servlet jsp 的 Servlet.service() 在调用 EJB bean 时抛出异常 java.lang.NullPointerException答案

【问题标题】：Servlet.service() for servlet jsp threw exception java.lang.NullPointerException while calling EJB beanservlet jsp 的 Servlet.service() 在调用 EJB bean 时抛出异常 java.lang.NullPointerException
【发布时间】：2011-12-01 21:47:21
【问题描述】：

在我的简单 jsp 页面上调用 EJB bean 时出现这样的错误：

 WARNING: StandardWrapperValve[jsp]: PWC1406: Servlet.service() for servlet jsp threw 
 exception java.lang.NullPointerException
at appjpa.ejb.UsersFacade.findAllStudents(UsersFacade.java:65)
at org.apache.jsp.Students_jsp._jspService(Students_jsp.java from :75)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:403)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:492)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:378)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98)
at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:330)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:174)
at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:828)
at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:725)
at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1019)
at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
at java.lang.Thread.run(Thread.java:662)

我的jsp页面的代码是这样的：

<%@page import="appjpa.entities.Users"%>
<%@page import="java.util.ArrayList"%>
<%@page import="socnet2.Student"%> <!-- class Student emulates real student -->
<%@page contentType="text/html" pageEncoding="UTF-8"%>
 <!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <jsp:useBean class="appjpa.ejb.UsersFacade" id="dao" scope="application"></jsp:useBean>
    <p><b>List of all students:</b></p>
    <form action="/AppJPA/Edit" method="POST">
       <jsp:scriptlet>
       int counter = 0;  if (dao != null){
       for (Users s : dao.findAllStudents()) {
        </jsp:scriptlet>
        <p> <jsp:expression> s.getName() + " " + s.getSurname()</jsp:expression></p>
        <input type="submit" name="Edit_<%=counter%>" value="Edit" />
        <input type="submit" name="Delete_<%=counter%>" value ="Delete"/> 
         <jsp:scriptlet>
                counter++;
            }}
        </jsp:scriptlet>
        <input type="submit" name="Add_new_student" value ="Add new student"/>
    </form>
</body>

由 NetBeans 7.0.1 向导为用户自动生成的实体类如下所示：

 @Entity
@Table(name = "USERS")
@NamedQueries({
@NamedQuery(name = "Users.findAll", query = "SELECT u FROM Users u"),
@NamedQuery(name = "Users.findById", query = "SELECT u FROM Users u WHERE u.id = :id"),
@NamedQuery(name = "Users.findByName", query = "SELECT u FROM Users u WHERE u.name =      :name"),
@NamedQuery(name = "Users.findBySurname", query = "SELECT u FROM Users u WHERE u.surname = :surname"),
@NamedQuery(name = "Users.findByLogin", query = "SELECT u FROM Users u WHERE u.login = :login"),
@NamedQuery(name = "Users.findByPassword", query = "SELECT u FROM Users u WHERE u.password = :password"),
@NamedQuery(name = "Users.findByRole", query = "SELECT u FROM Users u WHERE u.role = :role")})
public class Users implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
// @NotNull
@Column(name = "ID")
private Integer id;
@Size(max = 40)
@Column(name = "NAME")
private String name;
@Size(max = 40)
@Column(name = "SURNAME")
private String surname;
@Size(max = 40)
@Column(name = "LOGIN")
private String login;
@Size(max = 40)
@Column(name = "PASSWORD")
private String password;
@Size(max = 40)
@Column(name = "ROLE")
private String role;
@ManyToMany(mappedBy = "usersList")
private List<SubjectGroup> subjectGroupList;
@OneToMany(cascade = CascadeType.ALL, mappedBy = "idCreator")
private List<SubjectGroup> subjectGroupList1;
@OneToMany(cascade = CascadeType.ALL, mappedBy = "idSender")
private List<Messages> messagesList;

public Users() {
}

public Users(Integer id) {
    this.id = id;
}

public Integer getId() {
    return id;
}

public void setId(Integer id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getSurname() {
    return surname;
}

