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66

Chapter 3 Handling the Client Request: Form Data

the data one way for 

GET

 requests (in traditional CGI, this is usually through

the 

QUERY_STRING

 environment variable) and a different way for 

POST

requests (by reading the standard input in traditional CGI). Second, you have

to chop the pairs at the ampersands, then separate the parameter names (left

of the equal signs) from the parameter values (right of the equal signs).

Third, you have to URL decode the values. Alphanumeric characters are sent

unchanged, but spaces are converted to plus signs and other characters are

converted to %XX where XX is the ASCII (or ISO Latin 1) value of the char 

acter, in hex. Then, the server side program has to reverse the process. For

example, if someone enters a value of  

~hall, ~gates, and ~mcnealy

into a textfield with the name 

users

 in an HTML form, the data is sent as

users=%7Ehall%2C+%7Egates%2C+and+%7Emcnealy

 , and the

server side program has to reconstitute the original string. Finally, the fourth

reason  that  parsing  form  data  is  tedious  is  that  values  can  be  omitted  (e.g.,

param1=val1¶m2=¶m3=val3

 ) or a parameter can have more

than one value (e.g.,  

param1=val1¶m2=val2¶m1=val3

 ), so

your parsing code needs special cases for these situations.

3.2 Reading Form Data from 

Servlets

One of the nice features of servlets is that all of this form parsing is handled

automatically. You simply call the 

getParameter

 method of the 

Http 

ServletRequest

, supplying the case sensitive parameter name as an argu 

ment. You use 

getParameter

 exactly the same way when the data is sent by

GET

 as you do when it is sent by 

POST

. The servlet knows which request

method was used and automatically does the right thing behind the scenes.

The return value is a 

String

 corresponding to the URL decoded value of

the first occurrence of that parameter name. An empty 

String

 is returned if

the parameter exists but has no value, and 

null

 is returned if there was no

such parameter. If the parameter could potentially have more than one value,

you should call 

getParameterValues

 (which returns an array of strings)

instead of 

getParameter

 (which returns a single string). The return value of

getParameterValues

 is 

null

 for nonexistent parameter names and is a

one element array when the parameter has only a single value. 

Parameter names are case sensitive so, for example, 

request.get 

Parameter("Param1")

 and 

request.getParameter("param1")

 are

not interchangeable. 

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

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