How detect and remove (during a session) unused @ViewScoped beans that can’t be garbage collected

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.

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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.