Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.

306

Chapter 13 Using JavaBeans with JSP

error at page translation time when the 

page

 directive stipulates 

that the current page is not participating in sessions. (See 

Section 11.4,  The session Attribute. )

request

This value signifies that, in addition to being bound to a local 

variable, the bean object should be placed in the 

ServletRequest

 object for the duration of the current request, 

where it is available by means of the 

getAttribute

 method. 

This value is only a slight variation of the per request scope 

provided by 

scope="page"

 (or by default when no 

scope

 is 

specified).

Conditional Bean Creation

To make bean sharing more convenient, there are two situations where

bean related elements are evaluated conditionally.

First, a 

jsp:useBean

 element results in a new bean being instantiated

only if no bean with the same 

id

 and 

scope

 can be found. If a bean with the

same 

id

 and 

scope

 is found, the preexisting bean is simply bound to the vari 

able referenced by 

id

. A typecast is performed if the preexisting bean is of a

more specific type than the bean being declared, and a 

ClassCastExcep 

tion

 results if this typecast is illegal.

Second, instead of 

you can use

statements

The point of using the second form is that the statements between the

jsp:useBean

 start and end tags are executed only if a new bean is created,

not if an existing bean is used. This conditional execution is convenient for

setting initial bean properties for beans that are shared by multiple pages.

Since you don't know which page will be accessed first, you don't know which

page should contain the initialization code. No problem: they can all contain

the code, but only the page first accessed actually executes it. For example,

Listing 13.7 shows a simple bean that can be used to record cumulative

access counts to any of a set of related pages. It also stores the name of the

first page that was accessed. Since there is no way to predict which page in a

set will be accessed first, each page that uses the shared counter has state 

ments like the following:

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

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