java.lang.NullPointerException解决方案

import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.ResultSet;import javax.sql.DataSource;import javax.naming.Context;import javax.naming.InitialContext;import javax.naming.*;import java.sql.SQLException;import java.util.Properties;import java.util.List;import java.util.ArrayList;import java.util.Iterator;import myjdbc.util.WrapperUtil;public class DatabaseOperation {    private static DatabaseOperation dbo = null;    private static final int FETCH_ALL = Integer.MAX_VALUE;    private static final int NO_OFFSET = -1;    private static final int DEFAULT_PAGE_SIZE = 20;    private Connection conn = null;    private PreparedStatement pst = null;    private ResultSet rs = null;    private DataSource ds = null;    private static String staticTableConfigPath = "src/conf"; //    private static String staticDataSourceName = "DataSource";    //私有构在函数    private DatabaseOperation() {        try {            Context ctx = getWeblogicServerInitialContext();            ds = (DataSource) ctx.lookup(staticDataSourceName);        } catch (NamingException ex) {            ex.printStackTrace();        } catch (Exception e) {            e.printStackTrace();        }    }    //获得weblogic的上下文    private final Context getWeblogicServerInitialContext() throws Exception {        String url = "t3://127.0.0.1:7001";        String user = null;        String password = null;        Properties properties;        try {            properties = new Properties();            //  properties.put("tableConfigPath","F:/jbuilder_project/writebook/relation_manage/src/conf");            properties.put(Context.INITIAL_CONTEXT_FACTORY,                           "weblogic.jndi.WLInitialContextFactory");            properties.put(Context.PROVIDER_URL, url);            if (user != null) {                properties.put(Context.SECURITY_PRINCIPAL, user);                properties.put(Context.SECURITY_CREDENTIALS,                               password == null ? "" : password);            }            return new javax.naming.InitialContext(properties);        } catch (Exception e) {            e.printStackTrace();            throw e;        }    }private void openConnection() {        if (conn == null) {            try {                conn = ds.getConnection();            } catch (SQLException e) {                e.printStackTrace();            }        }    } public static void main(String args[]) throws Exception {        List result = new ArrayList();        String sql = "select * from orderitem as a";//    sql = sql.toLowerCase();        DatabaseOperation database = DatabaseOperation.getInstance();        //   database.openConnection();        result = database.executeSQL(sql);        System.out.println("asdf=====>" + result.size());