CODEWITHSUNDEEP
javaeclipsejspservletsderby
Sergey Scopin
Sergey Scopin

Reputation: 2245

Getting information from database via servlet

I'm trying to get some information from servlet. Here is code of my jsp page.

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"
    import="java.util.*, DB.DBServlet"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link href="css/bootstrap.min.css" rel="stylesheet">
<title>Lab4</title>
</head>
<body>
<div id='chekboxOfCompanies'>
 <%Iterator itr;%>
 <% List data= (List)request.getAttribute("country");
                for (itr=data.iterator(); itr.hasNext(); ) {
                    out.println("<input type='checkbox'>");
           }%>
</div>
<div id=''></div>
</body>
</html>

Here is code of servlet responsible for getting information.

public class DBServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    public String page = "main.jsp";   
    /**
     * @see HttpServlet#HttpServlet()
     */
    public DBServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        String protocol = "jdbc:derby:";
        Connection conn = null;
        ArrayList<Statement> statements = new ArrayList<Statement>(); 
        Statement s = null;
        ResultSet rs = null;
        String dbName = "Lab4DB";
        ArrayList<String> dataList = new ArrayList<String>();
        try {
            conn = DriverManager.getConnection(protocol + dbName);
            System.out.println("Connected to database " + dbName);
            conn.setAutoCommit(false);
            s = conn.createStatement();
            statements.add(s);
            s.executeQuery("SELECT company from companies");
            rs = s.getResultSet();
            while(rs.next()){
                dataList.add(rs.getString("country"));
            }
            rs.close();
            s.close();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        request.setAttribute("country", dataList);
        RequestDispatcher dispatcher = request.getRequestDispatcher(page);

        if (dispatcher != null) {

            dispatcher.forward(request, response);

        }
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
    }

}

When I try to run this code there is some Null pointer exceptions. Output of console.

SEVERE: Servlet.service() for servlet [jsp] in context with path [/Lab4DWP] threw exception [java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at org.apache.jsp.main_jsp._jspService(main_jsp.java:80)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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(AccessLogValve.java:953)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:695)
10.02.2014 15:11:12 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [jsp] in context with path [/Lab4DWP] threw exception [java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at org.apache.jsp.main_jsp._jspService(main_jsp.java:80)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    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(AccessLogValve.java:953)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:695)

What variable get's null value? How can I avoid it?

Upvotes: 0

Views: 610

Answers (1)

Gyan
Gyan

Reputation: 1176

Try printing out the list of countries in the console first, just to be sure. And btw, you are missing the itr.next() call for the iterator, that gives you the elements as well as the closing tag for the checkbox. I think that is what is giving you the null pointer.

P:S: I believe the way you are connecting to the database should not be working. getConnection(protocol +"//" + host-address + "/" + dbName) should be the correct way

Based on the comments :

First , please try the proper code to connect to the database and retrieve information.
For a good tutorial, please visit : Vogella tutorial
Second, please try and close all open tags. Even input tags are closed by using a "/>", if not the complete closing tag "< /input>".
Third, In tomcat, you can find the generated jsp code under work directory (might be hidden based on configuration). Jump to line 80, in the generated jsp(main_jsp.java) to find which line is exactly throwing that error.

Upvotes: 1

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