我很难弄清楚为什么我的代码出错了。下面是我的JSP文件（问题出在第二种形式上）：
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Book Drivers</title>
</head>
<body>
<h1>Book Demands</h1>
<form method="POST" action="BookDriver.do">
<br>View a table </br>
<input type="radio" name="tbl" value="ListTodaysDemands">List Todays Demands<br />
<input type="radio" name="tbl" value="ListAllDemands">List All Demands<br />
<input type=submit value="Go!"> <br />
</form>
</body>
<body>
<h2>Demands</h2>
<%=(String)(request.getAttribute("query"))%>
</body>
<body>
<h2>Journeys</h2>
<%=(String)(request.getAttribute("query1"))%>
</body>
<body>
<h2>Drivers</h2>
<%=(String)(request.getAttribute("query2"))%>
</body>
<body>
<h2>Book taxi</h2>
<form method="POST" action="BookDriver.do">
<table>
<tr>
<th></th>
<th>Please provide your following details</th>
</tr>
<tr>
<td>Name:</td>
<td><input type="text" name="name"/></td>
</tr>
<tr>
<td>Address:</td>
<td><input type="text" name="address"/></td>
</tr>
<tr>
<td>Destination:</td>
<td><input type="text" name="destination"/></td>
</tr>
<tr>
<td>Date:</td>
<td><input type="text" name="date"/></td>
</tr>
<tr>
<td>Time:</td>
<td><input type="text" name="time"/></td>
</tr>
<tr>
<td> <input type="submit" value="Book"/></td>
</tr>
</table>
</form>
</body>
下面是我的控制器：
public class BookDriver extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
LocalDate date = LocalDate.now();
String qry1 = "select * from CUSTOMER";
String qry2 = "select * from DRIVERS";
String qry3 = "select * from DEMANDS where date = '"+date+"'";
String qry4 = "select * from DEMANDS";
String qry5 = "select * from JOURNEY";
//String qry4 = "SELECT Drivers.Name, Drivers.Registration FROM Drivers LEFT JOIN Journey ON Journey.Registration = Drivers.Registration LEFT JOIN Demands ON Demands.Time = Journey.Time WHERE Demands.id IS NULL";
//String qry4 = "SELECT Drivers.Name, Drivers.Registration FROM Drivers LEFT JOIN Journey ON Journey.Registration = Drivers.Registration LEFT JOIN Demands ON Demands.Date = Journey.Date LEFT JOIN Demands ON Demands.Time = Journey.Time WHERE Demands.id IS NULL";
response.setContentType("text/html;charset=UTF-8");
HttpSession session = request.getSession(false);
Jdbc dbBean = new Jdbc();
dbBean.connect((Connection)request.getServletContext().getAttribute("connection"));
session.setAttribute("dbbean", dbBean);
if((Connection)request.getServletContext().getAttribute("connection")==null)
request.getRequestDispatcher("/WEB-INF/conErr.jsp").forward(request, response);
else if (request.getParameter("tbl").equals("ListTodaysDemands")){
String msg="No current demands";
String msg2="No current journeys";
String msg3="No current journeys";
try {
msg = dbBean.retrieve(qry3);
msg2 = dbBean.retrieve(qry5);
msg3 = dbBean.retrieve(qry2);
} catch (SQLException ex) {
Logger.getLogger(BookDriver.class.getName()).log(Level.SEVERE, null, ex);
}
request.setAttribute("query", msg);
request.setAttribute("query1", msg2);
request.setAttribute("query2", msg3);
request.getRequestDispatcher("/WEB-INF/bookDemands.jsp").forward(request, response);
}
else if (request.getParameter("tbl").equals("ListAllDemands")){
String msg="No current demands";
String msg2="No current journeys";
String msg3="No current drivers";
try {
msg = dbBean.retrieve(qry4);
msg2 = dbBean.retrieve(qry5);
msg3 = dbBean.retrieve(qry2);
} catch (SQLException ex) {
Logger.getLogger(BookDriver.class.getName()).log(Level.SEVERE, null, ex);
}
request.setAttribute("query", msg);
request.setAttribute("query1", msg2);
request.setAttribute("query2", msg3);
request.getRequestDispatcher("/WEB-INF/bookDemands.jsp").forward(request, response);
}
///////////THIS PART NOT WORKING//////////////////////////
String [] query = new String[5];
query[0] = (String)request.getParameter("name");
query[1] = (String)request.getParameter("address");
query[2] = (String)request.getParameter("destination");
query[3] = (String)request.getParameter("date");
query[4] = (String)request.getParameter("time");
Jdbc jdbc = (Jdbc)session.getAttribute("dbbean");
if (jdbc == null)
request.getRequestDispatcher("/WEB-INF/conErr.jsp").forward(request, response);
if(query[0].equals("") ) {
request.setAttribute("msg", "Username cannot be NULL");
}
request.getRequestDispatcher("/WEB-INF/bookDemands.jsp").forward(request, response);
}
JSP中的第一种形式绝对可以正常工作，第二种形式存在问题。每当我使用按钮“ Book”时，我都会得到一个空指针异常，并且我无法弄清楚为什么以及如果我将所有代码都注释为servlet中的第一种形式，那么它不再抛出该异常，并且可以正常工作。
真的很感谢您的帮助，因为我已经花了数小时在网上寻找解决方案，而且我仍然在努力解决问题上。
干杯
答案 0 :(得分：0)
使用request.getParameterValues（“ name”）..，其他也一样，也许当前值是