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16.9 Hidden Fields

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16.9 Hidden Fields

Hidden fields do not affect the appearance of the page that is presented to

the user. Instead, they store fixed names and values that are sent unchanged

to the server, regardless of user input. Hidden fields are typically used for

three purposes. 

First, they are one method of tracking users as they move around within a

site (see Section 9.1,  The Need for Session Tracking ). Servlet authors typi 

cally rely on the servlet session tracking API (Section 9.2) rather than

attempting to implement session tracking at this low level. 

Second, hidden fields are used to provide predefined input to a server side

program when a variety of static HTML pages act as front ends to the same

program on the server. For example, an on line store might pay commissions

to people who refer customers to their site. In this scenario, the referring

page could let visitors search the store's catalog by means of a form, but

embed a hidden field listing its referral ID. 

Third, hidden fields are used to store contextual information in pages that

are dynamically generated. For example, in the order confirmation page of

the on line store developed in Section 9.4, each row in the table corresponds

to a particular item being ordered (see Figure 9 6). The user can modify the

number of items ordered, but there is no visible form element that stores the

item ID. So, a hidden form is used (see Listing 9.5).

HTML Element:

(No End Tag)

Attributes:

NAME

 (required), 

VALUE

This element stores a name and a value, but no graphical element is created

in the browser. The name/value pair is added to the form data when the form

is submitted. For instance, with the following example, 

itemID=hall001

 will

always get sent with the form data.

Note that the term  hidden  does not mean that the field cannot be dis 

covered by the user, since it is clearly visible in the HTML source. Because

there is no reliable way to  hide  the HTML that generates a page, authors

are cautioned not to use hidden fields to embed passwords or other sensitive

information.

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