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Thread: a null pointer exception

  1. #1
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    Default a null pointer exception

    hi guys, I'm writing some servlets for a website, but I'm new to JSP, therefore I made alot mistakes when I was writing them, I've been fixing them for weeks, but this null pointer exception, I just don't know where the problem is, and what caused it..... so here is the code, thanks alot guys!

    package Servlet;
     
    import javax.servlet.*;
    import javax.servlet.http.*;
     
    import Model.DAO;
     
    import java.io.*;
    import java.sql.*;
     
    public class LoginServlet extends HttpServlet
    {
    	Connection conn=null;
    	Statement stmt=null;
     
    	String username;
    	String password;
     
    	public void init(ServletConfig config)throws ServletException
    	{
    		super.init();
    	}
     
    	public void service(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException
    	{
    		DAO.getConnection(conn);
    		try {
    			stmt=conn.createStatement();
    		} catch (SQLException e1) {
    		}
    		username=request.getParameter("username");
    		password=request.getParameter("password");
    		if(username=="admin")
    		{
    			try
    			{
    				ResultSet rs=stmt.executeQuery("select '"+username+"' from tb_user");
    				if(rs.next())
    				{
    					ResultSet vpass=stmt.executeQuery("select '"+password+"' from tb_user where id = '"+username+"'");
    					if(vpass.next())
    					{
    						request.getRequestDispatcher("./manage/manage_index.jsp").forward(request,response);
     
    					}
    					else
    					{
    						request.getRequestDispatcher("http://localhost:8080/form_apply_system/account/log_fail.jsp").forward(request,response);
    					}
    				}
     
    			}catch(Exception e)
    			{
    				e.printStackTrace();
    			}
    		}
    		else
    		{
    			try
    			{
    				ResultSet rs=stmt.executeQuery("select '"+username+"' from tb_user");
    				if(rs.next()==true)
    				{
    					ResultSet vpass=stmt.executeQuery("select '"+password+"' from tb_user where id = '"+username+"'");
    					if(vpass.next()==true)
    					{
    						request.getRequestDispatcher("http://localhost:8080/form_apply_system/account/log_success.jsp").forward(request,response);
    					}
    					else
    					{
    						request.getRequestDispatcher("http://localhost:8080/form_apply_system/account/log_fail.jsp").forward(request,response);
    					}
    				}
    				else
    				{
    					request.getRequestDispatcher("http://localhost:8080/form_apply_system/account/log_fail.jsp").forward(request,response);
    				}
    			}catch(Exception e)
    			{
    				e.printStackTrace();
    			}
    		}
    	}
    }

    I know this servlet sucks, so please just ignore its poor design and hard codind.....

    --- Update ---

    eclipse says that the null pointer exception occured at line 28, which is
    stmt=conn.createStatement();

    and DAO.getConnection is just a normal method used to get connection with MySQL database, here is the code of it:
    [hightlight=Java]
    package Model;

    import java.sql.*;

    public class DAO
    {
    public static void getConnection(Connection conn)
    {
    String driver="com.mysql.jdbc.Driver";
    String url="jdbc:mysql://localhost:3306/web";
    try
    {
    Class.forName(driver);
    conn=DriverManager.getConnection(url,"root","12345 6");
    }catch(Exception e)
    {
    e.printStackTrace();
    }
    }
    }
    [/highlight]


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: a null pointer exception

    Do you have the full text of the error message? Copy it and paste it here so we can see what statement the error is on.

    --- Update ---

    null pointer exception occured at line 28, which is
    If looks like conn has a null value.
    Where does the code assign a non-null value to the conn variable?
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: a null pointer exception

    java.lang.NullPointerException
    at Servlet.LoginServlet.service(LoginServlet.java:28)
    at javax.servlet.http.HttpServlet.service(HttpServlet .java:728)
    at org.apache.catalina.core.ApplicationFilterChain.in ternalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.do Filter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invo ke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invo ke(StandardContextValve.java:123)
    at org.apache.catalina.core.StandardHostValve.invoke( StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke (ErrorReportValve.java:99)
    at org.apache.catalina.valves.AccessLogValve.invoke(A ccessLogValve.java:953)
    at org.apache.catalina.core.StandardEngineValve.invok e(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.servic e(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.p rocess(AbstractHttp11Processor.java:1023)
    at org.apache.coyote.AbstractProtocol$AbstractConnect ionHandler.process(AbstractProtocol.java:589)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProce ssor.run(JIoEndpoint.java:312)
    at java.util.concurrent.ThreadPoolExecutor.runWorker( ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:724)

    Sure, here it is

    --- Update ---

    you mean the conn in the DAO or in the servlet?

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: a null pointer exception

    conn where the NPE happens on line 28
    If you don't understand my answer, don't ignore it, ask a question.

  5. The Following User Says Thank You to Norm For This Useful Post:

    alskdwq (September 22nd, 2013)

  6. #5
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    Default Re: a null pointer exception

    Quote Originally Posted by Norm View Post
    conn where the NPE happens on line 28
    Thanks alot man!!! You said about the non null value, so I re checked my codes trying to find such mistakes, and I found that the getConnection method didn't return an object, so the conn I'm using in the servlet is a null value, thanks!!!

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