Introduction
If you've ever used a Web search engine, visited an on-line bookstore, tracked stocks on-line, or asked a Web-based site for quotes on plane tickets, you've probably seen funny looking URLs like
http://host/path?user=Marty+Hall&origin=bwi&dest=lax.
The part after the question mark (i.e. user=Marty+Hall&origin=bwi&dest=lax) is known as form data, and is the most common way to get data from a Web page to a server-side program. It can be attached to the end of the URL after a question mark (as above), for GET requests, or sent to the server on a separate line, for POST requests.
Extracting the needed information from this form data is traditionally one of the most tedious parts of CGI programming. First of all, you have to read the data one way for GET requests (in traditional CGI, this is usually via the QUERY_STRING environment
variable), and another way for POST requests (usually be reading the standard input). Second, you have to chop the pairs at the ampersands, then separate the parameter names (left of the equals signs) from the parameter values (right of the equals signs).
Third, you have to URL-decode the values. Alphanumeric characters get sent unchanged, but spaces get converted to plus signs and other characters get converted to %XX where XX is the ASCII (or ISO Latin-1) value of the character, in hex. For example,
if someone entered a value of "~hall, ~gates, and ~mcnealy" into a textfield with the name "users" in an HTML form,the data would get sent as
"users=%7Ehall%2C+%7Egates%2C+and+%7Emcnealy".
Finally, the fourth reason that parsing form data is tedious is that values can be omitted (e.g. param1=val1¶m2=¶m3=val3) and a parameter can have more than one value by appearing more than once (e.g. param1=val1¶m2=val2¶m1=val3).
One of the nice features of Java servlets is that all of this form parsing is handled automatically. You simply call the getParameter method of the HttpServletRequest, supplying the parameter name as an argument. Note that parameter names are case sensitive. You do this exactly the same way when the data is sent via GET as you do when it is sent via POST.
The return value is a String corresponding to the uudecoded 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, as in the example above, you should call getParameterValues instead of getParameter. This returns an array of strings. Finally, although in real applications your servlet probably has a specific set of
parameter names it is looking for, for debugging purposes it is sometimes useful to get a full list. Use getParameterNames for this, which returns an Enumeration, each entry of which can be cast to a String and used in a getParameter call.
NEXT
This tutorial is now available as a book: Core Servlets and JavaServer Pages by Marty Hall, published by Sun Microsystems Press.
Read all about it at CoreServlets.com
Server-Side Web Applications using Java Servlets versions 2.1/2.2 and JavaServer Pages (JSP) version 1.0: A Tutorial
© 1999-2000 Marty Hall.
All source code freely available for unrestricted use.
Created for work in the Research and Technology Development Center of the Johns Hopkins University Applied Physics Lab, for courses in the Johns Hopkins Part-Time MS Program in Computer Science, and for various industry seminars and on-site Java short courses.
Please note that this is a first draft of the tutorial, so please send corrections, comments, and suggestions to me at hall@apl.jhu.edu.
Reprinted with permission from the author. Click here to visit the original version
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