Dependency Injection inside of HttpSessionListener implementation

Question

Problem: This injected dependency will always return 0 from SimpleController

Why does the context get lost for this bean when trying to do dependency injection into an HttpSessionListener implementation?
What is principles behind this am I missing/confusing for this not to be working?
How do I fix this?

Consider the following:

SessionCounterListener

public class SessionCounterListener implements HttpSessionListener {

  @Autowired
  private SessionService sessionService;

  @Override
  public void sessionCreated(HttpSessionEvent arg0) {
    sessionService.addOne();
  }

  @Override
  public void sessionDestroyed(HttpSessionEvent arg0) {
    sessionService.removeOne();
  } 
}

web.xml

<web-app ...>
    <listener>
        <listener-class>com.stuff.morestuff.SessionCounterListener</listener-class>
    </listener>

</web-app>

applicationContext.xml

<xml ...>

   <!-- Scan for my SessionService & assume it has been setup correctly by spring-->
   <context:component-scan base-package="com.stuff"/>

</beans>

Service: SessionService

@Service
public class SessionService{

  private int counter = 0;

  public SessionService(){}

  public void addOne(){
    coutner++;
  }

  public void removeOne(){
    counter--;
  }

  public int getTotalSessions(){
     return counter;
  }

}

Controller: SimpleController

@Component
public SimpleController
{
  @Autowired
  private SessionService sessionService;

  @RequestMapping(value="/webAppStatus")
  @ResponseBody
  public String getWebAppStatus()
  {
     return "Number of sessions: "+sessionService.getTotalSessions();
  }

}

score 4 · Answer 1 · edited May 23 '17 at 11:44

4

When you declare a <listener> in web.xml like so

<listener>
    <listener-class>com.stuff.morestuff.SessionCounterListener</listener-class>
</listener>

you are telling your Servlet container to instantiate the class specified in the listener-class element. In other words, this instance will not be managed by Spring and it will therefore not be able to inject anything and the field will remain null.

There are workarounds to this. And some more.

Note that this

<!-- Scan for my SessionService & assume it has been setup correctly by spring-->
<context:component-scan base-package="com.stuff"/>

is not a valid entry in web.xml. I don't know if that was a copy mistake on your part.

edited May 23 '17 at 11:44

Community

1
1

answered Dec 09 '13 at 18:14

Sotirios Delimanolis

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1

@stackoverflow See the workarounds. You don't necessarily need to remove it from web.xml. If you do, you need to register it differently (see `ServletContext`, `ServletContainerInitializer`, and `WebApplicationInitializer`). Otherwise, you can simply get the current `WebApplicationContext` from the `ServletContext` attributes. – Sotirios Delimanolis Dec 09 '13 at 18:24
So (bare with me) If understand the answers supplied by your 'workarounds' link. Essentially by just removing the listener from from the web.xml and implementing the ServletContextListener interface (with appropriate annations). I should be all set? – stackoverflow Dec 09 '13 at 18:38
1

@stack No. `ServletContextListener` has a different purpose than an `HttpSessionListener` (see the javadoc) but they are registered the same way, ie. ``. In the second link, it shows how to use `WebApplicationListener`. That interface gives you access to the `ServletContext` where you can programatically register a listener. Since you have access to the Spring context, you can have it provide a `SessionCounterListener` bean and register that. – Sotirios Delimanolis Dec 09 '13 at 18:42
1

@stackoverflow Don't confuse `web.xml` and spring context files, that start with a `` declaration. `` is a Spring element used in a spring context file, not in a web deployment descriptor. – Sotirios Delimanolis Dec 09 '13 at 19:08
The only way i can get my context is if every DI instance I call SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext(this); This seems like a terrible, clumsy way of doing this. So now to get things working I make this call about 8 different times through out my project (on requestMappings) – stackoverflow Dec 14 '13 at 17:34
@stackoverflow You can always go the `WebApplicationInitializer` (or its many implementations types) way. – Sotirios Delimanolis Dec 15 '13 at 00:34

score 0 · Answer 2 · answered Jul 31 '18 at 09:01

This is an answer here that shows the actual solution.

You should modify SessionCountListener like this, and the above example will work:

public class SessionCounterListener implements HttpSessionListener {

  @Autowired
  private SessionService sessionService;

  @Override
  public void sessionCreated(HttpSessionEvent arg0) {
    getSessionService(se).addOne();
  }

  @Override
  public void sessionDestroyed(HttpSessionEvent arg0) {
    getSessionService(se).removeOne();
  }

  private SessionService getSessionService(HttpSessionEvent se) {
    WebApplicationContext context = 
      WebApplicationContextUtils.getWebApplicationContext(
        se.getSession().getServletContext());
    return (SessionService) context.getBean("sessionService");
  } 
}

Dependency Injection inside of HttpSessionListener implementation

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