width+2*box.height}"/>

When the JSP compiler sees "

${}

"After formatting, it generates code to evaluate the expression and produce a substitute for the expression's value.

You can also use expression languages in the label's template text. For example

 <jsp:text>

The tag simply inserts the text in its body into the JSP output:

 <jsp:text><h1>Hello JSP!</h1></jsp:text>

now, in

 <jsp:text>

Use an expression in the body of the tag, like this:

 <jsp:text>Box Perimeter is: ${2*box.width + 2*box.height}</jsp:text>

Parentheses can be used to organize subexpressions in EL expressions. For example

 ${(1 + 2) * 3}

equals 9, but

 ${1 + (2 * 3)}

Equal to 7.

To disable the evaluation of EL expressions, use

 page

Instructions will

 isELIgnored

The attribute value is set to

 true

：

 <%@ page isELIgnored ="true|false" %>

This way, EL expressions are ignored. If set to

 false

, the container will evaluate the EL expression.

Basic operators in EL

EL expressions support most of the arithmetic and logical operators provided by Java:

Operator	describe
.	Visit a Bean Attribute or a map entry
[]	Access elements of an array or linked list
( )	Organize a subexpression to change precedence
+	add
-	minus or negative
*	take
/ or div	remove
% or mod	Take a mold
== or eq	Test for equality
!= or ne	Test if unequal
< or lt	Test if less than
> or gt	Test whether greater than
<= or le	Test if less than or equal to
>= orge	Test if greater than or equal to
&& or and	Test logical AND
\|\| or or	Test logical OR
! or not	test negation
empty	Test for null value

Functions in JSP EL

JSP EL allows you to use functions in expressions. These functions must be defined in the custom tag library. The syntax for using the function is as follows:

 ${ns:func(param1, param2, ...)}

ns refers to the namespace,

 func

Refers to the name of the function,

 param1

Refers to the first parameter,

 param2

refers to the second parameter, and so on. For example, there is a function

 fn: length

, defined in the JSTL library, can be used to get the length of a string as follows:

 ${fn:length("Get my length")}

To use functions from any tag library, you need to install these libraries in the server and then use

 <taglib>

Tags include these libraries in JSP files.

JSP EL implicit object

JSP EL supports the implicit objects listed in the following table:

Hidden object	describe
pageScope	page scope
requestScope	request scope
sessionScope	session scope
applicationScope	application scope
param	Parameters of Request object, string
paramValues	Parameters of Request object, string collection
header	HTTP header, string
headerValues	HTTP header, string collection
initParam	Context initialization parameters
cookies	Cookie value
pageContext	pageContext of the current page

You can use these objects in expressions just like variables. Next, several examples will be given to better understand this concept.

pageContext object

 pageContext

The object is in JSP

 pageContext

Object reference. Pass

 pageContext

Object that you can access

 request

Object. For example, visit

 request

The query string passed in by the object, like this:

 ${pageContext.request.queryString}

Scope object

 pageScope

 requestScope

 sessionScope

 applicationScope

Variables are used to access variables stored at various scope levels.

For example, if you need to explicitly access the

 applicationScope

Layered

box

Variables can be accessed like this:

applicationScope.box

param and paramValues objects

 param

and

 paramValues

Object used to access parameter values, by using

 request.getParameter

Methods and

 request.getParameterValues

Method.

For example, accessing a file named

 order

parameters, you can use expressions like this:

 ${param.order}

, or

 ${param["order"]}

The following example shows how to access the request in

 username

Parameters:

 <%@ page import="java.io.*,java.util.*" %><% String title = "Accessing Request Param"; %><html><head><title><% out.print(title ); %></title></head><body><center><h1><% out.print(title); %></h1></center><div align="center"><p>${param["username"]}</p></div></body></html>

The param object returns a single string, while

 paramValues

Object returns an array of strings.

header and headerValues objects

 header

and

 headerValues

Object is used to access the information header, by using

 request.getHeader

Methods and

 request.getHeaders

Method.

For example, to access a file named

 user-agent

information header, you can use the expression like this:

 ${header.user-agent}

, or

 ${header["user-agent"]}

The following example shows how to access

 user-agent

Information header:

 <%@ page import="java.io.*,java.util.*" %><% String title = "User Agent Example"; %><html><head><title><% out.print(title ); %></title></head><body><center><h1><% out.print(title); %></h1></center><div align="center"><p>${header["user-agent"]}</p></div></body></html>

The running results are as follows:

The header object returns a single value, while headerValues returns an array of strings.