JSP Implicit Objects

Implicit Objects in JSP
The JSP expression language defines a set of implicit objects, that are available with all JSP pages. Below are some JSP implicit objects and their description:

• 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

JSP 2.1 provides two EL resolvers to handle expressions that reference these objects: ImplicitObjectELResolver and ScopedAttributeELResolver.

A variable that matches one of the implicit objects is evaluated by ImplicitObjectResolver, which returns the implicit object. This resolver only handles expressions with a base of null. What this means for the following expression is that the ImplicitObjectResolver resolves the sessionScope implicit object only. Once the implicit object is found, the MapELResolver instance resolves the profile attribute because the profile object represents a map.

${sessionScope.profile}

ScopedAttributeELResolver evaluates a single object that is stored in scope. Like ImplicitObjectELResolver, it also only evaluates expressions with a base of null. This resolver essentially looks for an object in all of the scopes until it finds it, according to the behavior of PageContext.findAttribute(String). For example, when evaluating the expression ${product}, the resolver will look for product in the page, request, session, and application scopes and will return its value. If product is not found, null is returned.

When an expression references one of the implicit objects by name, the appropriate object is returned instead of the corresponding attribute. For example, ${pageContext} returns the PageContext object, even if there is an existing pageContext attribute containing some other value.

Reference

•  JSP Implicit Objects