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From "Jon Wingfield" <jon.wingfi...@mkodo.com>
Subject RE: NullPointerException when using JDBC ResultSet next() method
Date Tue, 09 Apr 2002 07:52:09 GMT
Maybe reading the code wrong but you're declaring the variable rs on the
third line of doGet(...) but putting the return value of the
stmt.executeQuery() in a variable called rset. Then you call next() on the
original variable, which is null ;)

A few other points:
You are definately loading the driver otherwise a ClassNotFoundException
would be thrown from Class.forName(...). You also would not have been able
to get a Connection or prepare a Statement object.
Mixing application logic and presentation code in a servlet can lead to a
maintenance nightmare. You can use custom jsp iterator tags to separate out
the application code. We use these a lot ;)
Also, from a maintenance point of view you may want to encapsulate the
database connection/sql generation code into domain specific classes. The
connection parameters and, possibly, the sql could then be placed in
configuration files. Makes changing database easy ;)

Hope this helps,
Jon

-----Original Message-----
From: Philip Kazmier, CEM R&D [mailto:philip.kazmier@nice.com]
Sent: 08 April 2002 22:55
To: 'Tomcat Users List'
Subject: RE: NullPointerException when using JDBC ResultSet next()
method


I apologize but I posted the wrong code.  Here is the correct code.  Line 45
is rs.next.  I am not sure that the driver has loaded properly.  How can I
tell?

Thanks.

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class ListAllOpenBugs extends HttpServlet
{
	public void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException
	{
		Connection con = null;
		//Statement stmt = null;
		ResultSet rs = null;
		int i = 0;

		res.setContentType("text/html");
		PrintWriter out = res.getWriter();

		try
		{
			// Load Oracle driver
			//DriverManager.registerDriver (new
oracle.jdbc.driver.OracleDriver());

			// Load the MySQL driver
			Class.forName("org.gjt.mm.mysql.Driver");

			// Connect to the local database
			//Connection conn =

//DriverManager.getConnection("jdbc:oracle:thin:172.24.230.20:1521:ORCL",
"system", "manager");

			// Get a connection to the database
			con =
DriverManager.getConnection("jdbc:mysql://localhost:3306/pssoftware?user=phi
lk&password=kirov");

			// Query the employee names
			Statement stmt = con.createStatement ();
			//ResultSet rset = stmt.executeQuery ("select
b.bug_row_id, b.status, ps.name, b.short_desc, b.date_opened from bug b,
ps_software ps where b.software_id = ps.pss_row_id and b.status = 1");
			ResultSet rset = stmt.executeQuery ("select sc_id
from bug_status_codes");

			// Display the result set as a list
			out.println("<HTML><HEAD><TITLE>All Open
Bugs</TITLE>");
			out.println("<LINK REL=STYLESHEET
HREF=\"..\\NICEStyle.css\">");
			out.println("</HEAD>");
			out.println("<BODY>");
			out.println("<TABLE>");

			while(rs.next())
			{
			//	if (i == 1)
			//	{
			//		i=0;
					//out.println("<TR><TD>" +
rs.getString("bug_row_id") + "</TD><TD>" + rs.getString("name") +
"</TD><TD>" + rs.getString("status") + "</TD><TD>" +
rs.getString("date_opened") + "</TD><TD>" + rs.getString("short_desc") +
"</TD></TR>");
					//out.println("<TR><TD>" +
rs.getString("bug_row_id") + "</TD></TR>");
					out.println("<TR><TD>" +
rs.getString("sc_id") + "</TD></TR>");
			//	}
			//	else
            //  {
			//		i=1;
					//out.println("<TR><TD>" +
rs.getString("bug_row_id") + "</TD><TD>" + rs.getString("name") +
"</TD><TD>" + rs.getString("status") + "</TD><TD>" +
rs.getString("date_opened") + "</TD><TD>" + rs.getString("short_desc") +
"</TD></TR>");
					//out.println("<TR><TD>" +
rs.getString("bug_row_id") + "</TD></TR>");
			//	}
			}
			out.println("</TABLE>");
			out.println("</BODY></HTML>");

		}
		catch (ClassNotFoundException e)
		{
			out.println("Couldn't load the database driver-> " +
e.getMessage());
		}
		catch(SQLException e)
		{
			out.println("SQLException caught: " +
e.getMessage());
		}

		finally
		{
			try
			{
				if (con != null) con.close();
			}
			catch(SQLException ignored) { }
		}
	}
};

-----Original Message-----
From: August Detlefsen [mailto:augustd123@yahoo.com]
Sent: Monday, April 08, 2002 1:19 PM
To: Tomcat Users List
Subject: Re: NullPointerException when using JDBC ResultSet next()
method


The stack trace says that your NullPointer occurs on line 45, but line
45 is blank. Is there some more code that you didn't ost? Import
statements, perhaps?

