EJB注入时的空指针异常 [英] Null pointer exception while EJB injection
问题描述
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。
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.
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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