我有一个登录页面，当我在其中输入正确的凭据时，会显示org.apache.jasper.JasperException: java.lang.NullPointerException错误。
这是Authenticate.java（POJO类），其中包含用于验证和读取数据库表中数据的方法：
/*
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* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package com.Exoticatravels;
import java.util.Iterator;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
/**
*
* @author sufiyaan
*/
public class Authenticate {
private String firstName;
private String pass;
EntityManagerFactory emf = Persistence.createEntityManagerFactory("Exotica_TravelsPU");
EntityManager em = emf.createEntityManager();
public boolean findUser(String enteredEmail){
Query query = em.createQuery("SELECT u FROM Users u WHERE u.email =" + "'" + enteredEmail + "'");
List results = query.getResultList();
if(results.isEmpty()){
return false;
}
Iterator it = results.iterator();
while (it.hasNext()){
Users user = (Users) it.next();
firstName = user.getFirstName().toString();
pass = user.getPassword().toString();
}
em.close();
emf.close();
return true;
}
public boolean authUser(String enteredPass){
if(enteredPass.equals(pass)){
return true;
}
return false;
}
public String getFirstName() {
return firstName;
}
public String getPass() {
return pass;
}
}
这是我的新手，我怀疑在将值分配给firstName变量时，此处是否存在错误。
user = (Users) it.next();
firstName = user.getFirstName().toString();
这是我的登录Servlet：
/*
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* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package com.Exoticatravels;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
/**
*
* @author sufiyaan
*/
public class Login extends HttpServlet {
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String email = request.getParameter("email");
String pass = request.getParameter("pass");
Authenticate auth = new Authenticate();
String firstName = auth.getFirstName();
boolean isUser = auth.findUser(email);
boolean isAuthenticated = auth.authUser(pass);
if(isUser == false || isAuthenticated == false){
RequestDispatcher rd = request.getRequestDispatcher("login.html");
rd.forward(request, response);
}
HttpSession session = request.getSession();
session.setAttribute("firstName", firstName);
RequestDispatcher rd = request.getRequestDispatcher("welcome.jsp");
rd.forward(request, response);
}
}
以下是该程序的日志，如果有帮助的话：
Warning: StandardWrapperValve[Login]: Servlet.service() for servlet Login threw exception
java.lang.NullPointerException
at org.apache.jsp.welcome_jsp._jspService(welcome_jsp.java:80)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:875)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:739)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:575)
at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:546)
at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:428)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:378)
at com.Exoticatravels.Login.doPost(Login.java:51)
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.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
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:416)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:283)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571)
at java.lang.Thread.run(Thread.java:748)
请帮助！预先感谢！
答案 0 :(得分：0)
您的代码在Login servlet中应该是这样的，这是因为您正在为会话设置空字符串firstName，我认为这可以解决问题
boolean isUser = auth.findUser(email);
boolean isAuthenticated = auth.authUser(pass);
String firstName = auth.getFirstName();