I think it probably happened here:

stmt = con.createStatement();

-IE: You were not able to get a Connection, con is null and calling its
methods will give you the NullPointer. Are you sure the driver loaded
properly?


--- "Philip Kazmier, CEM R&D" <philip.kazmier@nice.com> wrote:
>
>
> I am getting this error in a servlet compiled on Win2K, using
> JDK1.3.1 and
> Tomcat 3.3 with MySQL 3.23.47:
>
> Location: /PSSoftware/servlet/ListAllOpenBugs
> Internal Servlet Error:
> java.lang.NullPointerException
> 	at ListAllOpenBugs.doGet(ListAllOpenBugs.java:45)
> 	at javax.servlet.http.HttpServlet.service(HttpServlet.java)
> 	at javax.servlet.http.HttpServlet.service(HttpServlet.java)
> 	at org.apache.tomcat.facade.ServletHandler.doService(Unknown Source)
> 	at org.apache.tomcat.core.Handler.invoke(Unknown Source)
> 	at org.apache.tomcat.core.Handler.service(Unknown Source)
> 	at org.apache.tomcat.facade.ServletHandler.service(Unknown Source)
> 	at org.apache.tomcat.core.ContextManager.internalService(Unknown
> Source)
> 	at org.apache.tomcat.core.ContextManager.service(Unknown Source)
> 	at
>
org.apache.tomcat.modules.server.Http10Interceptor.processConnection(Unknown
> Source)
> 	at org.apache.tomcat.util.net.TcpWorkerThread.runIt(Unknown Source)
> 	at
> org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(Unknown
> Source)
> 	at java.lang.Thread.run(Thread.java:484)
>
>
> Here is the servlet code:
>
> public class MySQLTest extends HttpServlet
> {
> 	public void doGet(HttpServletRequest req, HttpServletResponse res)
> throws ServletException, IOException
> 	{
> 		Connection con = null;
> 		Statement stmt = null;
> 		ResultSet rs = null;
>
> 		res.setContentType("text/html");
> 		PrintWriter out = res.getWriter();
>
> 		try
> 		{
> 			// Load the MySQL driver
> 			Class.forName("org.gjt.mm.mysql.Driver");
> 			//try {
>
> //Class.forName("twz1.jdbc.mysql.jdbcMysqlDriver");
> 			//}
> 			//catch(Exception e){out.println(e);}
>
> 			// Get a connection to the database
> 			con =
>
DriverManager.getConnection("jdbc:mysql://localhost:3306/test?user=root&pass
> word=typhoon");
>
> 			// Create a statement object
> 			stmt = con.createStatement();
>
> 			// Execute and SQL query, get a result set
> 			rs = stmt.executeQuery("SELECT * from test");
>
> 			// Display the result set as a list
>
> out.println("<HTML><HEAD><TITLE>Test</TITLE></HEAD>");
> 			out.println("<BODY>");
> 			out.println("<UL>");
> 			while(rs.next())
> 			{
> 				out.println("<LI>" + rs.getString("test"));
> 			}
> 			out.println("</UL>");
> 			out.println("</BODY></HTML>");
>
> 		}
> 		catch (ClassNotFoundException e)
> 		{
> 			out.println("Couldn't load the database driver-> " +
> e.getMessage());
> 		}
> 		catch(SQLException e)
> 		{
> 			out.println("SQLException caught: " +
> e.getMessage());
> 		}
>
> 		finally
> 		{
> 			try
> 			{
> 				if (con != null) con.close();
> 			}
> 			catch(SQLException ignored) { }
> 		}
> 	}
> };
>
> As you can see I tried this with Oracle.  I get the same result when
> I used
> Oracle.
>
> Thanks.
>
> Phil Kazmier
> Developer,  CEM
> NICE Systems
> Denver, Colorado
> Office (720) 264-4284
>
>
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