Expression Languages

Before we dive into JSTL, we will examine one of its most important features: expression languages . Expression languages give JSP developers a mechanism for embedding expressions to be evaluated in place of constants or scriptlets. Consider the following example, in which a JSP must display the amount attribute contained in the JavaBean called PaymentDetail:

<jsp:useBean id="catalog" class="com.officemin.Catalog" scope="application" />
The number of items in the catalog is <%=catalog.getItems().getSize() %>
<!-Other JSP code->

Here, a page author has to use an expression <%= somevalue %> to access the properties of the JavaBean component. If the properties are nested, the expression becomes even more complex:

<%= catalog.getItems().getItem(0).getName()%>

An expression language allows a page author to access an object using a simplified syntax such as <x:sometag att="${aName}"> for a simple variable or

<x:sometag att="${outer.inner.innermost}"> for nested properties.

JSTL defines the syntax for such expression languages, based on ECMA-Script and XPath. To be more precise, the JSP 2.0 developed under JSR-152 owns the syntax from which the JSTL expressions are derived.

The expression language defines a set of implicit objects. When an expression references one of these objects by name, the appropriate object is returned instead of the corresponding attribute. For example, ${pageContext} returns the PageContext object. Most of the implicit objects are modeled as a java.util.Map and hold some name value mapping. Table B.1 lists the implicit objects available.

Table B.1: Implicit objects in JSTL
Implicit Object	Description
pageContext	The PageContext object in JSPs.
pageScope	Maps page-scoped attribute names to their values.
requestScope	Maps request-scoped attribute names to their values. The attributes are available only for the duration of the request.
sessionScope	Maps session-scoped attribute names to their values. Attributes are available until the session is invalidated.
applicationScope	Maps application-scoped attribute names to their values. Attributes are available throughout the application for the life of the container. These map to a ServletContext .getAttribute().
param	Maps parameter names to a single String parameter value using request.getParameter (name).
paramValues	Maps parameter names to a String[] of all values for that parameter using request.getParameter (name).
header	Maps header names to a single String header value using request.getHeader (name).
headerValues	Maps header names to a String[] of all values for the headers.
cookie	Maps cookie names to a single cookie object that is returned using the request.getCookies() method.
initParam	Maps context initialization parameter names to their values using the getInitParameter(name).

For example, to access the URI from the request, the implicit object request can be used, like ${pageContext.request.requestURI}. This will obtain the URI of the request, and the container will map this to HTTPServletRequest.getRequestURI().

To obtain a catalog object from the session, ${sessionScope:catalog} can be used, and the container will call pageContext.getAttribute("catalog", PageContext.SESSION_SCOPE).