錯(cuò)誤信息：

四月 26, 2016 2:15:22 下午 org.apache.catalina.core.StandardWrapperValve invoke

嚴(yán)重: Servlet.service() for servlet [LoginServlet] in context with path [/MessageBoard] threw exception

java.lang.NullPointerException

at servlet.LoginServlet.doPost(LoginServlet.java:61)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)

at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)

at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)

at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:522)

at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1095)

at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:672)

at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2500)

at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2489)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

at java.lang.Thread.run(Thread.java:745)

username = lisi ?password= 123 ?params長(zhǎng)度：2

代碼如下：

1. servlet包下的

package servlet;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.ArrayList;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import service.LoginService;

/**

?* Servlet implementation class LoginServlet

?*/

@WebServlet("/LoginServlet")

public class LoginServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

private LoginService service;

? ? ? ?

? ? /**

? ? ?* @see HttpServlet#HttpServlet()

? ? ?*/

? ? public LoginServlet() {

? ? ? ? super();

? ? ? ? // TODO Auto-generated constructor stub

? ? }

? ??

? ? public void destroy() {

super.destroy(); // Just puts "destroy" string in log

// Put your code here

}

/**

* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

*/

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

// TODO Auto-generated method stub

doPost(request,response);

}

/**

* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

*/

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

// TODO Auto-generated method stub?

String path = request.getContextPath(); ?

? ? ? ? request.setCharacterEncoding("UTF-8"); ?

? ? ? ? response.setContentType("text/html; charset=utf-8"); ?

? ? ? ? PrintWriter out = response.getWriter(); ??

?

String username = request.getParameter("username");

String password = request.getParameter("password");

? ? ? ?

List<String> params = new ArrayList<String>(); ?

? ? ? ? params.add(username); ?

? ? ? ? params.add(password);

? ? ? ? System.out.println("username = " +username+ " ?password= " + password +" ?params長(zhǎng)度："+params.size());

boolean flag=service.loginUser(params);

if(flag)

{

response.sendRedirect(path+"/login_success.jsp");

out.println("成功");

}

else

{

response.sendRedirect(path+"/login_failure.jsp");

out.println("失敗");

}

}

public void init() throws ServletException {

// Put your code here

}

}

2.接口

package service;

import java.util.List;

public interface LoginService {

? ?public boolean loginUser(List<String> params);

}

3.實(shí)現(xiàn)接口

package dao;

import java.sql.Connection;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.util.List;

import service.LoginService;

import util.JdbcUtils;

public class UsersDao implements LoginService{

public boolean loginUser(List<String> params){

PreparedStatement stmt = null;

ResultSet rs = null;

boolean flag = false;

? ? ? ??

//Connection conn;

try {

Connection conn = JdbcUtils.getConnection();

String sql = "select * from zoey_oper where usrname=? and password=?"; // SQL語(yǔ)句

stmt = conn.prepareStatement(sql);

int index=1;

if(params != null && !params.isEmpty())

{

for(int i=0; i<params.size(); i++){ ?

? ? ? ? ? ? ? ?stmt.setString(index++, params.get(i));?

? ? ? ? ? ? ? ?System.out.println(params.get(i));

? ? ? ? ? ?}

rs = (ResultSet) stmt.executeQuery();

if (rs.next())

? {

? ?flag = true;

? ?rs.close();

? ?stmt.close();

? }

? else?

? {

? ?flag = false ;

? ?rs.close();

? ?stmt.close();

? }

? conn.close();

? return flag;

}

} catch (Exception e1) {

// TODO Auto-generated catch block

e1.printStackTrace();

}

return flag;

}

}