急求jsp servlet 中这个错误是怎么为事？

我运行时，报这个错误？请大家告诉我，怎么处理？先谢谢！
java.lang.NullPointerException
cn.com.jobedu.blog.UserServlet.login(UserServlet.java:55)
cn.com.jobedu.blog.UserServlet.doPost(UserServlet.java:31)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

下面是UserServlet代码：
Java code:


package cn.com.jobedu.blog;

import java.io.IOException;
import java.sql.SQLException;
import java.util.List;

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

import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanListHandler;

public class UserServlet extends HttpServlet {
private static final long serialVersionUID = -1767037483934953543L;

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String method = request.getParameter("method");
if (