Question

我是JSP的新手，我试图开发一个简单的程序来测试我到目前为止学到的东西我所做的是一个使用Bean的JSP页面，并尝试使用用户提供的信息来保存实体。 Web应用程序部署在glassfish4服务器上

实体是一个带有setter和getter的简单User.java类（我可以在servlet中使用它，所以应该有任何问题）。

Bean是以下UserEJB.java类

@LocalBean
@Stateless
public class UserEJB implements Serializable {

    @PersistenceContext(unitName="scuoleSciPU")
    private EntityManager em; 

    public List<Request> findUsers() {
        Query query = em.createNamedQuery("findAllUsers");
        return query.getResultList();
    }

    public User createUser(User user) {
        em.persist(user); <-- here is where NullPointer is throw
        return user;
    }
}

JSP页面是以下页面，用户在提交表单后被重定向。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<jsp:useBean id="userEjb" type="com.storassa.javaee.scuolesci.UserEJB" class="com.storassa.javaee.scuolesci.UserEJB"
        scope="session">
</jsp:useBean>
<%@ page import="com.storassa.javaee.scuolesci.User" %>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Successful registration</title>
</head>
<BODY>
<h1>You have successfully registered to this website</h1>
<% 
    User user = new User();
    user.setEmail((String)request.getParameter("email"));
    user.setName((String)request.getParameter("name"));
    user.setSurname((String)request.getParameter("surname"));
    user.setPassword((String)request.getParameter("password"));
    user.setYearOfBirth(Integer.parseInt((String)request.getParameter("yearOfBirth")));
    userEjb.createUser(user);

    session.setAttribute("logged", "true");
    session.setAttribute("name", user.getName());%>
<a href="carousel.jsp"><h3>Return to main page</h3></a>
</BODY>
</html>

当加载JSP页面时，抛出NullPointerException（在em.persist（）中，如上面的代码所述）。这是完整的错误堆栈

StandardWrapperValve[jsp]: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
    at com.storassa.javaee.scuolesci.UserEJB.createUser(UserEJB.java:32)
    at org.apache.jsp.registered_jsp._jspService(registered_jsp.java:73)
    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.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:357)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
    at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
    at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
    at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
    at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
    at java.lang.Thread.run(Thread.java:745)

Answer 1

当您使用jsp：useBean时，servlet容器在请求/会话/ servlet上下文中查找具有该名称（或类型）的bean。如果找不到，它将实例化一个。即使您将其声明为SessionBean，您的bean也不在这些上下文中，它位于EJB容器中。因此，您从JSP引用的是该类的简单实例，而不是实际的EJB。因此，依赖注入机制不起作用，因此EntityManager变量为null。您应该查找EJB以获取正确的EJB实例（完成所有DI）或使用CDI。

在JSP上持久化实体：EntityManager上的NullPointerException

1 个答案: