即使我知道nationalID不为null，我的程序仍会给我一个空指针异常。它告诉我roster.delete（nationalID）; line为空，但是当我在Netbeans中运行调试器时，它甚至都没有进入删除状态。我还发布了我发现相关的display.jsp部分和Del.jsp这是删除页面。
名册不为空
Servlet侦听器
public class FoamServletListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
ServletContext sc = sce.getServletContext();
final String path = sc.getRealPath("/WEB-INF/");
Roster.initialize(path + "\\roster.dat");
sc.log("Passed roster initialization");
try {
sc.setAttribute("roster", Roster.getInstance());
} catch (RosterException ex) {
Logger.getLogger(FoamServletListener.class.getName()).log(Level.SEVERE, null, ex);
}
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
sce.getServletContext().removeAttribute("roster");
}
}
初始化方法
Roster roster;
@Override
public void init() throws ServletException {
super.init();
roster = (Roster) getServletContext().getAttribute("roster");
}
Servlet
String url = "/Del.jsp";
String action = request.getParameter("action");
if (action == null) {
action = "Delete";
}
switch (action) {
case "QueryAthlete":
nationalID = request.getParameter("nationalID");
try {
Athlete athlete = roster.find(nationalID);
// attach athlete and forward to Delete.jsp
request.setAttribute("athlete", athlete);
url = "/Del.jsp";
} catch (RosterException ex) {
Logger.getLogger(EditServlet.class.getName()).log(Level.SEVERE, null, ex);
// Logic to go back to display page
url = "/Display.jsp";
}
break;
case "Delete":
// Get Parameters
nationalID = request.getParameter("nationalID");
try {
roster.delete(nationalID);
url = "/Del.jsp";
} catch (RosterException ex) {
Logger.getLogger(DeleteServlet.class.getName()).log(Level.SEVERE, null, ex);
errMsg = "Cannot delete this Athlete.";
}
request.setAttribute("errMsg", errMsg);
break;
case "Cancel":
url = "/Display.jsp";
break;
}
request.getRequestDispatcher(url).forward(request, response);
Display.jsp
<td><a href="<c:url value ='/DeleteServlet'/>?action=QueryAthlete&nationalID=${athlete.nationalID}">Delete</a></td>
Del.jsp
<form name="name" action="<c:url value='/DeleteServlet'/>" method="POST">
<table border="1" cellpadding="2">
<thead></thead>
<tbody>
<tr>
<td>National ID</td>
<td>${athlete.nationalID}</td>
</tr>
<tr>
<td>First Name</td>
<td>${athlete.firstName}</td>
</tr>
<tr>
<td>Last Name</td>
<td>${athlete.lastName}</td>
</tr>
<tr>
<td>Date Of Birth</td>
<td>${athlete.dateOfBirth}</td>
</tr>
<tr>
<td>Are you sure you want to delete this Athlete?
<input type="submit" value="Delete" name="action" />
<input type="submit" value="Cancel" name="action" /></td>
</tr>
</tbody>
</table>
<c:if test="${errMsg} != null" var="errMsg">
</c:if>
</form>
这是StackTrace：
Info: action = null
Warning: StandardWrapperValve[DeleteServlet]: Servlet.service() for servlet DeleteServlet threw exception
java.lang.NullPointerException
at java.util.TreeMap.getEntry(TreeMap.java:347)
at java.util.TreeMap.remove(TreeMap.java:603)
at edu.saintpaul.csci2466.foam.roster.Roster.delete(Roster.java:143)
at servlets.DeleteServlet.processRequest(DeleteServlet.java:83)
at servlets.DeleteServlet.doPost(DeleteServlet.java:126)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilt erChain.java:344)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain. java:214)
at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.jav a:393)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilt erChain.java:256)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain. java:214)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:3 16)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:1 60)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:415)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:282)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(Con tainerMapper.java:459)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java :167)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:201)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:175)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.ja va:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilter Chain.java:284)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFil terChain.java:201)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain. java:133)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain. java:112)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport. java:561)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrate gy.java:112)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrat egy.java:117)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadI OStrategy.java:56)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run (WorkerThreadIOStrategy.java:137)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThread Pool.java:565)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPoo l.java:545)
at java.lang.Thread.run(Thread.java:745)
最佳答案
您得到的是空指针异常，不是becoz为null，而是becoz名册在roster.delete(nationalID);中为null
关于
当我在Netbeans中运行调试器时，它甚至都没有进入
删除大小写。
可能是您的代码未正确编译和部署，但异常是您从已部署的代码中实际得到的结果
关于java - 空指针异常roster.delete(nationalID);，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/29236865/