JSP implicit objects

JSP technology makes available to any JSP page a set of implicit objects. These are Java class instances that are created automatically by the JSP mechanism and that allow interaction with the underlying servlet environment.

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.
When an expression references one of the implicit objects by name, the appropriate object is returned instead of the corresponding attribute.

Using an Implicit Object

Any of the implicit objects discussed in the preceding section may be useful. The following example uses the request object to retrieve and display the value of the username parameter from the HTTP request:

<H3>Hello World<%= request.getParameter("username") %>!</H3>

References By: oracle | oracle share on :        :  

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