java - EJB注入(inject)时出现空指针异常

Question

我用一个无状态 Ejb 编写了非常简单的 Web 应用程序，当我实现接口(interface)时有趣的时刻来了——ejb 依赖注入(inject)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>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 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(){
        return "axaxaxa!";
    }
}

界面

package com;

import javax.ejb.Remote;

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

因此，当我取消注释“implements Simple”时，我得到了 NullPointerException(在接口(interface)中带有 @Remote 或 @Local 注释)，另一种情况下我得到了“axaxaxa!”。为什么会这样？我读到 Ejb 类必须实现接口(interface)。

我使用 TommEEcas 应用服务器。

Answer 1

问题通过改变 bean 变量类型(到接口(interface)类型而不是类类型)来解决

带有 NullPointerException 的版本

private SimpleBean bean;

工作版本

private Simple bean;