Abstract servlet

[sameera liyanage]

there is a abstract servlet in my project and made the database connection in init() method.
.in my subclass servlet extend the abstract servlet and overide th init() method.
and call super class init() method.
but if i remove calling super.init() it gives an ERROR.
what is the reason?
when extending my servlet to the super servlet it should call super init() method?

[Seetharaman Venkatasamy]

aruna sameera wrote:
when extending my servlet to the super servlet it should call super init() method?

Not required . what error you are getting ? . and better to post your base servlet code using the code tag.

[sameera liyanage]

this is my abstract servlet

package servlets;

import java.io.IOException;
import java.sql.*;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


 public abstract class Connector extends javax.servlet.http.HttpServlet implements javax.servlet.Servlet {
    	
	 Connection con = null;
	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
	}  	
	
	
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
	}   	  	  
	
	
	public void init() throws ServletException {
		System.out.println("super.init();super.init();");
	    String url = "jdbc:mysql://localhost:3306/";
	    String db = "student";
	    String driver = "com.mysql.jdbc.Driver";
	    String user = "root";
	    String pass = "admin";
	    try{
	      Class.forName(driver).newInstance();
	      con = DriverManager.getConnection(url+db, user, pass);
	      
	    }
	    catch (Exception e){
	      e.printStackTrace();
	    }
	}   
}

This is my sub seervlet class

package servlets;

import java.io.IOException;
import java.sql.*;


import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


 public class StudentResults extends servlets.Connector implements javax.servlet.Servlet {
   
   
   
   
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
	}  	
	
	
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
	}   	  	  
	
	public void init() throws ServletException {
		
		//super.init();
		
		try{
	        Statement st =con.createStatement();
	        ResultSet res = st.executeQuery("SELECT * FROM  details");
	  
	        while (res.next()) {
	        	
	          String i = res.getString("name");
	          int s = res.getInt("age");
	          System.out.println(i + "\t\t" + s);
	         
	        }
	        con.close();
	      }
	      catch (SQLException s){
	        System.out.println("SQL code does not execute.");
	      }    
	}   
}

this is the error
[code]


java.lang.NullPointerException
	at servlets.StudentResults.init(StudentResults.java:31)
	at javax.servlet.GenericServlet.init(GenericServlet.java:211)
	at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1105)
	at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:757)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:130)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
	at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
	at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
	at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
	at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
	at java.lang.Thread.run(Unknown Source)
Apr 21, 2009 1:48:37 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Allocate exception for servlet StudentResults
java.lang.NullPointerException
	at servlets.StudentResults.init(StudentResults.java:31)
	at javax.servlet.GenericServlet.init(GenericServlet.java:211)
	at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1105)
	at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:757)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:130)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
	at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
	at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
	at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
	at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
	at java.lang.Thread.run(Unknown Source)

[/code]

[Paul Sturrock]

It is required if you want your init code to be run in Connector. If you override a method it replaces the method it overrides, if you still want to call the original method call super.themethodName() in the overridden method.

[Seetharaman Venkatasamy]

that is because of your Connection is null , since the DB Connection logic is inside your BaseServlet's[Connector] init().
so, you need to call the method

[Albareto McKenzie]

Hi there,

What do you thing that can be null in your code with the super.init() commented? Why it works when you decomment it? (I don't know if decomment exist but I think all of you understand :P )

The exception is quite logic if you think about it!

[Albareto McKenzie]

Oh, seetharaman, answered to you.

Other thing I have seen is that you have an abstract class with no abstract declarations... anyway, here the problem has nothing to do with being abstract servlet or "normal" one.