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Handling Form Data

Example: Listing All Form Data
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Note: Chapter 3 of Core Servlets and JavaServer Pages contains much more detailed and up to date information on this topic. The full text is now available online in PDF for free access.

1. 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&param2=&param3=val3) and a parameter can have more than one value in that the same parameter can appear more than once (e.g. param1=val1&param2=val2&param1=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

You can also download the source or try it on-line. Note: also uses ServletUtilities.java, shown earlier.

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" +
                "  <LI>param1: "
                + request.getParameter("param1") + "\n" +
                "  <LI>param2: "
                + request.getParameter("param2") + "\n" +
                "  <LI>param3: "
                + request.getParameter("param3") + "\n" +
                "</UL>\n" +
                "</BODY></HTML>");
  }

  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
    doGet(request, response);
  }
}

2.2 ThreeParams Output

3. Example: Listing All Form Data

Here's an example that looks up all the parameter names that were sent and puts them in a table. It highlights parameters that have zero values as well as ones that have multiple values. First, it looks up all the parameter names via the getParameterNames method of HttpServletRequest. This returns an Enumeration. Next, it loops down the Enumeration in the standard manner, using hasMoreElements to determine when to stop and using nextElement to get each entry. Since nextElement returns an Object, it 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 they are displayed in a bulleted list. Otherwise the one main value is just placed into the table.

3.1 ShowParameters.java

Note: also uses ServletUtilities.java, shown earlier.

package hall;

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.
 *  
 *  Part of tutorial on servlets and JSP that appears at
 *  http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/
 *  1999 Marty Hall; may be freely used or adapted.
 */

public class ShowParameters extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String title = "Reading All Request Parameters";
    out.println(ServletUtilities.headWithTitle(title) +
                "<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.println("<TR><TD>" + paramName + "\n<TD>");
      String[] paramValues = request.getParameterValues(paramName);
      if (paramValues.length == 1) {
        String paramValue = paramValues[0];
        if (paramValue.length() == 0)
          out.print("<I>No Value</I>");
        else
          out.print(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);
  }
}

3.2 Front End to ShowParameters

Here's an HTML form that sends a number of parameters to this servlet. Right click on the source code link to download the HTML. Left click on the link to try it out on-line. It uses POST to send the data (as should all forms that have PASSWORD entries), demonstrating the value of having servlets include both a doGet and a doPost. However, just for the sake of illustration, a version using GET can also be downloaded or tried out on-line.

PostForm.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/hall.ShowParameters"
      METHOD="POST">
  Item Number:
  <INPUT TYPE="TEXT" NAME="itemNum"><BR>
  Quantity:
  <INPUT TYPE="TEXT" NAME="quantity"><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>
    <INPUT TYPE="RADIO" NAME="cardType"
                     VALUE="Visa">Visa<BR>
    <INPUT TYPE="RADIO" NAME="cardType"
                     VALUE="Master Card">Master Card<BR>
    <INPUT TYPE="RADIO" NAME="cardType"
                     VALUE="Amex">American Express<BR>
    <INPUT TYPE="RADIO" NAME="cardType"
                     VALUE="Discover">Discover<BR>
    <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>

3.3 Submission Result

Result of Sending Data to ShowParameters