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When to use applets instead of HTML forms: Three easy techniques

Using applets for web page menus and database interactions

By Mark O. Pendergast, PhD, JavaWorld.com, 04/22/13

Page 6 of 7

Using applets instead of HTML POST forms

The method described in the previous section performs the equivalent of an HTML GET operation. All data is encoded in the URL. Using the get method is not appropriate for all applications, however. Applications that have sensitive data or ones that would result in an encoded URL longer than 2048 characters (that is, the maximum length supported by some browsers) must be implemented using the post method. Communication between an applet and a servlet requires that the applet create an HTTPURLConnection with the web server. The process of creating the connection causes the web server to run the servlet. The HTTPURLConnection contains both an input and an output stream. These streams can then be used by the applet to transmit data to the servlet and receive data back from it.

To open a connection with a servlet, an applet first creates a URL, and then asks the URL to open the connection:

URL link = new URL(getCodeBase()+"FindCustomer"); // url of a servlet
HttpURLConnection urlconnection = (HttpURLConnection) link.openConnection();
Once the connection is open you should set its properties to provide two-way communications in binary mode.
urlconnection.setDoOutput(true);   // turn on output
urlconnection.setDoInput(true);     // turn on input
urlconnection.setUseCaches (false);   // turn off caching
urlconnection.setDefaultUseCaches (false);
// set to binary transmission
urlconnection.setRequestProperty ("Content-Type", "application/octet-stream");

At this point a standard input and output stream are available to the applet for communication with the servlet. Simply call the urlConnection.getOutputStream() or urlConnection.getInputStream() methods to access them. Depending on the needs of the applet, these streams can then be converted to ObjectInput/OutputStreams for object-oriented communication or DataInput/OutputStreams for transmitting primitives. I generally use ObjectInput/OutputStreams since they can perform all the functions of DataInput/OutputStreams:

ObjectOutputStream oos = new ObjectOutputStream(urlconnection.getOutputStream());
  ObjectInputStream ois = new ObjectInputStream(urlconnection.getInputStream());

Once the conversion is complete, the applet can use the output stream to transmit request parameters to the servlet and the input stream to read the data back. All data is transmitted directly between the applet and the servlet. No JavaScript or IFrames are required. Applet signing or special privileges are also not required because the applet is communicating with a servlet residing on its own web server.

The applet-side code presented in Listing 11 is invoked when the user presses the Find button on the Customer Search tab (shown in Figure 4).

Listing 11. Applet Find Customer method

/**
 *
 *  Find customers based on partial last name entered by the user
 * @param evt
 */
    private void findCustomerButtonActionPerformed(java.awt.event.ActionEvent evt) {
        // TODO add your handling code here:


     try{
       DefaultTableModel tm = (DefaultTableModel)resultsTable.getModel();
       String lastName = searchField.getText();
       URL link = new URL(getCodeBase()+"FindCustomer");
       clearResults();
       setMessageLabel("Processing request ...");
       HttpURLConnection urlconnection = (HttpURLConnection) link.openConnection();

       urlconnection.setDoOutput(true);
       urlconnection.setDoInput(true);
       urlconnection.setUseCaches (false);
       urlconnection.setDefaultUseCaches (false);

       // Specify the content type that we will send binary data
       urlconnection.setRequestProperty ("Content-Type", "application/octet-stream");

       ObjectOutputStream oos = new ObjectOutputStream(urlconnection.getOutputStream());
       oos.writeObject(lastName);

       ObjectInputStream ois = new ObjectInputStream(urlconnection.getInputStream());
       while(true)
       {
        Customer c = (Customer)ois.readObject();
        if(c.id.length() == 0)  // empty id indicates last customer
             break;
        else
        {
         Object[] row = {c.id,c.firstName,c.lastName,c.address1,c.address2,c.city,c.state,c.zip,c.phone};
         tm.addRow(row);
        }
       }
       oos.close();
       ois.close();
       setMessageLabel(tm.getRowCount()+" records found");
     }
     catch(Exception ex)
     {
      setMessageLabel("Error processing request : "+ex.toString());
     }

}

With this method we create an HttpURLConnection and set it up for two-way binary communication. The connection is then turned into separate ObjectInputStream and ObjectOutputStreams. The applet method transmits the search text that the user had entered into a searchField. On the server side, the servlet sets the content type to binary and converts the streams to ObjectInputStream and ObjectOutputStream types. The search text is read in, an SQL Select operation is performed, and the resulting data is sent back one record at a time, encapsulated in a Customer object, as shown in Listing 12. Once the last record has been sent the servlet sends a Customer object with a 0 length ID, signaling the end of the data. The stream is then flushed and closed.

