How to write CSV file and open it in browser

Question

Two things. First, replace the file writer with the response writer and make sure you tell the response to expect the correct output format. Also, be sure to close the writer if an exception occurs. Anyway, here is my guess for what your code should probably look like:

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package gov.ca.water.preciptemp;

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 *
 * @author hdunsford
 */
public class DailyHydroServlet1 extends HttpServlet {

    private static final Logger logger = Logger.getLogger(DailyHydroServlet1.class.getName());

    /**
     * Processes requests for both HTTP <code>GET</code> and <code>POST</code> 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("text/html;charset=UTF-8");
        Connection conn = null;
        ResultSet result = null;
        PreparedStatement selectStmt = null;
        PrintWriter out;
        try {
            out = response.getWriter();
            conn = DBUtil.getConnection();
            if (request.getParameter("param1").equals("RED9")) {
                selectStmt = conn.prepareStatement("select * from TableName");
            } else if (request.getParameter("param1").equals("RED921")) {
                selectStmt = conn.prepareStatement("select * from TableName");
                result = selectStmt.executeQuery();

                while (result.next()) {

                    out.append(result.getString("PARTC_SSN"));
                    out.append(',');

                    out.append("\n");
                }
                out.flush();
                out.close();
            }
        } catch (Exception ex) {
            try{
                out.close();
            }catch(Exception e){
                //ignore
            }
            logger.log(Level.SEVERE, ex.getMessage(), ex);
        } finally {
            try {
                if (result != null) {
                    result.close();
                    result = null;
                }
                if (selectStmt != null) {
                    selectStmt.close();
                    selectStmt = null;
                }
                if (conn != null) {
                    conn.close();
                    conn = null;
                }

            } catch (SQLException e) {
                DBUtil.log("Exception from ExcelReportServlet class, Method -createNoEcmatsRegDoc "
                                + e);
            }
        }

    }


    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Handles the HTTP <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>

}