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 by 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 in that the same parameter can appear 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 servlets probably have a specific set of parameter names they are 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.
2. Example: Reading Three Parameters
Here's a simple example that reads parameters named param1 , param2 , and param3 , listing their values in a bulleted list. Note that, although you are required to specify response settings (content type, status line, other HTTP headings) before beginning to generate the content, there is no requirement that you read the request parameters at any particular time.
Also note you can easily make servlets that can handle both GET and POST data, simply by having its doPost method call doGet or by overriding service (which calls doGet , doPost , doHead , etc.). This is good standard practice, since it requires very little extra work and permits flexibility on the part of the client. If you're used to the traditional CGI approach where you read POST data via the standard input, you should note that there is a similar way with servlets by first calling getReader or getInputStream on the HttpServletRequest . This is a bad idea for regular parameters, but might be of use for uploaded files or POST data being sent by custom clients rather than via HTML forms. Note, however, that if you read the POST data in that manner, it might no longer be found by getParameter .
2.1 ThreeParams.java
package hall; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.*; public class ThreeParams extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html); PrintWriter out = response.getWriter(); String title Reading Three Request Parameters; out.println(ServletUtilities.headWithTitle(title) + <BODY\n+ <H1 ALIGN=CENTER+ title + </H1\n+ <UL\n+ <LIparam1: + request.getParameter("param1) + \n+ <LIparam2: + request.getParameter("param2) + \n+ <LIparam3: + request.getParameter("param3) + \n+ </UL\n+ </BODY</HTML); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }