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EJB3.1 的新手，我正在关注http://netbeans.org/kb/docs/javaee/javaee-entapp-ejb.html上的教程示例，完成后，我运行项目但得到一个NullPointerException; 当我使用该教程中的源代码 zip 时也会发生同样的情况，所以我想这不是我自己的源代码中的错字。

问题是newsEntityFacade我的ListNewsservlet 中的 是null，因此在List news = newsEntityFacade.findAll();

（Java7、Glassfish 3.1.2、Netbeans 7.1.2）

错误日志：

WARNING: StandardWrapperValve[ListNews]: PWC1406: Servlet.service() for servlet ListNews threw exception
java.lang.NullPointerException
    at web.ListNews.processRequest(ListNews.java:53)
    at web.ListNews.doGet(ListNews.java:83)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:668)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:770)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1542)
    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 org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:161)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:331)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
    at com.sun.enterprise.v3.services.impl.ContainerMapper$AdapterCallable.call(ContainerMapper.java:317)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:195)
    at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:849)
    at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:746)
    at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1045)
    at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:228)
    at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
    at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
    at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
    at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
    at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
    at java.lang.Thread.run(Thread.java:722)

ListNews.java servlet：

package web;

[...]

@WebServlet(name = "ListNews", urlPatterns = {"/ListNews"})
public class ListNews extends HttpServlet {
    @EJB
    private SessionManagerBean sessionManagerBean;
    @EJB
    private NewsEntityFacade newsEntityFacade;

    /**
     * Processes requests for both HTTP
     * <code>GET</code> and
     * <code>POST</code> methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.getSession(true);
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        try {
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet ListNews</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Servlet ListNews at " + request.getContextPath() + "</h1>");

            List news = newsEntityFacade.findAll();
            for (Iterator it = news.iterator(); it.hasNext();) {
                NewsEntity elem = (NewsEntity) it.next();
                out.println(" <b>"+elem.getTitle()+" </b><br />");
                out.println(elem.getBody()+"<br /> ");
            }
            out.println("<a href='PostMessage'>Add new message</a>");

            out.println("<br><br>");
            out.println(sessionManagerBean.getActiveSessionsCount() + " user(s) reading the news.");

            out.println("</body>");
            out.println("</html>");
        } finally {            
            out.close();
        }
    }

    [...]
}

新闻实体外观：

package ejb;

import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless
public class NewsEntityFacade extends AbstractFacade<NewsEntity> {
    @PersistenceContext(unitName = "NewsApp-ejbPU")
    private EntityManager em;

    @Override
    protected EntityManager getEntityManager() {
        return em;
    }

    public NewsEntityFacade() {
        super(NewsEntity.class);
    }

}

它从 AbstractFacade 扩展而来：

package ejb;

import java.util.List;
import javax.persistence.EntityManager;

public abstract class AbstractFacade<T> {
    private Class<T> entityClass;

    public AbstractFacade(Class<T> entityClass) {
        this.entityClass = entityClass;
    }

    protected abstract EntityManager getEntityManager();

    public void create(T entity) {
        getEntityManager().persist(entity);
    }

    public void edit(T entity) {
        getEntityManager().merge(entity);
    }

    public void remove(T entity) {
        getEntityManager().remove(getEntityManager().merge(entity));
    }

    public T find(Object id) {
        return getEntityManager().find(entityClass, id);
    }

    public List<T> findAll() {
        javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
        cq.select(cq.from(entityClass));
        return getEntityManager().createQuery(cq).getResultList();
    }

    public List<T> findRange(int[] range) {
        javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
        cq.select(cq.from(entityClass));
        javax.persistence.Query q = getEntityManager().createQuery(cq);
        q.setMaxResults(range[1] - range[0]);
        q.setFirstResult(range[0]);
        return q.getResultList();
    }

    public int count() {
        javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
        javax.persistence.criteria.Root<T> rt = cq.from(entityClass);
        cq.select(getEntityManager().getCriteriaBuilder().count(rt));
        javax.persistence.Query q = getEntityManager().createQuery(cq);
        return ((Long) q.getSingleResult()).intValue();
    }

}
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