在Java Web App Servlet中使用MySQL类

Question

This is my first attempt at a Java web app with servlets (and Java itself). I'm trying to create a MySQL class that will handle queries, connection, etc. For whatever reason. I can get the connection but once I try to execute a query it gives a

java.lang.NullPointerException

I'm pretty certain that I'm making some kind of rookie mistake. but it's not clear to me what it is.

Here is the SQL class

package coreservlets;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
//import com.mysql.jdbc.*;


public class DBInterface {
    private static Connection conn;
    private Statement statement;

    public static Connection Conn() throws SQLException, ClassNotFoundException {
        String  url = "jdbc:mysql://localhost:3306/cmwebapp";
        String user = "user";
        String pw = "password";

        try{
            //Class.forName("com.mysql.jdbc.Driver").newInstance();
            Class.forName("com.mysql.jdbc.Driver");
        conn = (Connection) DriverManager.getConnection(url, user, pw);
        //System.out.println(conn.getAutoCommit());
        } catch (SQLException e) {
            System.out.println("MySQL Connection failed.");
            e.printStackTrace();
        } 
        return conn;
    }

    public ResultSet Query(String query){
        try{
        statement = (Statement) conn.createStatement();
        ResultSet rs = statement.executeQuery(query);
        System.out.println(rs);
        return rs;
        }catch(Exception e){
            System.out.println("TestServlet.java SQL connection failed.");
            e.printStackTrace();
            return null;
        }
    }
}

The class I'm trying to call it in

package internalLogic;
import coreservlets.*;

import java.io.*;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class TheWork {
    private DBInterface conn;
    private String param;
    private ResultSet set;

    public TheWork(String p){
        try {
              conn = (DBInterface) DBInterface.Conn();
              param = "SELECT * FROM test WHERE index =" + p ;
            } catch(Exception e) {
                System.out.println("In TheWork ctor SQL connection or variable assignment failed.");
                e.printStackTrace();
            } 
    }

    public String Execute(){
        set = conn.Query(param);
        System.out.println(set);
        Object s = null;
        String ret = "";
        try{
            while (set.next()) {
                // Make container class for SQL result sets.
                if(s instanceof String){
                    s = set.getObject("index");         
                }
                if (s instanceof Integer){
                    s = String.valueOf(s);
               }
                ret += s + ", ";
            }
            return ret;
        }catch(Exception e){
            System.out.println("In TheWork Execute SQL connection or variable assignment failed.");
            e.printStackTrace();
            return null;
        }
    }
}

Thanks to anyone who wants to help me out.

Oct 29, 2013 9:51:03 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [Test] in context with path [/test-app] threw exception
java.lang.NullPointerException
    at internalLogic.TheWork.Execute(TheWork.java:26)
    at testPackage.TestServlet.doGet(TestServlet.java:32)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    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:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1002)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:585)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Answer 1

In your Execute() method the instance conn is probably null . I think that in your TheWork class:

private DBInterface conn;

should be

private Connection conn;

and in the constructor

conn = (DBInterface) DBInterface.Conn();

should be simply

conn = DBInterface.Conn();

UPDATE: (The suggested above corrections are not useful)

After your last comment, I have realized what is happening; the real problem is that you need to return an instance of DBInterface from your Conn() method. If you take your original code and change just the method Conn() like I did below, it should fix the problem ...

public static DBInterface Conn() throws SQLException, ClassNotFoundException {
    String  url = "jdbc:mysql://localhost:3306/cmwebapp";
    String user = "user";
    String pw = "password";
    DBInterface dbInterface = new DBInterface();
    try {
        Class.forName("com.mysql.jdbc.Driver");
        dbInterface.conn = (Connection) DriverManager.getConnection(url, user, pw);
    } catch (SQLException e) {
        System.out.println("MySQL Connection failed.");
        e.printStackTrace();
    }
    return dbInterface;
}