EJB注入时的空指针异常 [英] Null pointer exception while EJB injection

问题描述

我写了一个非常简单的Web应用程序与一个无状态的Ejb，有趣的时刻到来时，我实现接口 - ejb依赖注入不起作用：

web。 xml

 <？xml version =1.0？> 
< web-app xmlns =http://java.sun.com/xml/ns/javaee
 xmlns：xsi =http://www.w3.org/2001/XMLSchema -instance
 xsi：schemaLocation =http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd
 version =3.0> 
< display-name>原型创建的Web应用< / display-name> 
< servlet> 
< servlet-name> simpleController< / servlet-name> 
< servlet-class> com.SimpleController< / servlet-class> 
< / servlet> 
< servlet-mapping> 
< servlet-name> simpleController< / servlet-name> 
< url-pattern> *。job< / url-pattern> 
< / servlet-mapping> 
< / web-app> 
  

Servlet类

  package com; 
 import javax.ejb.EJB; 
 import javax.servlet.ServletException; 
 import javax.servlet.http.HttpServlet; 
 import javax.servlet.http.HttpServletRequest; 
 import javax.servlet.http.HttpServletResponse; 
 import java.io.IOException; 
 
 public class SimpleController extends HttpServlet {
 
 @EJB 
 private SimpleBean bean; 
 
 @Override 
 protected void doPost（HttpServletRequest req，HttpServletResponse resp）throws ServletException，IOException {
 resp.getWriter（）。println（bean.getSomeString（））; 
} 
 
 @Override 
 protected void doGet（HttpServletRequest req，HttpServletResponse resp）throws ServletException，IOException {
 doPost（req，resp）; 
} 
} 
  

bean

  package com; 
 
 import javax.ejb.Stateless; 
 
 @Stateless 
 public class SimpleBean / * implements Simple * / {
 public String getSomeString（）{
 returnaxaxaxa！ 
} 
} 
  

界面

  package com; 
 
 import javax.ejb.Remote; 
 
 // @ Local 
 @Remote 
 public interface Simple {
 String getSomeString（）; 
} 
  

所以当我不注意实现简单时，我得到NullPointerException（使用@Remote或@Local注释在界面），另外一种情况我得到axaxaxa！。为什么会发生我读了Ejb类必须实现内容。

我使用Tommeecas应用服务器。

解决方案

通过更改bean变量类型（接口类型而不是类类型）解决问题

使用NullPointerException

  private SimpleBean bean; 
  

工作版

  private Simple bean; 
  

I wrote very simple web application with one stateless Ejb, the interesting moment comes when I implement interface - ejb dependency injection does not work:

web.xml

<?xml version="1.0" ?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
         version="3.0">
  <display-name>Archetype Created Web Application</display-name>
    <servlet>
        <servlet-name>simpleController</servlet-name>
        <servlet-class>com.SimpleController</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>simpleController</servlet-name>
        <url-pattern>*.job</url-pattern>
    </servlet-mapping>
</web-app>

Servlet class

package com;
import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class SimpleController extends HttpServlet {

    @EJB
    private SimpleBean bean;

    @Override
    protected void doPost (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().println(bean.getSomeString());
    }

    @Override
    protected void doGet (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }
}

bean

package com;

import javax.ejb.Stateless;

@Stateless
public class SimpleBean /*implements Simple*/ {
    public String getSomeString(){
        return "axaxaxa!";
    }
}

inteface

package com;

import javax.ejb.Remote;

//@Local
@Remote
public interface Simple {
    String getSomeString();
}

So when I uncoment "implements Simple" I got NullPointerException (with @Remote or @Local annotation in interface), another case I get "axaxaxa!". Why it happens? I read the Ejb class must implement intefaces.

I use TommEEcas Application Server.

解决方案

Problem solves by changing bean variable type (to interface type instead of class type)

Version with NullPointerException

private SimpleBean bean;

Working version

private Simple bean;

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