解决方案：变量仅在if / elseif语句的范围内初始化，因此如果未输入范围，则getAttribute调用null。
我验证给定的数字是在我本地存储的SQL数据库中。如果是，我将该行的数据存储在一个对象中，存储在ArrayList中并传递给JSP。如果没有，我将布尔变量设置为true并将其传递给我的JSP。
这样做的时候，我的布尔变量得到了NullPointerException，我不知道为什么。我知道那个例外意味着什么，我不确定为什么它适用于我的情况。任何建议，或替代我如何接近这一点都会很棒。
我的JSP
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1" import="java.util.List, java.sql.ResultSet, ca.package.beans.*"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Scan</title>
</head>
<body>
<% List<Products> productsList = (List<Products>)request.getAttribute("productsList"); %>
<h1>List of Scanned Items</h1>
<table>
<tr>
<td><b>Item Code</b></td>
<td><b>Name</b></td>
<td><b>Price</b></td>
<td><b>Tax</b></td>
</tr>
<% boolean invalidCode = (boolean)session.getAttribute("invalidCode"); %>
<% if (!productsList.isEmpty()) { %>
<% for(Products product : productsList) { %>
<tr>
<td><%= product.getCode() %></td>
<td><%= product.getName() %></td>
<td><%= product.getPrice() %></td>
<td><% if(product.isTaxable()) { %>
<%= product.getPrice() * 0.13 %>
<% } else if (product.isTaxable() == false) { %>
<% out.print("0"); %>
<% } %></td>
</tr>
<% } %>
</table>
<% } else if (invalidCode == true) { %>
<p>The entered code is wrong</p>
<% } %>
<div class='codeBox'>
<form action="eShop" method="Post">
<table>
<tr>
<td align="left"><b>Item Code:</b><input type="text" name="code"><br><br></td>
</tr>
<tr>
<td align="left"><input type="submit" name="action" value="Scan!" style="width: 150px"></td>
</tr>
</table>
</form>
</div>
</body>
</html>
我的Servlet
@WebServlet("/eShop")
public class eShop extends HttpServlet {
private static final long serialVersionUID = 1L;
List<Products> productsList = new CopyOnWriteArrayList<Products>();
/**
* @see HttpServlet#HttpServlet()
*/
public eShop() {
super();
// TODO Auto-generated constructor stub
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
HttpSession session = request.getSession();
String itemCode = request.getParameter("code");
boolean invalidCode;
try{
Class.forName("com.mysql.jdbc.Driver");
Connection con=DriverManager.getConnection(
"jdbc:mysql://localhost/assignment2","root","1111");
Statement stmt=con.createStatement();
ResultSet rs=stmt.executeQuery("select * from ProductCatalogue where code = "+itemCode);
int codeCheck = Integer.parseInt(itemCode);
DataAccessImpl da = new DataAccessImpl();
if(itemExists(rs)) {
Products temp = new Products(rs.getString(1), rs.getString(2), rs.getDouble(3), rs.getBoolean(4));
productsList.add(temp);
System.out.println("In the if");
request.setAttribute("productsList", productsList);
request.getRequestDispatcher("Scan.jsp").forward(request, response);
}
else if(da.codeExists(codeCheck) == false){
invalidCode = true;
System.out.println("THIS IS EXECUTING!");
session.setAttribute("invalidCode", invalidCode);
request.getRequestDispatcher("Scan.jsp").forward(request, response);
}
con.close();
}catch(Exception e){ e.printStackTrace();}
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}
private boolean itemExists(ResultSet rs) {
try {
if(rs.next()) {
return true;
}
}catch(Exception e) {
}
return false;
}
}
堆栈追踪：
SEVERE: Servlet.service() for servlet [jsp] threw exception
java.lang.NullPointerException
at org.apache.jsp.Scan_jsp._jspService(Scan_jsp.java:141)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:444)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:459)
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:384)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312)
at ca.sheridancollege.controllers.eShop.doGet(eShop.java:64)
at ca.sheridancollege.controllers.eShop.doPost(eShop.java:76)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:651)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:417)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:754)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1376)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.base/java.lang.Thread.run(Unknown Source)
org.apache.jasper.JasperException: An exception occurred processing [/Scan.jsp] at line [23]
20:
21: <% boolean invalidCode = (boolean)(request.getAttribute("invalidCode")); %>
22:
23: <% if (!productsList.isEmpty()) { %>
24: <% for(Products product : productsList) { %>
25: <tr>
26: <td><%= product.getCode() %></td>
Stacktrace:
at org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:593)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:482)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:459)
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:384)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312)
at ca.sheridancollege.controllers.eShop.doGet(eShop.java:64)
at ca.sheridancollege.controllers.eShop.doPost(eShop.java:76)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:651)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:417)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:754)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1376)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at org.apache.jsp.Scan_jsp._jspService(Scan_jsp.java:141)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:444)