NullPointerException - connecting it mySQL

[Jeremiah Elliott]

hey guys, here is the deal: I am a student, who is tryin to learn java. If there is a good tutorial some where,just post a link and I will be glad to RTFM. I am trying to connect to a mySql database from tomcat, but get the following error message:

 type Exception report
message Internal Server Error
description The server encountered an internal error (Internal Server Error) that prevented it from fulfilling this request.
exception
java.lang.NullPointerException
at bean.com.list.ConnectionBean.executeQuery(ConnectionBean.java:46)
at org.apache.jsp.checkpass$jsp._jspService(checkpass$jsp.java:116)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.jasper.servlet.JspServlet$JspServletWrapper.service(JspServlet.java:202)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:382)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:474)
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 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.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:2343)
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:1012)
at org.apache.catalina.connector.http.HttpProcessor.run(HttpProcessor.java:1107)
at java.lang.Thread.run(Thread.java:484)
 

The code for the connection class is here:

 package bean.com.list;
import java.sql.*;
import javax.servlet.http.*;
public class ConnectionBean implements HttpSessionBindingListener
        {
  private Connection connection;
  private Statement statement;
  private static final String login="root";
  private static final String password="";
  private static String strUrl = "jdbc:mysql://localhost/thelist";
   

        public ConnectionBean(){
          try{
      Class.forName("org.gjt.mm.mysql.Driver");
      Connection conn = DriverManager.getConnection (strUrl);
          }
            catch (ClassNotFoundException e){
            System.err.println("ConnectionBean: Driver unavailable");
            connection = null;
          }
            catch(SQLException e){
            System.err.print("ConnectionBean: Driver not loaded");
            connection = null;
            }
          }
          public Connection GetConnection(){
            return connection;
          }
          public void commit() throws SQLException{
            connection.commit();
          }
          public void roleback() throws SQLException{
            connection.rollback();
          }
    public ResultSet executeQuery(String sql) throws SQLException{
return statement.executeQuery(sql);
    }
          public int executeUpdate(String sql) throws SQLException{
            return statement.executeUpdate(sql);
          }
          public void valueBound(HttpSessionBindingEvent event){
            System.err.println("ConnectionBean: in the ValueBond method");
            try{
              if (connection == null || connection.isClosed()){
  //connection = DriverManager.getConnection(dbURL,login,password);
  Connection conn = DriverManager.getConnection (strUrl);
                statement = connection.createStatement();
              }
            }
            catch(SQLException e){connection=null;}
          }
          public void valueUnBound(HttpSessionBindingEvent enten){
            try{
              connection.close();
            }
            catch (SQLException e){}
            finally{
              connection = null;
            }
          }
          protected void finalize(){
            try{
              connection.close();
            }
            catch(SQLException e){}
            }
            
            public void valueUnbound(javax.servlet.http.HttpSessionBindingEvent p1) {
            }
            
          } 

[Jamie Robertson]

I believe your problem code is here:

if (connection == null || connection.isClosed()){
//connection = DriverManager.getConnection(dbURL,login,password);
Connection conn = DriverManager.getConnection (strUrl);
[B]//problem is here.  
statement = connection.createStatement();[/B]              
}

What your code is doing is:
if connection is null then perform "statement = connection.createStatement();" in which connection is null. You do create a new Connection called "conn" if connection is null, but never use it ( you use the old null connection )
Jamie

[Jeremiah Elliott]

Thanks for the tip, however it has not solved the problem. this is what I have done:

  
package bean.com.list;

import java.sql.*;
import javax.servlet.http.*;
public class ConnectionBean implements HttpSessionBindingListener
        {
        private Connection connection;
        private Statement statement;
        private static final String driver = "org.gjt.mm.mysql.Driver";
        private static final String dbURL="jdbc:mysql://localhost/thelist";
        private static final String login="root";
        private static final String password="";
        public ConnectionBean(){
          try{
            Class.forName(driver);
            connection=DriverManager.getConnection(dbURL,login,password);
            statement=connection.createStatement();
          }
            catch (ClassNotFoundException e){
            System.err.println("ConnectionBean: driver unavailable");
            connection = null;
          }
            catch(SQLException e){
            System.err.print("ConnectionBean: Driver not loaded");
            connection = null;
            }
          }
          public Connection GetConnection(){
            return connection;
          }
          public void commit() throws SQLException{
            connection.commit();
          }
          public void roleback() throws SQLException{
            connection.rollback();
          }
          public void setAutoCommit(boolean autoCommit)
            throws SQLException{
            connection.setAutoCommit(autoCommit);
          }
          public ResultSet executeQuery(String sql) throws SQLException{
            return statement.executeQuery(sql);
          }
          public int executeUpdate(String sql) throws SQLException{
            return statement.executeUpdate(sql);
          }
          public void valueBound(HttpSessionBindingEvent event){
            System.err.println("ConnectionBean: in the ValueBond method");
            try{
              if (connection == null || connection.isClosed()){
                connection = DriverManager.getConnection(dbURL,login,password);
                statement = connection.createStatement();
              }
            }
            catch(SQLException e){connection=null;}
          }
          public void valueUnBound(HttpSessionBindingEvent enten){
            try{
              connection.close();
            }
            catch (SQLException e){}
            finally{
              connection = null;
            }
          }
          protected void finalize(){
            try{
              connection.close();
            }
            catch(SQLException e){}
            }
            
            public void valueUnbound(javax.servlet.http.HttpSessionBindingEvent p1) {
            }
            
          }
  

the error is on line 51, based on the tomcat error message. line 51 is:
return statement.executeQuery(sql);
I also tried this code where you where saying the error was:

 Connection conn = DriverManager.getConnection (strUrl);
                statement = conn.createStatement(); 

but it had the same results.
thanks for your time, Jeremiah

[Jamie Robertson]

Jeremiah: This line of code worries me. Maybe someone can verify my thoughts.

if (connection == null || connection.isClosed()){

If I am right, both conditions get evaluated every time. If this is the case, the order of events would be 1. connection == null evaluates to true 2. It tries to evaluate connection.isClosed() but since it is null, it throws a null pointer exception. maybe you can rework your code like this:

if ( connection == null )
{
    connection = ...
    statement = ...
}
else if ( connection.isClosed())
{
    connection = ...
    statement = ....
}

This would ensure that your connection is not null before you call the method isClosed().
let me know if this is the case
Jamie

[Dave Wingate]

Actually, imho, the line you mention uses the logical short-circuit operator and, as such, should never try to invoke the method of connection if the value of connection is null:
if (connection == null || connection.isClosed()){
the short-circuit "or" essentially says "if the first statement is true, just stop looking and return true." So, if it is the case that the value of connection is null, evaluation of the boolean immediately stops and returns true.
had the statement been:
if (connection == null | connection.isClosed())
I think your concern would be valid.

[Bosun Bello]

Yep! You are corret Dave. The second condition only gets evaluated if the first one is false.

[Jeremiah Elliott]

Thanks everyone!!
I got it working now.