如何检测和删除(在会话期间)不能被垃圾收集的未使用的 @ViewScoped bean [英] How detect and remove (during a session) unused @ViewScoped beans that can't be garbage collected

问题描述

codebulb.ch 在这篇文章中很好地解释和确认了这个问题提出的问题，包括 JSF @ViewScoped、CDI @ViewSCoped 之间的一些比较，以及 Omnifaces @ViewScoped，并明确声明 JSF @ViewScoped 是设计泄漏": 中实现在其文档中如下:

<块引用>

在某些情况下，当调用浏览器 unload 事件时，也可能需要立即销毁视图范围的 bean.IE.当用户通过 GET 导航离开或关闭浏览器选项卡/窗口时.两个 JSF 2.2 视图范围注释都不支持这一点.从 OmniFaces 2.2 开始，此 CDI 视图范围注释将保证在浏览器卸载时也会调​​用带注释的 @PreDestroy 方法.这个技巧是通过一个自动包含的帮助脚本 omnifaces:unload.js 的同步 XHR 请求来完成的.然而，有一个小警告:在缓慢的网络和/或较差的服务器硬件上，最终用户卸载页面的操作与所需的结果之间可能存在明显的滞后.如果这是不可取的，那么最好坚持 JSF 2.2 自己的视图范围注释并接受延迟的销毁.

自 OmniFaces 2.3 以来，卸载已得到进一步改进，以便在服务器端状态保存的情况下也从 JSF 实现的内部 LRU 映射中物理删除关联的 JSF 视图状态，从而进一步降低 ViewExpiredException 的风险在之前创建/打开的其他视图上.作为此更改的副作用，在与 OmniFaces CDI 视图范围 bean 相同的视图中引用的任何标准 JSF 视图范围 bean 的 @PreDestroy 注释方法也将保证在浏览器卸载时被调用.

你可以在这里找到相关的源代码:

• 卸载脚本初始化器:ViewScopeManager#registerUnloadScript()
• 卸载脚本本身:unload.unminified.js
• 卸载视图处理程序:OmniViewHandler#unloadView()
• JSF 视图状态销毁器:Hacks#removeViewState()

卸载脚本将运行 在窗口的 beforeunload 事件期间，除非它是由任何基于 JSF (ajax) 的表单提交引起的.至于 commandlink 和/或 ajax 提交，这是特定于实现的.目前 Mojarra、MyFaces 和 PrimeFaces 已被识别.

卸载脚本将 触发器 navigator.sendBeacon 在现代浏览器上并回退到同步 XHR(异步会失败，因为页面可能在请求实际到达服务器之前被卸载).

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";如果(navigator.sendBeacon){//在卸载事件期间不推荐使用同步 XHR，现代浏览器为此提供了 Beacon API，它基本上会触发并忘记请求.navigator.sendBeacon(url, new Blob([query], {type: contentType}));}别的 {var xhr = new XMLHttpRequest();xhr.open(POST", url, false);xhr.setRequestHeader(X-Requested-With", XMLHttpRequest");xhr.setRequestHeader("Content-Type", contentType);xhr.send(查询);}

卸载视图处理程序将 显式 销毁所有 @ViewScoped bean，包括标准 JSF bean(请注意，卸载脚本仅在视图引用至少一个 OmniFaces @ViewScoped 豆).

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

然而，这不会破坏 HTTP 会话中的物理 JSF 视图状态，因此下面的 使用案例会失败:

1. 将物理视图数设置为 3(在 Mojarra 中，使用 com.sun.faces.numberOfLogicalViews 上下文参数，在 MyFaces 中使用 org.apache.myfaces.NUMBER_OF_VIEWS_IN_SESSION 上下文参数).
2. 创建一个引用标准 JSF @ViewScoped bean 的页面.
3. 在标签页中打开此页面并始终保持打开状态.
4. 在另一个标签页中打开同一页面，然后立即关闭此标签页.
5. 在另一个标签页中打开同一页面，然后立即关闭此标签页.
6. 在另一个标签页中打开同一页面，然后立即关闭此标签页.
7. 在第一个选项卡中提交表单.

这会因 ViewExpiredException 而失败，因为之前关闭的选项卡的 JSF 视图状态在 PreDestroyViewMapEvent 期间并未物理销毁.他们仍然坚持在会议上.OmniFaces @ViewScoped 实际上会销毁它们.然而，销毁 JSF 视图状态是特定于实现的.这至少解释了应该实现这一目标的 Hacks 类中相当 hacky 的代码.

这个具体案例的集成测试可以在 ViewScopedIT#destroyViewState() 在 ViewScopedIT.xhtml 这是 当前 运行 WildFly 10.0.0、TomEE 7.0.1 和 Payara4.1.1.163.

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简而言之:只需将 javax.faces.view.ViewScoped 替换为 org.omnifaces.cdi.ViewScoped.其余部分是透明的.

import javax.inject.Named;导入 org.omnifaces.cdi.ViewScoped；@命名@ViewScoped公共类 Bean 实现 Serializable {}

我至少已经努力提出一个公共 API 方法来物理销毁 JSF 视图状态.也许它会出现在 JSF 2.3 中，然后我应该能够消除 OmniFaces Hacks 类中的样板.一旦在 OmniFaces 中完善，它可能最终会出现在 JSF 中，但不会在 2.4 之前.

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

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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.

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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.

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解决方案

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.

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