Re: Mysterious null pointer exception

[Jon Wingfield]

Your instance variable conn will always be null.
In your init method you are assigning the result of the 
ds.getConnection() to a local variable. Change it from

>            Connection conn = ds.getConnection();
to
>            conn = ds.getConnection();
and you may be ok.
Of course, conn may also be null if your servlet can't find the 
jdbc/empdb datasource...

HTH,
Jon
Vamsee Kanakala wrote:
Hello list users,
   Maybe this is not the best place to ask a general servlet doubt, but 
I'm hoping someone can point out the mistake I'm making. I'm attaching a 
servlet file (FetchEmployeeServlet.java), which for some strange reason, 
gives a Null-Pointer Exception when accessing a function. This tells me 
that the class is not instantiated or something, but I'm new to 
servlets, so I can't figure out why.

TIA,
Vamsee.
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package com.vamsee.empdb;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.sql.*;
import javax.sql.*;
import java.util.*;
import javax.naming.*;
public class FetchEmployeeServlet extends HttpServlet {
    private Connection conn = null;
    private ServletContext context;
    private PreparedStatement pstmt = null;
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        context = config.getServletContext();
        try {
            Context initContext = new InitialContext();
            Context envContext = (Context)initContext.lookup("java:comp/env");
            if (envContext == null)
                throw new Exception("Panic: No Context!");
            DataSource ds = (DataSource)envContext.lookup("jdbc/empdb");
            if (ds != null) {
                Connection conn = ds.getConnection();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    public void service(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        String jsp;
        String cmd = req.getParameter("cmd");
        String idString = req.getParameter("id");
        int id;
        try {
            id = Integer.parseInt(idString);
        } catch (NumberFormatException e) {
            id = 0;
        }
        if ("get".equals(cmd)) {
            EmployeeBean bean = this.fetchEmployee(id);
            req.setAttribute("employee", bean);
            jsp = "/employee.jsp";
        } else {
            List list = this.fetchAll();
            req.setAttribute("list", list);
            jsp = "/list.jsp";
        }
        RequestDispatcher dispatcher;
        dispatcher = context.getRequestDispatcher(jsp);
        dispatcher.forward(req, res);
    }
    public EmployeeBean makeBean(ResultSet results)
        throws SQLException {
        EmployeeBean bean = new EmployeeBean(results.getInt("id"));
        bean.setFirstName(results.getString("fname"));
        bean.setLastName(results.getString("lname"));
        bean.setEmail(results.getString("email"));
        bean.setDepartment(results.getString("department"));
        bean.setImage(results.getString("image"));
        return bean;
    }
    public EmployeeBean fetchEmployee(int id) {
        EmployeeBean bean = null;
        try {
            ResultSet results;
            String sql = "select * from people_table where id = ?";
            PreparedStatement pstmt = conn.prepareStatement(sql);
            synchronized (pstmt) {
                pstmt.clearParameters();
                pstmt.setInt(1, id);
                results = pstmt.executeQuery();
            }
            if (results.next())
                bean = makeBean(results);
            if (results != null)
                results.close();
        } catch (SQLException se) {
            se.printStackTrace();
        }
        return bean;
    }
    public List fetchAll() {
        List list = new ArrayList();
        try {
            ResultSet results;
            Statement st = conn.createStatement();
            System.out.println("Okay until create statement");
            results = st.executeQuery("select * from people_table");
            System.out.println("Okay until execute query");
            while (results.next())
                list.add(makeBean(results));
        } catch (SQLException se) {
            se.printStackTrace();
        }
        return list;
    }
    public void destroy() {
        try {
            if (conn != null)
                conn.close();
        } catch (SQLException e) { }
    }
}

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