public void setSurname(String surname) {
    this.surname = surname;
}

public String getLogin() {
    return login;
}

public void setLogin(String login) {
    this.login = login;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

public String getRole() {
    return role;
}

public void setRole(String role) {
    this.role = role;
}

public List<SubjectGroup> getSubjectGroupList() {
    return subjectGroupList;
}

public void setSubjectGroupList(List<SubjectGroup> subjectGroupList) {
    this.subjectGroupList = subjectGroupList;
}

public List<SubjectGroup> getSubjectGroupList1() {
    return subjectGroupList1;
}

public void setSubjectGroupList1(List<SubjectGroup> subjectGroupList1) {
    this.subjectGroupList1 = subjectGroupList1;
}

public List<Messages> getMessagesList() {
    return messagesList;
}

public void setMessagesList(List<Messages> messagesList) {
    this.messagesList = messagesList;
}

@Override
public int hashCode() {
    int hash = 0;
    hash += (id != null ? id.hashCode() : 0);
    return hash;
}

@Override
public boolean equals(Object object) {
    // TODO: Warning - this method won't work in the case the id fields are not set
    if (!(object instanceof Users)) {
        return false;
    }
    Users other = (Users) object;
    if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
        return false;
    }
    return true;
}

@Override
public String toString() {
    return "appjpa.entities.Users[ id=" + id + " ]";
}

}

最后，UsersFacade 类是这样的：

@Stateless
public class UsersFacade extends AbstractFacade<Users> implements UsersFacadeLocal {
@PersistenceContext(unitName = "AppJPAPU")
private EntityManager em;

protected EntityManager getEntityManager() {
    return em;
}

public UsersFacade() {
    super(Users.class);
}

@Override
public Users checkInBase(String login, String password) {
    Users u = (Users)(em.createNamedQuery("Users.findByLogin").setParameter("login",      login).getSingleResult());

    return u;
}

@Override
public void removeUserFromSubjectGroup(Users user, SubjectGroup group) {
    Users u = em.find(Users.class, user.getId());
    SubjectGroup g = em.find(SubjectGroup.class, group.getId());
    if (u != null && g != null) {
        user.getSubjectGroupList().remove((SubjectGroup)group);
        group.getUsersList().remove(user);
        em.merge(user);
        em.merge(group);
    }
}

@Override
public void addUserToSubjectGroup(Users user, SubjectGroup group) {
    Users u = em.find(Users.class, user.getId());
    SubjectGroup g = em.find(SubjectGroup.class, group.getId());
    if (u != null && g != null) {
        user.getSubjectGroupList().add((SubjectGroup)group);
        group.getUsersList().add(user);
        em.merge(user);
        em.merge(group);
    }
}

@Override
public List<Users> findAllStudents() {
    String role = new String("student");
     List<Users> students = (List<Users>)(em.createNamedQuery("SELECT u FROM Users u WHERE u.role = :role").setParameter("role", role).getResultList());
     if (students == null){
         System.out.println("Students are null!!!");
     }

    return students;
}

@Override
public boolean deleteStudent(Users user) {
    Users u = em.find(Users.class, user.getId());
    em.remove(user);
    return true;
}

@Override
public boolean updateStudent(Users studentToEdit, String newName, String newSurname, String newLogin, String newPassword) {
    Users u = em.find(Users.class, studentToEdit.getId());
    u.setName(newName);
    u.setSurname(newSurname);
    u.setLogin(newLogin);
    u.setPassword(newPassword);
     em.merge(studentToEdit);
    return true;
}

@Override
public void insertStudenttoDB(Users student, String newName, String newSurname, String newLogin, String newPassword) {
   em.persist(student);
   Users u = em.find(Users.class, student.getId());
    if (u != null) {
    student.setName(newName);
    student.setSurname(newSurname);
    student.setLogin(newLogin);
    student.setPassword(newPassword);  
    em.merge(student);
    }
}     
}

我对 EJB 很陌生，也许我没有正确使用它们。我尝试在其他页面从 UsersFaced 调用其他方法，但收到相同的错误。请解释一下我做错了什么......

