Can anyone tell me why i'm getting this: java.lang.NullPointerException

Question

Please lend me your expertise.I hava a feeling its got something to
do with the session.When I run the servlet I get this error:
java.lang.NullPointerException
     at ConnectionPerClient.doGet(connectionperclient.java:85)
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:740)
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)
     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)
     at filters.ExampleFilter.doFilter(ExampleFilter.java:149)
     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213)
     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)
     at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:243)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
     at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
     at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
     at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:190)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
     at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:475)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
     at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
     at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
     at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2347)
     at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:180)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
     at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:170)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
     at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:170)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
     at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:468)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
     at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
     at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
     at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:174)
     at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
     at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
     at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
     at org.apache.catalina.connector.http.HttpProcessor.process(HttpProcessor.java:1027)
     at org.apache.catalina.connector.http.HttpProcessor.run(HttpProcessor.java:1125)
     at java.lang.Thread.run(Thread.java:484)
Here is my code:
import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
class ConnectionHolder implements HttpSessionBindingListener
private Connection con = null;
public ConnectionHolder(Connection con)
this.con = con;
try
con.setAutoCommit(false);
catch(SQLException e)
public Connection getConnection()
return con;
public void valueBound(HttpSessionBindingEvent event)
public void valueUnbound(HttpSessionBindingEvent event)
try
if(con!=null)
con.rollback();
con.close();
catch(SQLException e)
public class ConnectionPerClient extends HttpServlet
public void init(ServletConfig config)throws ServletException
super.init(config);
try
Class.forName("oracle.jdbc.driver.OracleDriver");
catch(ClassNotFoundException e)
public void doGet(HttpServletRequest req, HttpServletResponse res)throws ServletException, IOException
res.setContentType("text/plain");
PrintWriter out = res.getWriter();
HttpSession session = req.getSession(true);
ConnectionHolder holder = (ConnectionHolder) session.getAttribute("servletapp.connection");
if(holder == null)
try
holder = new ConnectionHolder(DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:macha","username","password"));
session.setAttribute("servletapp.connection",holder);
catch(SQLException e)
Connection con = holder.getConnection();
try
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery("SELECT * FROM david.Emp");
//Print start of table and column headers
out.println("<TABLE CELLSPACING=\"0\" CELLPADDING=\"3\" BORDER=\"1\">");
out.println("<TR><TH>EMPNO</TH><TH>ENAME</TH><TH>JOB</TH><TH>SAL</TH></TR>");
//Loop through results of query.
while(rs.next())
out.println("<TR>");
out.println(" <TD>" + rs.getString("EMPNO") + "</TD>");
out.println(" <TD>" + rs.getString("ENAME") + "</TD>");
     out.println(" <TD>" + rs.getString("JOB") + "</TD>");
out.println(" <TD>" + rs.getString("SAL") + "</TD>");
out.println("</TR>");
out.println("</TABLE>");
catch(SQLException e)
out.println("SQLException: " + e.getMessage() + "<BR>");
while((e = e.getNextException()) != null)
out.println(e.getMessage() + "<BR>");
finally
//Clean up resources, close the connection.
if(con != null)
try
con.close();
catch (Exception ignored) {}

Answer 1

Take this code:
ConnectionHolder holder = (ConnectionHolder) session.getAttribute("servletapp.connection");
if(holder == null)
try
holder = new ConnectionHolder(DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:macha","username","password"));
session.setAttribute("servletapp.connection",holder);
catch(SQLException e)
}and change it to:
ConnectionHolder holder = (ConnectionHolder) session.getAttribute("servletapp.connection");
if(holder == null)
try
holder = new ConnectionHolder(DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:macha","username","password"));
session.setAttribute("servletapp.connection",holder);
catch(SQLException e)
    e.printStackTrace(); // THIS IS THE LINE TO ADD IN