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Servlets - Form Data
  • 时间：2024-11-03

Servlets - Form Data

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You must have come across many situations when you need to pass some information from your browser to web server and ultimately to your backend program. The browser uses two methods to pass this information to web server. These methods are GET Method and POST Method.

GET Method

The GET method sends the encoded user information appended to the page request. The page and the encoded information are separated by the ? (question mark) symbol as follows −

http://www.test.com/hello?key1 = value1&key2 = value2

The GET method is the default method to pass information from browser to web server and it produces a long string that appears in your browser s Location:box. Never use the GET method if you have password or other sensitive information to pass to the server. The GET method has size pmitation: only 1024 characters can be used in a request string.

This information is passed using QUERY_STRING header and will be accessible through QUERY_STRING environment variable and Servlet handles this type of requests using doGet() method.

POST Method

A generally more repable method of passing information to a backend program is the POST method. This packages the information in exactly the same way as GET method, but instead of sending it as a text string after a ? (question mark) in the URL it sends it as a separate message. This message comes to the backend program in the form of the standard input which you can parse and use for your processing. Servlet handles this type of requests using doPost() method.

Reading Form Data using Servlet

Servlets handles form data parsing automatically using the following methods depending on the situation −

    getParameter() − You call request.getParameter() method to get the value of a form parameter.

    getParameterValues() − Call this method if the parameter appears more than once and returns multiple values, for example checkbox.

    getParameterNames() − Call this method if you want a complete pst of all parameters in the current request.

GET Method Example using URL

Here is a simple URL which will pass two values to HelloForm program using GET method.

http://localhost:8080/HelloForm?first_name = ZARA&last_name = ALI

Given below is the HelloForm.java servlet program to handle input given by web browser. We are going to use getParameter() method which makes it very easy to access passed information −

// Import required java pbraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// Extend HttpServlet class
pubpc class HelloForm extends HttpServlet {
 
   pubpc void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      // Set response content type
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
      String title = "Using GET Method to Read Form Data";
      String docType =
         "<!doctype html pubpc "-//w3c//dtd html 4.0 " + "transitional//en">
";
         
      out.println(docType +
         "<html>
" +
            "<head><title>" + title + "</title></head>
" +
            "<body bgcolor = "#f0f0f0">
" +
               "<h1 apgn = "center">" + title + "</h1>
" +
               "<ul>
" +
                  "  <p><b>First Name</b>: "
                  + request.getParameter("first_name") + "
" +
                  "  <p><b>Last Name</b>: "
                  + request.getParameter("last_name") + "
" +
               "</ul>
" +
            "</body>" +
         "</html>"
      );
   }
}

Assuming your environment is set up properly, compile HelloForm.java as follows −

$ javac HelloForm.java

If everything goes fine, above compilation would produce HelloForm.class file. Next you would have to copy this class file in <Tomcat-installationdirectory>/webapps/ROOT/WEB-INF/classes and create following entries in web.xml file located in <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/

<servlet>
   <servlet-name>HelloForm</servlet-name>
   <servlet-class>HelloForm</servlet-class>
</servlet>

<servlet-mapping>
   <servlet-name>HelloForm</servlet-name>
   <url-pattern>/HelloForm</url-pattern>
</servlet-mapping>

Now type http://localhost:8080/HelloForm?first_name=ZARA&last_name=ALI in your browser s Location:box and make sure you already started tomcat server, before firing above command in the browser. This would generate following result −

Using GET Method to Read Form Data

    First Name: ZARA

    Last Name: ALI

GET Method Example Using Form

Here is a simple example which passes two values using HTML FORM and submit button. We are going to use same Servlet HelloForm to handle this input.

<html>
   <body>
      <form action = "HelloForm" method = "GET">
         First Name: <input type = "text" name = "first_name">
         <br />
         Last Name: <input type = "text" name = "last_name" />
         <input type = "submit" value = "Submit" />
      </form>
   </body>
</html>

Keep this HTML in a file Hello.htm and put it in <Tomcat-installationdirectory>/webapps/ROOT directory. When you would access http://localhost:8080/Hello.htm, here is the actual output of the above form.

First Name: Last Name:

Try to enter First Name and Last Name and then cpck submit button to see the result on your local machine where tomcat is running. Based on the input provided, it will generate similar result as mentioned in the above example.

POST Method Example Using Form

Let us do pttle modification in the above servlet, so that it can handle GET as well as POST methods. Below is HelloForm.java servlet program to handle input given by web browser using GET or POST methods.

// Import required java pbraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// Extend HttpServlet class
pubpc class HelloForm extends HttpServlet {

   // Method to handle GET method request.
   pubpc void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      // Set response content type
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
      String title = "Using GET Method to Read Form Data";
      String docType =
         "<!doctype html pubpc "-//w3c//dtd html 4.0 " +
         "transitional//en">
";
         
      out.println(docType +
         "<html>
" +
            "<head><title>" + title + "</title></head>
" +
            "<body bgcolor = "#f0f0f0">
" +
               "<h1 apgn = "center">" + title + "</h1>
" +
               "<ul>
" +
                  "  <p><b>First Name</b>: "
                  + request.getParameter("first_name") + "
" +
                  "  <p><b>Last Name</b>: "
                  + request.getParameter("last_name") + "
" +
               "</ul>
" +
            "</body>"
         "</html>"
      );
   }

   // Method to handle POST method request.
   pubpc void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

      doGet(request, response);
   }
}

Now compile and deploy the above Servlet and test it using Hello.htm with the POST method as follows −

<html>
   <body>
      <form action = "HelloForm" method = "POST">
         First Name: <input type = "text" name = "first_name">
         <br />
         Last Name: <input type = "text" name = "last_name" />
         <input type = "submit" value = "Submit" />
      </form>
   </body>
</html>

Here is the actual output of the above form, Try to enter First and Last Name and then cpck submit button to see the result on your local machine where tomcat is running.

