JPA 中的 NullPointerException [英] NullPointerException in JPA
问题描述
我正在使用 jpa 创建一个 Java EE 应用程序.
在服务器上运行代码时出现以下错误
[2/25/12 22:56:31:371 IST] 00000047 SystemOut O hello[2/25/12 22:56:31:375 IST] 00000047 SystemOut O 错误...:([2/25/12 22:56:31:371 IST] 00000047 SystemErr R java.lang.NullPointerException[2/25/12 22:56:31:371 IST] 00000047 SystemErr R at plh.service.ejb.UserBean.getUserDetails(UserBean.java:41)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:45)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R 在 java.lang.reflect.Method.invoke(Method.java:599)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at com.ibm.etools.utc.model.ReflectionMethodModel.invoke(ReflectionMethodModel.java:65)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at com.ibm.etools.utc.servlet.InvokeServlet.invoke(InvokeServlet.java:113)[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at com.ibm.etools.utc.servlet.InvokeServlet.doPost(InvokeServlet.java:374)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R 在 javax.servlet.http.HttpServlet.service(HttpServlet.java:738)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R 在 javax.servlet.http.HttpServlet.service(HttpServlet.java:831)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1443)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:790)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:443)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapperImpl.handleRequest(ServletWrapperImpl.java:175)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.CacheServletWrapper.handleRequest(CacheServletWrapper.java:91)[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:859)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.WSWebContainer.handleRequest(WSWebContainer.java:1557)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:173)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:455)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewInformation(HttpInboundLink.java:384)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.http.channel.inbound.impl.HttpICLReadCallback.complete(HttpICLReadCallback.java:83)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:165)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:138)[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:202)[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:766)[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:896)[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1527)
UserBean.java 中的第 41 行是:
//在UserBean.java中,包含一个无状态EJB的UserBean
公共类 UserBean 实现 UserBeanInterface {/*** 默认构造函数.*/@PersistenceContext私有 EntityManager 管理器;公共用户豆(){}公共字符串身份验证(字符串名称){返回空;}公共字符串更改密码(){返回空;}公共字符串 setUserDetails(){返回空;}公共字符串 getUserDetails(){System.out.println("你好");尝试{41=>查询 query=manager.createQuery("from User");列表<用户>结果 = query.getResultList();如果(结果.大小()!= 0){迭代器<用户>stIterator=results.iterator();while(stIterator.hasNext()){用户 st=stIterator.next();System.out.print("用户ID:"+st.getUserId());System.out.print("类型:"+st.getUserType());System.out.print("详细信息:"+st.getUserProfile());}}别的{System.out.println("没有找到记录.");}}捕获(例外 e){e.printStackTrace();System.out.println("错误...:(");}返回嗨";}}
User 类,User 是从数据库表中自动生成的.数据库中有一个 USER 表.请帮助说明此错误的可能原因.
Your EntityManager
manager
是注入到客户端 UserBean 的无接口 Enterprise Java Bean代码>.EJB 应该注入到生命周期由应用服务器管理的类中.
客户端必须是 Web 组件或另一个企业 bean.在您的情况下,客户端 UserBean
是一个 POJO(Plain Old Java Object).有关如何使用 EJB 的更多详细信息,请参阅此处.>
一个可能的简单解决方案是通过在类定义之前添加注释 javax.ejb.Stateless 使 UserBean 成为无状态会话 Bean.您最终应该使用 EJB 注释将 UserBean 注入到 ManagedBean 中.
I am creating a Java EE application using jpa.
I am getting the following error when the code is run on server
[2/25/12 22:56:31:371 IST] 00000047 SystemOut O hello
[2/25/12 22:56:31:375 IST] 00000047 SystemOut O Error...:(
[2/25/12 22:56:31:371 IST] 00000047 SystemErr R java.lang.NullPointerException
[2/25/12 22:56:31:371 IST] 00000047 SystemErr R at plh.service.ejb.UserBean.getUserDetails(UserBean.java:41)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:45)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at java.lang.reflect.Method.invoke(Method.java:599)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at com.ibm.etools.utc.model.ReflectionMethodModel.invoke(ReflectionMethodModel.java:65)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at com.ibm.etools.utc.servlet.InvokeServlet.invoke(InvokeServlet.java:113)
[2/25/12 22:56:31:372 IST] 00000047 SystemErr R at com.ibm.etools.utc.servlet.InvokeServlet.doPost(InvokeServlet.java:374)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at javax.servlet.http.HttpServlet.service(HttpServlet.java:738)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at javax.servlet.http.HttpServlet.service(HttpServlet.java:831)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1443)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:790)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:443)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.ServletWrapperImpl.handleRequest(ServletWrapperImpl.java:175)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.servlet.CacheServletWrapper.handleRequest(CacheServletWrapper.java:91)
[2/25/12 22:56:31:373 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:859)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.WSWebContainer.handleRequest(WSWebContainer.java:1557)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:173)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:455)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewInformation(HttpInboundLink.java:384)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.http.channel.inbound.impl.HttpICLReadCallback.complete(HttpICLReadCallback.java:83)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:165)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
[2/25/12 22:56:31:374 IST] 00000047 SystemErr R at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)
[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:138)
[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:202)
[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:766)
[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:896)
[2/25/12 22:56:31:375 IST] 00000047 SystemErr R at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1527)
Line no 41 in UserBean.java is :
//within UserBean.java, containing UserBean a stateless EJB
public class UserBean implements UserBeanInterface {
/**
* Default constructor.
*/
@PersistenceContext
private EntityManager manager;
public UserBean() {
}
public String authenticate(String name){
return null;
}
public String changePassword(){
return null;
}
public String setUserDetails(){
return null;
}
public String getUserDetails(){
System.out.println("hello");
try{
41=> Query query=manager.createQuery("from User");
List<User> results = query.getResultList();
if(results.size()!=0){
Iterator<User> stIterator=results.iterator();
while(stIterator.hasNext()){
User st=stIterator.next();
System.out.print("User Id:"+st.getUserId());
System.out.print("Type:"+st.getUserType());
System.out.print("Detail:"+st.getUserProfile());
}
}
else
{
System.out.println("Record not found.");
}
}
catch(Exception e)
{
e.printStackTrace();
System.out.println("Error...:(");
}
return "hi";
}
}
The User class, User has been generated automatically from the database tables. There is a USER table in the database. Please help on what the possible reason for this error could be.
Your EntityManager
manager
is no-interface an Enterprise Java Bean injected in your client UserBean
. EJBs should be injected in classes whose lifecycle is managed by the application server.
The client must be either a web component or another enterprise bean. In your case, the client UserBean
is a POJO (Plain Old Java Object). See here for further details about how to use EJBs.
A possible simple solution is to make UserBean a Stateless Session Bean by adding the annotation javax.ejb.Stateless before the class definition. And you should eventually inject UserBean into your ManagedBean with the EJB annotation.
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