4.4 Example: Reading All Parameters

The previous example extracts parameter values from the form data according to prespecified parameter names . It also assumes that each parameter has 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. Although this approach is rarely used in production servlets (if you don't know the names of the form parameters, you probably don't know what to do with them), it is quite useful for debugging.

First, the servlet looks up all the parameter names with the getParameterNames method of HttpServletRequest . This method returns an Enumeration that contains the parameter names in an unspecified order. Next , the servlet loops down the Enumeration in the standard manner, using hasMoreElements to determine when to stop and using nextElement to get each parameter name . 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 single value is placed directly into the table.

The source code for the servlet is shown in Listing 4.3; Listing 4.4 shows the HTML code for a front end you can use to try out the servlet. Figures 4-3 and 4-4 show the result of the HTML front end and the servlet, respectively.

Listing 4.3 ShowParameters.java
 package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.*; /** Shows all the parameters sent to the servlet via either  *  GET or POST. Specially marks parameters that have  *  no values or multiple values.  */ public class ShowParameters extends HttpServlet {   public void doGet(HttpServletRequest request,                     HttpServletResponse response)       throws ServletException, IOException {     response.setContentType("text/html");     PrintWriter out = response.getWriter();     String docType =     "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +     "Transitional//EN\">\n";     String title = "Reading All Request Parameters";     out.println(docType +                 "<HTML>\n" +                 "<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +                 "<BODY BGCOLOR=\"#FDF5E6\">\n" +                 "<H1 ALIGN=CENTER>" + title + "</H1>\n" +                 "<TABLE BORDER=1 ALIGN=CENTER>\n" +                 "<TR BGCOLOR=\"#FFAD00\">\n" +                 "<TH>Parameter Name<TH>Parameter Value(s)");  Enumeration paramNames = request.getParameterNames();  while(  paramNames.hasMoreElements()  ) {  String paramName = (String)paramNames.nextElement();  out.print("<TR><TD>" + paramName + "\n<TD>");  String[] paramValues =   request.getParameterValues(paramName);  if (paramValues.length == 1) {         String paramValue = paramValues[0];         if (paramValue.length() == 0)           out.println("<I>No Value</I>");         else           out.println(paramValue);       } else {         out.println("<UL>");         for(int i=0; i<paramValues.length; i++) {           out.println("<LI>" + paramValues[i]);         }         out.println("</UL>");       }     }     out.println("</TABLE>\n</BODY></HTML>");   }   public void doPost(HttpServletRequest request,                      HttpServletResponse response)       throws ServletException, IOException {     doGet(request, response);   } } 
Figure 4-3. HTML form that collects data for the ShowParameters servlet.

graphics/04fig03.jpg

Figure 4-4. Result of the ShowParameters servlet.

graphics/04fig04.jpg

Notice that the servlet uses a doPost method that simply calls doGet . That's because we want it to be able to handle both GET and POST requests . This approach is a good standard practice if you want HTML interfaces to have some flexibility in how they send data to the servlet. See the discussion of the service method in Section 3.6 (The Servlet Life Cycle) for a discussion of why having doPost call doGet (or vice versa) is preferable to overriding service directly. The HTML form from Listing 4.4 uses POST , as should all forms that have password fields (for details, see Chapter 19, "Creating and Processing HTML Forms"). However, the ShowParameters servlet is not specific to that particular front end, so the source code archive site at http://www.coreservlets.com/ includes a similar HTML form that uses GET for you to experiment with.

Listing 4.4 ShowParametersPostForm.html
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE>A Sample FORM using POST</TITLE></HEAD> <BODY BGCOLOR="#FDF5E6"> <H1 ALIGN="CENTER">A Sample FORM using POST</H1> <FORM  ACTION="/servlet/coreservlets.ShowParameters"  METHOD="POST">   Item Number: <INPUT TYPE="TEXT" NAME="itemNum"><BR>   Description: <INPUT TYPE="TEXT" NAME="description"><BR>   Price Each: <INPUT TYPE="TEXT" NAME="price" VALUE="$"><BR>   <HR>   First Name: <INPUT TYPE="TEXT" NAME="firstName"><BR>   Last Name: <INPUT TYPE="TEXT" NAME="lastName"><BR>   Middle Initial: <INPUT TYPE="TEXT" NAME="initial"><BR>   Shipping Address:   <TEXTAREA NAME="address" ROWS=3 COLS=40></TEXTAREA><BR>   Credit Card:<BR>   &nbsp;&nbsp;<INPUT TYPE="RADIO" NAME="cardType"                      VALUE="Visa">Visa<BR>   &nbsp;&nbsp;<INPUT TYPE="RADIO" NAME="cardType"                      VALUE="MasterCard">MasterCard<BR>   &nbsp;&nbsp;<INPUT TYPE="RADIO" NAME="cardType"                      VALUE="Amex">American Express<BR>   &nbsp;&nbsp;<INPUT TYPE="RADIO" NAME="cardType"                      VALUE="Discover">Discover<BR>   &nbsp;&nbsp;<INPUT TYPE="RADIO" NAME="cardType"                      VALUE="Java SmartCard">Java SmartCard<BR>   Credit Card Number:   <INPUT TYPE="PASSWORD" NAME="cardNum"><BR>   Repeat Credit Card Number:   <INPUT TYPE="PASSWORD" NAME="cardNum"><BR><BR>   <CENTER><INPUT TYPE="SUBMIT" VALUE="Submit Order"></CENTER> </FORM> </BODY></HTML> 


Core Servlets and JavaServer Pages (Vol. 1.Core Technologies)
Core Servlets and Javaserver Pages: Core Technologies, Vol. 1 (2nd Edition)
ISBN: 0130092290
EAN: 2147483647
Year: 2002
Pages: 194

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