JPA中的NullPointerException

Question

我正在使用jpa创建Java EE应用程序.

当代码在服务器上运行时,我收到以下错误

    [2/25/12 22:56:31:371 IST] 00000047 SystemOut     O hello
    [2/25/12 22:56:31:375 IST] 00000047 SystemOut     O 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)

UserBean.java中的第41行是:

//在UserBean.java中,包含UserBean无状态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";

    }
    }

User类,User已从数据库表自动生成.数据库中有一个USER表.请帮助解决此错误的可能原因.

Answer 1

您EntityManager manager的客户端中注入的Enterprise Java Bean是无接口的UserBean.应将EJB注入其生命周期由应用程序服务器管理的类中.

客户端必须是Web组件或其他企业bean.在您的情况下,客户端UserBean是POJO(Plain Old Java Object).有关如何使用EJB的更多详细信息,请参见此处.

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来使UserBean成为无状态会话Bean.最后,您应该使用EJB注释将UserBean注入到ManagedBean中.