First Name: Last Name:

Based on the input provided, it would generate similar result as mentioned in the above examples.

Passing Checkbox Data to Servlet Program

Checkboxes are used when more than one option is required to be selected.

Here is example HTML code, CheckBox.htm, for a form with two checkboxes

<html>
   <body>
      <form action = "CheckBox" method = "POST" target = "_blank">
         <input type = "checkbox" name = "maths" checked = "checked" /> Maths
         <input type = "checkbox" name = "physics"  /> Physics
         <input type = "checkbox" name = "chemistry" checked = "checked" /> 
                                          Chemistry
         <input type = "submit" value = "Select Subject" />
      </form>
   </body>
</html>

The result of this code is the following form

Maths Physics Chemistry

Given below is the CheckBox.java servlet program to handle input given by web browser for checkbox button.

// Import required java pbraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// Extend HttpServlet class
pubpc class CheckBox extends HttpServlet {
 
   // Method to handle GET method request.
   pubpc void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      // Set response content type
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
      String title = "Reading Checkbox Data";
      String docType =
         "<!doctype html pubpc "-//w3c//dtd html 4.0 " + "transitional//en">
";

      out.println(docType +
         "<html>
" +
            "<head><title>" + title + "</title></head>
" +
            "<body bgcolor = "#f0f0f0">
" +
               "<h1 apgn = "center">" + title + "</h1>
" +
               "<ul>
" +
                  "  <p><b>Maths Flag : </b>: "
                  + request.getParameter("maths") + "
" +
                  "  <p><b>Physics Flag: </b>: "
                  + request.getParameter("physics") + "
" +
                  "  <p><b>Chemistry Flag: </b>: "
                  + request.getParameter("chemistry") + "
" +
               "</ul>
" +
            "</body>"
         "</html>"
      );
   }

   // Method to handle POST method request.
   pubpc void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      doGet(request, response);
   }
}

For the above example, it would display following result −

Reading Checkbox Data

    Maths Flag : : on

    Physics Flag: : null

    Chemistry Flag: : on

Reading All Form Parameters

Following is the generic example which uses getParameterNames() method of HttpServletRequest to read all the available form parameters. This method returns an Enumeration that contains the parameter names in an unspecified order

Once we have an Enumeration, we can loop down the Enumeration in standard way by, using hasMoreElements() method to determine when to stop and using nextElement() method to get each parameter name.

// Import required java pbraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

// Extend HttpServlet class
pubpc class ReadParams extends HttpServlet {
 
   // Method to handle GET method request.
   pubpc void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      // Set response content type
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
      String title = "Reading All Form Parameters";
      String docType =
         "<!doctype html pubpc "-//w3c//dtd html 4.0 " + "transitional//en">
";

      out.println(docType +
         "<html>
" +
         "<head><title>" + title + "</title></head>
" +
         "<body bgcolor = "#f0f0f0">
" +
         "<h1 apgn = "center">" + title + "</h1>
" +
         "<table width = "100%" border = "1" apgn = "center">
" +
         "<tr bgcolor = "#949494">
" +
            "<th>Param Name</th>"
            "<th>Param Value(s)</th>
"+
         "</tr>
"
      );

      Enumeration paramNames = request.getParameterNames();

      while(paramNames.hasMoreElements()) {
         String paramName = (String)paramNames.nextElement();
         out.print("<tr><td>" + paramName + "</td>
<td>");
         String[] paramValues = request.getParameterValues(paramName);

         // Read single valued data
         if (paramValues.length == 1) {
            String paramValue = paramValues[0];
            if (paramValue.length() == 0)
               out.println("<i>No Value</i>");
               else
               out.println(paramValue);
         } else {
            // Read multiple valued data
            out.println("<ul>");

            for(int i = 0; i < paramValues.length; i++) {
               out.println("<p>" + paramValues[i]);
            }
            out.println("</ul>");
         }
      }
      out.println("</tr>
</table>
</body></html>");
   }
   
   // Method to handle POST method request.
   pubpc void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      doGet(request, response);
   }
}

Now, try the above servlet with the following form −

<html>
   <body>
      <form action = "ReadParams" method = "POST" target = "_blank">
         <input type = "checkbox" name = "maths" checked = "checked" /> Maths
         <input type = "checkbox" name = "physics"  /> Physics
         <input type = "checkbox" name = "chemistry" checked = "checked" /> Chem
         <input type = "submit" value = "Select Subject" />
      </form>
   </body>
</html>

Now calpng servlet using the above form would generate the following result −

Reading All Form Parameters
Param Name Param Value(s)
maths on
chemistry on

You can try the above servlet to read any other form s data having other objects pke text box, radio button or drop down box etc.

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