我有一个jsp页面，用于从用户获取数据并使用servlet将它们添加到数据库中。当我尝试运行页面时出现null异常。但它没有任何错误。有人可以帮我弄清楚错误是什么吗？
错误 -
type Exception report
messageInternal Server Error
descriptionThe server encountered an internal error that prevented it from fulfilling this request.
exception
org.apache.jasper.JasperException: java.lang.NullPointerException
root cause
java.lang.NullPointerException
我的jsp -
<%@page import="java.sql.ResultSet"%>
<%@page import="java.sql.Statement"%>
<%@page import="java.sql.DriverManager"%>
<%@page import="java.util.ArrayList"%>
<%@page import="com.cert.classes.Incident"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Sri Lanka CERT</title>
<link rel="stylesheet" type="text/css" href="bootstrap/css/bootstrap.min.css" />
<link rel="stylesheet" type="text/css" href="font-awesome/css/font-awesome.min.css" />
<link rel="stylesheet" type="text/css" href="css/local.css" />
<script type="text/javascript" src="js/jquery-1.10.2.min.js"></script>
<script type="text/javascript" src="bootstrap/js/bootstrap.min.js"></script>
</head>
<body>
<div id="wrapper">
<nav class="navbar navbar-inverse navbar-fixed-top" role="navigation">
<div class="navbar-header">
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-ex1-collapse">
<span class="sr-only"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="index.html">Sri Lanka CERT</a>
</div>
<div class="collapse navbar-collapse navbar-ex1-collapse">
<ul class="nav navbar-nav side-nav">
<li><a href="main_page.jsp"><i class="fa fa-bullseye"></i> Dashboard</a></li>
<li><a href="search_edit.jsp"><i class="fa fa-tasks"></i> Search and Update</a></li>
</ul>
<ul class="nav navbar-nav navbar-right navbar-user">
<li class="dropdown messages-dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown"><i class="fa fa-envelope"></i> Messages<b class="caret"></b></a>
<ul class="dropdown-menu">
<li class="dropdown-header">2 New Messages</li>
<li class="message-preview">
<a href="#">
<span class="avatar"><i class="fa fa-bell"></i></span>
<span class="message">Security alert</span>
</a>
</li>
<li class="divider"></li>
<li class="message-preview">
<a href="#">
<span class="avatar"><i class="fa fa-bell"></i></span>
<span class="message">Security alert</span>
</a>
</li>
<li class="divider"></li>
<li><a href="#">Go to Inbox <span class="badge"></span></a></li>
</ul>
</li>
<li class="dropdown user-dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown"><i class="fa fa-user"></i><b class="caret"></b></a>
<ul class="dropdown-menu">
<li><a href="#"><i class="fa fa-gear"></i> Change Password</a></li>
<li class="divider"></li>
<li><a href="#"><i class="fa fa-power-off"></i> Log Out</a></li>
</ul>
<li class="divider-vertical"></li>
<li>
<form class="navbar-search">
<input type="text" value=<%=request.getSession().getAttribute("user") %>><br>
</form>
</li>
</li>
</ul>
</div>
</nav>
<div id="page-wrapper">
<div class="row">
<div class="col-lg-6">
<form role="form" action="/addticket" method="post">
<div class="form-group">
<label>First Name</label>
<input class="form-control" name="firstname">
</div>
<div class="form-group">
<label>Last Name</label>
<input class="form-control" name="lastname">
</div>
<div class="form-group">
<label>Phone</label>
<input class="form-control" name="phone">
</div>
<div class="form-group">
<label>Mode</label><br>
<label class="radio-inline">
<input type="radio" name="mode" value="call">
Call
</label>
<label class="radio-inline">
<input type="radio" name="mode" value="email">
Email
</label>
<label class="radio-inline">
<input type="radio" name="mode" value="visit">
Visit
</label>
<label class="radio-inline">
<input type="radio" name="mode" value="other">
Other
</label>
</div>
<div class="form-group">
<label>Status</label><br>
<label class="radio-inline">
<input type="radio" name="status" value="opened">
Opened
</label>
<label class="radio-inline">
<input type="radio" name="status" value="sent">
Document Sent
</label>
<label class="radio-inline">
<input type="radio" name="status" value="submitted">
Document Submitted
</label>
<label class="radio-inline">
<input type="radio" name="status" value="closed" disabled>
Closed
</label>
</div>
<div cla <label>Comment</label>
<textarea name="comment" class="form-control" rows="3"></textarea>
</div>
<input type="hidden" class="form-control" value=<%=request.getSession().getAttribute("user")%> name="handler">
<%
ArrayList<Incident> list1 = (ArrayList<Incident>) getServletContext().getAttribute("main_list");
ArrayList<Incident> list2 = (ArrayList<Incident>) getServletContext().getAttribute("sub_list");
ArrayList<Incident> list3 = (ArrayList<Incident>) getServletContext().getAttribute("incident_list");
%>
</div>
<div class="col-lg-6">
<div class="form-group">
<label>Main Category</label>
<select name="main_category" class="form-control">
<%
for(Incident item : list1){ %>
<option label="<%=item.getType()%>" value="<%=item.getType()%>" >
<%
}
%>
</select>
</div>
<div class="form-group">
<label>Sub Category</label>
<select name="sub_category" class="form-control">
<%
for(Incident item : list2){ %>
<option label="<%=item.getType()%>" value="<%=item.getType()%>" >
<%
}
%>
</select>
</div>
<div class="form-group">
<label>Incident Type</label>
<select name="incident_type" class="form-control">
<%
for(Incident item : list3){ %>
<option label="<%=item.getType()%>" value="<%=item.getType()%>" >
<%
}
%>
</select>
</div>
<div class="form-group has-error">
<label>Target</label>
<input type="text" class="form-control" name="target">
</div>
<div class="form-group has-warning">
<label>Forwarded</label>
<input type="text" class="form-control" name="forwarded">
</div>
<button type="submit" class="btn btn-default">Add Ticket</button>
<button type="reset" class="btn btn-default">Clear</button>
</form>
</div>
</div>
</div>
</div>
</body>
</html>
答案 0 :(得分：0)
此代码的问题是
ArrayList<Incident> list1 = (ArrayList<Incident>) getServletContext().getAttribute("main_list");
此行if(getServletContext().getAttribute("main_list")!=null)的第一个checknull条件，然后键入强制转换为ArrayList。如果我们为此行getServletContext().getAttribute("main_list")获取空值并且您尝试使用ArrayList进行类型转换，那么您将仅获得Null Pointer excetion。请使用JSTL标记而不是直接在jsp中放置java代码...