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70

Chapter 3 Handling the Client Request: Form Data

Figure 3 2

Output of ThreeParams servlet.

3.4 Example: Reading All 

Parameters

The previous example extracted parameter values from the form data based

upon prespecified parameter names. It also assumed that each parameter

had exactly one value. Here's an example that looks up all the parameter

names that are sent and puts their values in a table. It highlights parameters

that have missing values as well as ones that have multiple values. 

First, the servlet looks up all the parameter names by the 

getParameter 

Names

 method of 

HttpServletRequest

. This method returns an 

Enumera 

tion

 that contains the parameter names in an unspecified order. Next, the

servlet loops down the 

Enumeration

 in the standard manner, using 

has 

MoreElements

 to determine when to stop and using 

nextElement

 to get

each entry. Since 

nextElement

 returns an 

Object

, the servlet casts the result

to a 

String

 and passes that to 

getParameterValues

, yielding an array of

strings. If that array is one entry long and contains only an empty string, then

the parameter had no values and the servlet generates an italicized  No Value 

entry. If the array is more than one entry long, then the parameter had multiple

values and the values are displayed in a bulleted list. Otherwise, the one main

value is placed into the table unmodified. The source code for the servlet is

shown in Listing 3.3, while Listing 3.4 shows the HTML code for a front

end that can be used to try the servlet out. Figures 3 3 and 3 4 show the

result of the HTML front end and the servlet, respectively.

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

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