How detect and remove (during a session) unused @V

问题:

EDIT: The problem raised by this question is very well explained and confirmed in this article by codebulb.ch, including some comparison between JSF @ViewScoped, CDI @ViewSCoped, and the Omnifaces @ViewScoped, and a clear statement that JSF @ViewScoped is 'leaky by design': May 24, 2015 Java EE 7 Bean scopes compared part 2 of 2

---

EDIT: 2017-12-05 The test case used for this question is still extremely useful, however the conclusions concerning Garbage Collection in the original post (and images) were based on JVisualVM, and I have since found they are not valid. Use the NetBeans Profiler instead ! I am now getting completely consistent results for OmniFaces ViewScoped with the test app on forcing GC from within the NetBeans Profiler instead of JVisualVM attached to GlassFish/Payara, where I am getting references still held (even after @PreDestroy called) by field sessionListeners of type com.sun.web.server.WebContainerListener within ContainerBase$ContainerBackgroundProcessor, and they won't GC.

---

It is known that in JSF2.2, for a page that uses a @ViewScoped bean, navigating away from it (or reloading it) using any of the following techniques will result in instances of the @ViewScoped bean "dangling" in the session so that it will not be garbage collected, leading to endlessly growing heap memory (as long as provoked by GETs):

• Using an h:link to GET a new page.

• Using an h:outputLink (or an HTML A tag) to GET a new page.

• Reloading the page in the browser using a RELOAD command or button.

• Reloading the page using a keyboard ENTER on the browser URL (also a GET).

By contrast, passing through the JSF navigation system by using say an h:commandButton results in the release of the @ViewScoped bean such that it can be garbage collected.

This is explained (by BalusC) at JSF 2.1 ViewScopedBean @PreDestroy method is not called and demonstrated for JSF2.2 and Mojarra 2.2.9 by my small NetBeans example project at https://stackoverflow.com/a/30410401/679457, which project illustrates the various navigation cases and is available for download here. (EDIT: 2015-05-28: The full code is now also available here below.)

[EDIT: 2016-11-13 There is now also an improved test web app with full instructions and comparison with OmniFaces @ViewScoped and result table on GitHub here: https://github.com/webelcomau/JSFviewScopedNav]

I repeat here an image of the index.html, which summarises the navigation cases and the results for heap memory:

Q: How can I detect such "hanging/dangling" @ViewScoped beans caused by GET navigations and remove them, or otherwise render them garbage collectable ?

Please note that I am not asking how to clean them up when the session ends, I have already seen various solutions for that, I am looking for ways to clean them up during a session, so that heap memory does not grow excessively during a session due to inadvertent GET navigations.

---

回答1:

Basically, you want the JSF view state and all view scoped beans to be destroyed during a window unload. The solution has been implemented in OmniFaces @ViewScoped annotation which is fleshed out in its documentation as below:

There may be cases when it's desirable to immediately destroy a view scoped bean as well when the browser unload event is invoked. I.e. when the user navigates away by GET, or closes the browser tab/window. None of the both JSF 2.2 view scope annotations support this. Since OmniFaces 2.2, this CDI view scope annotation will guarantee that the @PreDestroy annotated method is also invoked on browser unload. This trick is done by a synchronous XHR request via an automatically included helper script omnifaces:unload.js. There's however a small caveat: on slow network and/or poor server hardware, there may be a noticeable lag between the enduser action of unloading the page and the desired result. If this is undesireable, then better stick to JSF 2.2's own view scope annotations and accept the postponed destroy.

Since OmniFaces 2.3, the unload has been further improved to also physically remove the associated JSF view state from JSF implementation's internal LRU map in case of server side state saving, hereby further decreasing the risk at ViewExpiredException on the other views which were created/opened earlier. As side effect of this change, the @PreDestroy annotated method of any standard JSF view scoped beans referenced in the same view as the OmniFaces CDI view scoped bean will also guaranteed be invoked on browser unload.

You can find the relevant source code here:

• The unload script initializer: ViewScopeManager#registerUnloadScript()
• The unload script itself: unload.unminified.js
• The unload view handler: OmniViewHandler#unloadView()
• The JSF view state destroyer: Hacks#removeViewState()

The unload script will run during window's beforeunload event, unless it's caused by any JSF based (ajax) form submit. As to commandlink and/or ajax submits, this is implementation specific. Currently Mojarra, MyFaces and PrimeFaces are recognized.