【问题讨论】：

标签： jsp exception servlets ejb-3.0 jpa-2.0

【解决方案1】：

如果您通过<jsp:useBean> 手动将EJB 类实例化为JSP 中的简单Java bean，而不是通过@EJB 获取容器管理实例，则不会注入@PersistenceContext。我不知道你是如何以及在哪里学会用这种方式编写 JSP 的，但它无疑是基于一本严重过时的书/教程。您需要对当前的无控制器模型视图方法进行重大更改。您需要添加一个控制器。扔掉所有那些<jsp:xxx> 标签并创建一个充当真正控制器的Servlet。

例如

@WebServlet("/students")
public class StudentsServlet extends HttpServlet {

    @EJB
    private UsersFacade usersFacade;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setAttribute("users", usersFacade.findAllStudents());
        request.getRequestDispatcher("/WEB-INF/students.jsp").forward(request, response);
    }

}

请注意，我将您的 JSP 移到了 /WEB-INF 文件夹中，这样就不可能在不调用 servlet 的情况下有意或无意地调用它。

现在，如下更改您的 JSP：

<%@page pageEncoding="UTF-8" %>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<!DOCTYPE html>
<html lang="en">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
    </head>
    <body>
        <h4>List of all students:</h4>
        <form action="${pageContext.request.contextPath}/edit" method="post">
            <c:forEach items="${users}" var="user" varStatus="loop">
                <p><c:out value="${user.name} ${user.surname}" /></p>
                <input type="submit" name="Edit_${loop.index}" value="Edit" />
                <input type="submit" name="Delete_${loop.index}" value ="Delete"/> 
            </c:forEach>
            <input type="submit" name="Add_new_student" value ="Add new student"/>
        </form>
    </body>
</html>

通过 servlet 的 URL http://localhost:8080/AppJPA/students 打开它，这样它的 doGet() 将执行正确的工作，并且 JSTL <c:forEach> 将相应地迭代 users。

顺便说一句，我对您命名按钮的方式表示怀疑。识别所选用户将非常笨拙。但这是另一个问题的主题。

另见：

【讨论】：

感谢您的回答。我的应用程序中有一个控制器 servlet，我的问题是在某些情况下我没有正确使用它（。我添加了 @EJB private UsersFacade usersFacade; 在 servlet 中，但现在我收到了javax.servlet.ServletException：PWC1392：实例化 servlet 类 control.Controler 时出错，根本原因：com.sun.enterprise.container.common.spi.util.InjectionException：尝试注入远程 ejb-ref name=control.Controler/usersFacade 时出现异常和其他一些根案例。
那么您的 EJB 设计/实现中存在问题，导致容器无法创建和管理它。堆栈跟踪的最根本原因包含此问题的答案。如果你实在想不通，就问一个新问题。这是这个问题的题外话。

【解决方案2】：

如果这是你的一整套命名查询，我猜这个代码：

 List<Users> students = (List<Users>)(em.createNamedQuery(
         "SELECT u FROM Users u WHERE u.role = :role").setParameter("role", role)
         .getResultList());

搞砸了。您正在尝试创建名称等于 SELECT u FROM Users u WHERE u.role = :role 的 named 查询。

您已在 Users 注释中定义了命名查询，而您尝试使用的是这个：

@NamedQuery(name = "Users.findByRole", 
            query = "SELECT u FROM Users u WHERE u.role = :role")})

因此，您应该使用它的名称 - Users.findByRole。此外，您不需要转换为List<Users>，因为有一个appropriate method parameter 来定义结果类型。应该是：

List<Users> students = em.createNamedQuery("Users.findByRole", Users.class)
                         .setParameter("role", role).getResultList();

您的代码中有一些不好的做法，例如：

为您的实体使用复数；应该是User 而不是Users，
使用小脚本；并且使用 <jsp:scriptlet> 并没有让它变得更好。

【讨论】：