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10.5 Predefined Variables

245

that 

out

 is used almost exclusively in scriptlets, since JSP expressions 

are automatically placed in the output stream and thus rarely need to 

refer to 

out

 explicitly. 

session

This variable is the 

HttpSession

 object associated with the request. 

Recall that sessions are created automatically, so this variable is bound 

even if there is no incoming session reference. The one exception is if 

you use the 

session

 attribute of the 

page

 directive (see Section 11.4) 

to turn sessions off. In that case, attempts to reference the 

session

variable cause errors at the time the JSP page is translated into a servlet. 

application

This variable is the 

ServletContext

 as obtained via 

getServletCon 

fig().getContext()

. Servlets and JSP pages can store persistent data 

in the 

ServletContext

 object rather than in instance variables. 

Serv 

letContext

 has 

setAttribute

 and 

getAttribute

 methods that let 

you store arbitrary data associated with specified keys. The difference 

between storing data in instance variables and storing it in the 

Servlet 

Context

 is that the 

ServletContext

 is shared by all servlets in the serv 

let engine (or in the Web application, if your server supports such a 

capability). For more information on the use of the 

ServletContext

, 

see Section 13.4 (Sharing Beans) and Chapter 15 (Integrating Servlets 

and JSP).

config

This variable is the 

ServletConfig

 object for this page. 

pageContext

JSP introduced a new class called 

PageContext

 to give a single point of 

access to many of the page attributes and to provide a convenient place 

to store shared data. The 

pageContext

 variable stores the value of the 

PageContext

 object associated with the current page. See Section 13.4 

(Sharing Beans) for a discussion of its use.

page

This variable is simply a synonym for 

this

 and is not very useful in the 

Java programming language. It was created as a place holder for the 

time when the scripting language could be something other than Java. 

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.