Entitymanager抛出NullPointerException

Question

我使用Netbeans.The实体管理器开发JavaEE应用程序时在应用程序中获得空值。不能注射电磁波。我仍然得到一个nullPointerException，因为em在createNamedQuery中得到一个空值。谁能让我知道我错过了什么。

JSP page- index.jsp：

<%@page import="managed.userBean" %>
 <jsp:useBean id="bean" class="managed.userBean" scope="session" />
.....
<input type="submit" value="Submit" onclick="<jsp:scriptlet> bean.validate();</jsp:scriptlet>" /> 

userBean.java

package managed;

import java.io.Serializable;
import javax.ejb.EJB;
import javax.faces.bean.ManagedBean;
import javax.faces.bean.SessionScoped;
import session.UserFacade;

@ManagedBean
@SessionScoped


public class userBean implements Serializable {

private String uid, password,role,fname,lname;
private String response="" ;
@EJB
UserFacade userFacade;
....
public String validate() {
    System.out.println("in validate going to user facade:  " + uid + password);
    response = userFacade.validateUser(uid,password);
 .....
}

UserFacade.java

package session;
import javax.ejb.EJB;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless

public class UserFacade extends AbstractFacade<User> {
@PersistenceContext (unitName = "ExamSysPU")
public EntityManager em;
String role;

@Override
protected EntityManager getEntityManager() {
    System.out.println("check if em is open");
    return em;
}

public UserFacade() {
    super(User.class);
}


    public String validateUser(String uid, String password) {
     ...
              List results = em.createNamedQuery("User.findByUid").setParameter("uid", uid).getResultList();
   ....
}

Persistence.xml：

   <?xml version="1.0" encoding="UTF-8"?>
  <persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   <persistence-unit name="ExamSysPU" transaction-type="JTA">
      <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider> 
     <jta-data-source>test</jta-data-source>
      <exclude-unlisted-classes>false</exclude-unlisted-classes>
      <properties/>
  </persistence-unit>
 </persistence>

谢谢，希尔巴

Answer 1

之所以会出现这个问题，是因为您使用<jsp:useBean>手动创建了UserBean，而不是使用由JSF为您创建和管理的依赖项(并因此自动注入了所有必需的依赖项)。本质上，如果您手动创建和管理bean，那么您也应该手动创建、管理和注入它的依赖项。

停止阅读那些十年前的JSP 1.x教程。在Java EE 6天内，它们不再适用，并且只会让您在这些日子里对正确的方法感到困惑。JSP已经被Facelets (XHTML)所取代。学习一些JSF2/Facelets教程。基本上，您的具体问题可以通过将JSP文件替换为至少包含以下内容的Facelets文件来解决：

<h:form>
    <h:commandButton value="Submit" action="#{bean.validate}" />
</h:form>

另请参阅：

• Our JSF wiki page -包含一个JSF2/Facelets Hello World启动示例和几个指向sane JSF2 tutorials

的链接