JSP Expression Language, or JSP EL, can easily access data stored in JavaBeans components.
JSP EL can create arithmetic and logical expressions.
Inside a JSP EL expression, we can use numbers, strings, boolean values, and null.
The following code shows how to use a string to set an attribute value in a JSP tag.
<jsp:setProperty name="box" property="height" value="100"/>
value="100" sets the constant value 100 to box's height property.
With JSP EL we can put an expression in the value attribute for jsp:setProperty in the
following syntax.
${expr}
expr specifies the expression.
The following code uses the JSP EL to in <jsp:setProperty> tag to double the width as the height.
<jsp:setProperty name="box" property="height" value="${2*box.width}"/>
You can use JSP EL expressions within template text for a tag. The following <jsp:text> declaration inserts <h1>Hello JSP!</h1> into the JSP output:
<jsp:text> <h1>Hello JSP!</h1> </jsp:text>
You can include a JSP EL expression in the body of a <jsp:text> tag or any other tag. For example:
<jsp:text>
Box heightis: ${2*box.width}
</jsp:text>
To deactivate the evaluation of EL expressions, we specify the isELIgnored attribute of the page directive as below:
<%@ page isELIgnored ="true|false" %>
JSP Expression Language supports most of the arithmetic and logical operators supported by Java.
| Operator | Description |
|---|---|
| . | Access a bean property |
| [] | Access an array or List element |
| ( ) | Group a expression to change the evaluation order |
| + | Add |
| - | Subtract |
| * | Multiply |
| / or div | Division |
| % or mod | Modulo |
| == or eq | equality |
| != or ne | Not equality |
| < or lt | less than |
| > or gt | greater than |
| <= or le | less than or equal |
| >= or gt | greater than or equal |
| && or and | logical AND |
| || or or | logical OR |
| ! or not | Unary Boolean complement |
| empty | Test for empty variable values |
We can define functions in JSP EL with the following syntax:
${ns:func(param1, param2, ...)}
ns is the namespace of the function, func
is the name of the function and param1 is the first parameter value.
For example, the function fn:length, which is part of the JSTL library can be used as follows to get the the length of a string.
${fn:length("hello")}
To use a function from tag libraries, install the libraries on your server and include the libraries in your JSP using <taglib> directive.
The JSP expression language supports the following implicit objects:
| Implicit object | Description |
|---|---|
| pageScope | page scoped variables |
| requestScope | request scoped variables |
| sessionScope | session scoped variables |
| applicationScope | application scoped variables |
| param | Request parameters as strings |
| paramValues | Request parameters as collections of strings |
| header | HTTP request headers as strings |
| headerValues | HTTP request headers as collections of strings |
| initParam | Context-initialization parameters |
| cookie | Cookie values |
| pageContext | The JSP PageContext object for the current page |
The pageContext object can access pageContext JSP object. For example, to access the incoming query string for a request, use the expression:
${pageContext.request.queryString}
To explicitly access the aVariable variable in the application scope,
access it through the applicationScope variable as applicationScope.aVariable.
The param and paramValues objects can access the parameter values available through the request.getParameter and request.getParameterValues methods.
To access a parameter named myVariable, use the expression ${param.myVariable} or ${param["myVariable"]}.
The following code shows how to access a request parameter named myVariable:
<%@ page import="java.io.*,java.util.*" %> <% String title = "Accessing Request Param"; %> <html> <body> <h1><% out.print(title); %></h1> <p>${param["myVariable"]}</p> </body> </html>
The header and headerValues objects can access the header values available through the request.getHeader and request.getHeaders methods respectively.
To access a header named user-agent, use the expression ${header.user-agent} or ${header["user-agent"]}.
<%@ page import="java.io.*,java.util.*" %> <html> <body> <p>${header["user-agent"]}</p> </body> </html>