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，并在完成后运行项目，但得到NullPointerException;当我使用该教程中的源代码zip时也会发生同样的事情，所以我想这不是我自己的源代码中的拼写错误。
问题是我的newsEntityFacade servlet中的ListNews为null，因此将NPE投放到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();
}
}
[...]
}
NewsEntityFacade：
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);
}
}
从AbstractFacade扩展：
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();
}
}