Chapter 24: Creating JavaServer Pages
When a JSP processes a request, it has access to a set of implicit objects, each of which is associated with a given scope. Other objects can be created in scripts. These created objects have a scope attribute that defines where the reference to that object is created and removed.
There are four scopes:
Page – accessible only in the page in which the object is created. Released when the response is returned or the request forwarded.
Request – accessible from pages processing the request in which the object is created. Released when the request has been processed.
Session – accessible from pages processing requests in the same session in which the object is created. Released when the session ends.
Application – accessible from pages processing requests in the same application in which the object is created. Released when the runtime environment reclaims the ServletContext.
References to the object are stored in the PageContext, Request, Session, or Application object, according to the object’s scope.
The following implicit objects are always available within scriptlets and expressions:
request – the request triggering the service invocation.
response – the response to the request.
pageContext – the page context for this JSP.
session – the session object created for the requesting client (if any).
application – the servlet context obtained from the servlet configuration, as in the call getservletConfig().getContext().
out – an object that writes to the output stream.
config – the ServletConfig for this JSP.
page – the instance of this page’s implementation class that is processing the current request. A synonym for this when the programming language is Java.
For information about the scope and type of each implicit object, see the JavaServer Pages Syntax Card .
If the JSP is an error page (the page directive’s isErrorPage attribute is set to true), the following implicit object is also available:
exception – the uncaught Throwable that resulted in the error page being invoked.
For more information, see “Error handling”.
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