Listing 12. FindCustomer Servlet

/**
 *
 * @author Mark Pendergast
 */
@WebServlet(urlPatterns = {"/FindCustomer"})
public class FindCustomer extends HttpServlet {

    /**
     * Processes requests for both HTTP
     * GET and
     * POST methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
                   response.setContentType("application/octet-stream");

         InputStream in = request.getInputStream();
         ObjectInputStream inputFromApplet = new ObjectInputStream(in);
         OutputStream outstr = response.getOutputStream();
         ObjectOutputStream oos = new ObjectOutputStream(outstr);

         try {
           // response.setContentType("application/x-java-serialized-object");
             String lastName = (String)inputFromApplet.readObject();
            if(lastName == null)
                lastName = "";  // default to all

            Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");  // load the driver
            Connection con = DriverManager.getConnection("jdbc:sqlserver://localhost:1433;databaseName=Customer;user=AppletTest;password=AppletTest;");
            PreparedStatement find = con.prepareStatement("select * from Customers where Last_Name like ?");
            find.setString(1, lastName+"%");
            ResultSet rs = find.executeQuery();
            while(rs.next())
            {
             Customer c = new Customer(rs.getString(1), rs.getString(3),rs.getString(2),rs.getString(4),
                               rs.getString(5),rs.getString(6),rs.getString(7),rs.getString(8), rs.getString(9));
             oos.writeObject(c);
            }
            rs.close();
            find.close();
            con.close();
          }
         catch(Exception ex)
         {
           System.out.println(ex.toString());
         }
         finally{
            Customer last = new Customer();
            oos.writeObject(last);  // customer with a blank id indicates the last one

            oos.flush();
        oos.close();

         }

    }

This approach should be appealing to hard-core Java programmers who prefer to avoid mixing Java, JavaScript, HTML, and CSS to create their applications, and to those of us who are frustrated with inconsistent implementations of JavaScript and HTML. Note that you may need to flush the streams to be sure data is sent in a timely manner.

In conclusion

In this article I have demonstrated three useful applications of applets for web development. First I showed you how to use applets to create a multi-level menu system, then I demonstrated techniques for using applets in place of HTML forms for get and post operations. My examples included discussion about three Java classes -- URLEncoder, AppletContext, and HTTPURLConnections -- and their capabilities. All of the techniques demonstrated in this article are relatively secure; that is, none of them require that the applet be signed because none directly accesses a database. While I've walked through the examples, you'll learn much more by studying the code in the article's source file, which contains the following NetBeans projects:

• MenuApplet demonstrates a simple News-Weather-Sports menu.
• URLEncoding-Applet is a client-side applet demonstrating the URL encoding (Get) method
• URLEncoding-JspPages uses server-side JSP pages to demonstrate the URL encoding (Get) method
• ServletStreams-Applet is a client-side applet demonstrating the HTTPURLConnection (Post) method
• ServletStreams-ServletPages uses servlets for the HTTPURLConnection (Post) method

About the author

Dr. Mark Pendergast is an associate professor at Florida Gulf Coast University and a returning JavaWorld contributor (see Resources). He received an MS and PhD from the University of Arizona and a BSE in electrical computer engineering from the University of Michigan. He has worked as an engineer for Control Data Corporation, Harris Corporation, Ventana Corporation, and taught at the University of Florida and Washington State University. His works have appeared in books and journals and he has presented his work at numerous conferences. He is a member of the ACM (Association for Computer Machinery) and IEEE (Institute of Electrical and Electronics Engineers) computer societies.

Read more about Enterprise Java in JavaWorld's Enterprise Java section.