【发布时间】：2014-02-28 15:02:21
【问题描述】：
我在尝试调用会话 bean 的方法时遇到问题，首先从 JSP 调用 servlet，然后 servlet 调用会话 bean 的方法来获取一些数据，它似乎得到了 NullPointerException。另一方面，如果我只是自己运行 servlet，它会完成这项工作并从会话 bean 中检索数据。
以下是servlet的代码；
package web;
import java.io.IOException;
import java.util.ArrayList;
import javax.annotation.PostConstruct;
import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import auction.ItemRegistrationSessionBeanRemote;
/**
* Servlet implementation class processItem
*/
@WebServlet("/processItem")
public class processItem extends HttpServlet {
private static final long serialVersionUID = 1L;
public ArrayList<String> categories = new ArrayList<String>();
@EJB ItemRegistrationSessionBeanRemote itemRegistrationSession;
/**
* @see HttpServlet#HttpServlet()
*/
public processItem() {
super();
}
@PostConstruct
public
void init() {
System.out.println("init()");
categories = itemRegistrationSession.getCategories();
System.out.println("init() categories = " + categories.toString());
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}
@PostConstruct
public void obtainCategories() {
categories = itemRegistrationSession.getCategories();
}
@PostConstruct
public ArrayList<String> getCategories() {
return categories;
}
}
这是调用servlet的JSP部分：
<jsp:useBean id="obj" scope="page" class="web.processItem" />
<select name="affiliation" id="categories-dropdown">
<% obj.init(); %>
<c:forEach var="aff" items="${obj.getCategories()}"
<%System.out.println("categories = " + obj.getCategories()); %>
<option value="${aff}">${aff}</option>
</c:forEach>
</select>
这是会话 Bean：
package auction;
import java.util.ArrayList;
import java.util.List;
import javax.ejb.LocalBean;
import javax.ejb.Stateful;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import com.sun.jersey.spi.resource.Singleton;
import entity.Category;
/**
* Session Bean implementation class ItemRegistrationSessionBean
*/
@Stateful @Singleton
public class ItemRegistrationSessionBean implements ItemRegistrationSessionBeanRemote {
@PersistenceContext(name = "MiniEbayEJB")
private EntityManager emgr;
/**
* Default constructor.
*/
public ItemRegistrationSessionBean() {
// TODO Auto-generated constructor stub
}
@Override
public ArrayList<String> getCategories() {
System.out.println("Hello from SessionBean getCategories");
ArrayList<String> categories = new ArrayList<String>();
List<Category> obtainedCategories = (List<Category>) emgr.createNamedQuery("Category.findAll", Category.class).getResultList();
for (int i = 0; i < obtainedCategories.size(); i++) {
categories.add(obtainedCategories.get(i).toString());
System.out.println("Current ArrayList of categories = " + categories);
}
return categories;
}
}
这是从 jsp->servlet->session bean 调用时的堆栈跟踪：
2014-02-28T14:48:21.696+0000|WARNING: StandardWrapperValve[jsp]: PWC1406: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at web.processItem.init(processItem.java:35)
at org.apache.jsp.insert_jsp._jspService(insert_jsp.java:166)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:770)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:770)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1550)
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:860)
at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:757)
at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1056)
at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:229)
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:744)
【问题讨论】：
标签： java jsp servlets ejb session-bean