getting parameters from jsp...plz help

i want to show a image using servlet,which is being called by a java file (beans).
in that java how will i get the parameter eg: poll-id=1,2 or 3...n from that jsp page?....here what i've done(.java file) but the its not showing the image(error: data not available):

package myapp.webwork.beans;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.*;
import org.jfree.data.jdbc.*;
import org.jfree.data.general.*;

import java.io.*;
import java.awt.*;
import java.util.*;
import java.awt.image.*;
import org.jfree.data.*;
import org.jfree.chart.*;
import org.jfree.chart.plot.*;
import org.jfree.chart.urls.*;
import org.jfree.chart.entity.*;
import javax.servlet.*;
import javax.servlet.http.*;
import org.jfree.data.general.DefaultPieDataset;
import org.jfree.chart.ChartFactory;
import org.jfree.chart.ChartUtilities.*;
import org.jfree.chart.imagemap.*;
import java.io.PrintWriter.*;
import java.lang.String.*;
import java.lang.Integer.*;
import org.jfree.chart.ChartRenderingInfo.*;



public class votepie {
    
    Connection con;
    PreparedStatement st;
    ResultSet rs;
    //int id;
    
    /**
     * @param args
     */
        
    public PieDataset readData(int id) {
        
        JDBCPieDataset data = null;
        String url = "jdbc:mysql://localhost/vote";
        //Connection con;
        try {
        Class.forName("com.mysql.jdbc.Driver");
        }
        catch (ClassNotFoundException e) {
        System.err.print("ClassNotFoundException: ");
        System.err.println(e.getMessage());
        }
         
        try {
            
            //id = rs.getString("poll_id");
        con = DriverManager.getConnection(url, "vote", "vote001");
        data = new JDBCPieDataset(con);
                
        st = con.prepareStatement("SELECT option_text, counter FROM VOTE_VOTES where poll_id=?");
        
        //PreparedStatement st= con.prepareStatement(sql);
        st.setInt(1, id);
        rs = st.executeQuery();
                
        //data.executeQuery(sql);
        con.close();
        st.close();
        rs.close();
        }
        catch (SQLException e) {
        System.err.print("SQLException: ");
        System.err.println(e.getMessage());
        }
        catch (Exception e) {
        System.err.print("Exception: ");
        System.err.println(e.getMessage());
        }
        return data;
        }
    
    
    public String getvoteserve(HttpServletRequest request, HttpServletResponse response) {
        votepie pd = new votepie();
        int id = new Integer (request.getParameter("poll_id"));
    
        // create the chart...
        JFreeChart chart = ChartFactory.createPieChart(
              "Vote Result Pie Chart",  // chart title
              pd.readData(id),             // data
              true,                // include legend
              true,
              false
        );

        // set the background color for the chart...
        chart.setBackgroundPaint(Color.cyan);
        PiePlot plot = (PiePlot) chart.getPlot();
        plot.setNoDataMessage("No data available");
         
        
        // OPTIONAL CUSTOMISATION COMPLETED.

        ChartRenderingInfo info = null;
        HttpSession session = request.getSession();
        try {

          //Create RenderingInfo object
          response.setContentType("text/html");
          info = new ChartRenderingInfo(new StandardEntityCollection());
          BufferedImage chartImage = chart.createBufferedImage(640, 400, info);

          // putting chart as BufferedImage in session, 
          // thus making it available for the image reading action Action.
          session.setAttribute("chartImage", chartImage);

          PrintWriter writer = new PrintWriter(response.getWriter());
          ChartUtilities.writeImageMap(writer, "imageMap", info, true);
          writer.flush();
        
        }
        catch (Exception e) {
           // handle your exception here
        }
       
        String pathInfo = "http://";
        pathInfo += request.getServerName();
        int port = request.getServerPort();
        pathInfo += ":"+String.valueOf(port);
        pathInfo += request.getContextPath();
        String chartViewer = pathInfo + "/servlet/voteserve";
        return chartViewer;
      }
    }

another thinh when I put thvalue of the "poll_id" manually into the SQL query the image is being shown. so may be the prob lies here. or is it so?....plz kindly suggest what to do....