16.2 Implicit JSP Scripting Objects

Scripting elements can use predefined variables that the container assigns as references to implicit objects (Table 16-1) to access request and application data. These objects are instances of classes defined by the servlet and JSP specifications. Appendix D contains complete descriptions of all methods for each class, and they are briefly introduced here and used in a number of examples in this chapter.

Table 16-1. Implicit JSP objects

Variable name	Java type
application	javax.servlet.ServletContext
config	javax.servlet.ServletConfig
exception	java.lang.Throwable
out	javax.servlet.jsp.JspWriter
page	java.lang.Object
pageContext	javax.servlet.jsp.PageContext
request	javax.servlet.http.HttpServletRequest
response	javax.servlet.http.HttpServletResponse
session	javax.servlet.http.HttpSession

These objects provide access to the same information (and more) as the implicit variables you can use in EL expressions, but it's not a one-to-one match:

pageContext

The pageContext variable contains a reference to an instance of the class named javax.servlet.jsp.PageContext. It provides methods for accessing references to all the other objects and attributes for holding data that is shared between components in the same page. It's the same object that you can access with the ${pageContext} EL expression. Attribute values for this object represent the page scope; they are the same objects as are available to the EL world as a Map represented by the ${pageScope} expression.

request

The request variable contains a reference to an instance of a class that implements an interface named javax.servlet.http.HttpServletRequest. It provides methods for accessing all the information that's available about the current request, such as request parameters, attributes, headers, and cookies. It's the same object that you can access with the ${pageContext.request} EL expression. Attribute values for this object represent the request scope; they are the same objects as are available to the EL world as a Map represented by the ${requestScope} expression.

response

The response variable contains a reference to an object representing the current response message. It's an instance of a class that implements the javax.servlet.http.HttpServletResponse interface, with methods for setting headers and the status code, and adding cookies. It also provides methods related to session tracking. These methods are the response methods you're most likely to use. The same object can be accessed with the ${pageContext.response} EL expression.

session

The session variable allows you to access the client's session data, managed by the server. It's assigned a reference to an instance of a class that implements the javax.servlet.http.HttpSession interface, which provides access to session data as well as information about the session, such as when it was created and when a request for the session was last received. It's the same object that you can access with the ${pageContext.session} EL expression. Attribute values for this object represent the session scope; they are the same objects as are available to the EL world as a Map represented by the ${sessionScope} expression.

application

The application variable contains a reference to the instance of a class that implements the javax.servlet.ServletContext interface that represents the application. This object holds references to other objects that more than one user may require access to, such as a database connection pool shared by all application users. It also contains log( ) methods you can use to write messages to the container's log file. It's the same object that you can access with the ${pageContext.servletContext} EL expression. Attribute values for this object represent the application scope; they are the same objects as are available to the EL world as a Map represented by the ${applicationScope} expression.

out

The out object is an instance of javax.servlet.jsp.JspWriter. You can use the print( ) and println( ) methods provided by this object to add text to the response message body. In most cases, however, you will just use template text and JSP action elements instead of explicitly printing to the out object.

exception

The exception object is available only in error pages and contains information about a runtime error. It's the same object that you can access with the ${pageContext.exception} EL expression.

The remaining two implicit objects (config and page) are so rarely used in scripting elements that I don't discuss them here. If you're interested, you can read about them in Appendix D.

All variable names listed in Table 16-1 are reserved for the implicit object references. If you declare your own variables in a JSP page, you must not use these reserved variable names.