從servlet調用會話bean與jsp-> servlet授予不同的結果

Question

我在嘗試調用會話bean的方法時，首先從JSP調用servlet，然後調用會話bean的方法到servlet獲取一些數據似乎得到一個NullPointerException。另一方面，如果我只是自己運行servlet，它會完成這項工作並從會話bean中檢索數據。

以下是servlet的代碼;

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; 
    } 
} 

這是調用servlet的JSP的部分：從JSP-致電時>

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; 
    } 

} 

這裏是堆棧跟蹤：

<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> 

這是會話Bean servlet->會話bean：

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) 

Answer 1

你不能像你那樣從JSP中調用「servlet」實例。 你實際做什麼用：

<jsp:useBean id="obj" scope="page" class="web.processItem" /> 

是創建web.processItem的一個「新」的實例，它是不是容器管理的根本 - 因此它沒有你的依賴。

由於容器確實創建並管理了processItem的實例，並且已注入了您的依賴關係，所以只能調用servlet。