The unload script will trigger navigator.sendBeacon on modern browsers and fall back to synchronous XHR (asynchronous would fail as page might be unloaded sooner than the request actually hits the server).

var url = form.action;
var query = "omnifaces.event=unload&id=" + id + "&" + VIEW_STATE_PARAM + "=" + encodeURIComponent(form[VIEW_STATE_PARAM].value);
var contentType = "application/x-www-form-urlencoded";

if (navigator.sendBeacon) {
    // Synchronous XHR is deprecated during unload event, modern browsers offer Beacon API for this which will basically fire-and-forget the request.
    navigator.sendBeacon(url, new Blob([query], {type: contentType}));
}
else {
    var xhr = new XMLHttpRequest();
    xhr.open("POST", url, false);
    xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
    xhr.setRequestHeader("Content-Type", contentType);
    xhr.send(query);
}

The unload view handler will explicitly destroy all @ViewScoped beans, including standard JSF ones (do note that the unload script is only initialized when the view references at least one OmniFaces @ViewScoped bean).

context.getApplication().publishEvent(context, PreDestroyViewMapEvent.class, UIViewRoot.class, createdView);

This however doesn't destroy the physical JSF view state in the HTTP session and thus the below use case would fail:

1. Set number of physical views to 3 (in Mojarra, use com.sun.faces.numberOfLogicalViews context param and in MyFaces use org.apache.myfaces.NUMBER_OF_VIEWS_IN_SESSION context param).
2. Create a page which references a standard JSF @ViewScoped bean.
3. Open this page in a tab and keep it open all time.
4. Open the same page in another tab and then immediately close this tab.
5. Open the same page in another tab and then immediately close this tab.
6. Open the same page in another tab and then immediately close this tab.
7. Submit a form in the first tab.

This would fail with a ViewExpiredException because the JSF view states of previously closed tabs aren't physically destroyed during PreDestroyViewMapEvent. They still stick around in the session. OmniFaces @ViewScoped will actually destroy them. Destroying the JSF view state is however implementation specific. That explains at least the quite hacky code in Hacks class which should achieve that.

The integration test for this specific case can be found in ViewScopedIT#destroyViewState() on ViewScopedIT.xhtml which is currently run against WildFly 10.0.0, TomEE 7.0.1 and Payara 4.1.1.163.

---

In a nutshell: just replace javax.faces.view.ViewScoped by org.omnifaces.cdi.ViewScoped. The rest is transparent.

import javax.inject.Named;
import org.omnifaces.cdi.ViewScoped;

@Named
@ViewScoped
public class Bean implements Serializable {}

I have at least made an effort to propose a public API method to physically destroy the JSF view state. Perhaps it will come in JSF 2.3 and then I should be able to eliminate the boilerplate in OmniFaces Hacks class. Once the thing is polished in OmniFaces, it will perhaps ultimately come in JSF, but not before 2.4.

回答2:

Okay, so I cobbled something together.

The Principle

The now-irrelevant viewscoped beans sit there, wasting everyone's time and space because in a GET navigation case, using any of the controls that you've highlighted, the server is not involved. If the server is not involved, it has no way of knowing the viewscoped beans are now redundant (that is until the session has died). So what's needed here is a way to tell the server-side that the view from which you're navigating, needs to terminate its view-scoped beans

The Constraints

The server-side should be notified as soon as the navigation is happening

1. beforeunload or unload in an <h:body/> would have been ideal but for the following problems

   • Browsers don't uniformly respect either of them

   • A solution using either of them will most likely require an AJAX solution that goes outside the JSF framework. JSF's ajax-ready script must be executed in the context of a form. You can't have <h:body/> inside a form. I prefer to keep it all inside JSF

2. You can't send an ajax request in onclick of a control, and also navigate in the same control. Not without a dirty popup anyway. So navigating onclick in a h:button or h:link is out of it

The dirty compromise

Trigger an ajax request onclick, and have a PhaseListener do the actual navigation and the viewscope cleanup

The Recipe

1. 1 PhaseListener (a ViewHandler would also work here; I'm going with the former because it's easier to setup)

2. 1 wrapper around the JSF js API

3. A medium helping of shame

Let's see:

1. The PhaseListener

   public ViewScopedCleaner implements PhaseListener{
   
       public void afterPhase(PhaseEvent evt){
            FacesContext ctxt = event.getFacesContext();
            NavigationHandler navHandler = ctxt.getApplication().getNavigationHanler();
            boolean isAjax =  ctx.getPartialViewContext().isAjaxRequest(); //determine that it's an ajax request
            Object target = ctxt.getExternalContext().getRequestParameterMap().get("target"); //get the destination URL
   
                   if(target !=null && !target.toString().equals("")&&isAjax ){
                        ctxt.getViewRoot().getViewMap().clear(); //clear the map
                        navHandler.handleNavigation(ctxt,null,target);//navigate
                    }
   
       }
   
       public PhaseId getPhaseId(){
           return PhaseId.APPLY_REQUEST_VALUES;
       }
   
   }
   

2. The JS wrapper

    function cleanViewScope(){
     jsf.ajax.request(this, null, {execute: 'someButton', target: this.href});
      return false;
     }
   

3. Putting it together

     <script>
        function cleanViewScope(){
            jsf.ajax.request(this, null, {execute: 'someButton', target: this.href}); return false;
         }
     </script>  
   
    <f:phaseListener type="com.you.test.ViewScopedCleaner" />
    <h:link onclick="cleanViewScope();" value="h:link: GET: done" outcome="done?faces-redirect=true"/>
   

To Do

1. Extend the h:link, possibly add an attribute to configure the clearing behaviour

2. The way the target url is being passed is suspect; might open up a hole