RFR: 8294589: MenuBarSkin: memory leak when changing skin

[Jeanette Winzenburg]

On Thu, 29 Sep 2022 23:00:17 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

> Fixed memory leak by using weak listeners and making sure to undo everything that has been done to MenuBar/Skin in dispose().

modules/javafx.controls/src/main/java/javafx/scene/control/skin/MenuBarSkin.java line 328:

> 326:         mouseEventHandler = t -> {
> 327:             if (getSkinnable() != null) {
> 328:                 Bounds containerScreenBounds = container.localToScreen(container.getLayoutBounds());

this null check is wrong: skinnable is guaranteed to never be null - except after dispose, at which time it's illegal to access any of the skin's methods/fields/state. So checking here is smearing over a precondition violation of the caller (most probably a dangling handler/listener) which has to be fixed.

Note we deliberately removed all null checks in all (cleaned) skins :) If we see any exceptions, the procedure is to 

- write a test exposing the exception (it's failing), this test belongs into SkinCleanTest
- find and fix the violator
- see the test passing

There might be other exceptions as well (like IOOB or similar) which require a similar procedure, see the available tests. Actually, I think it's a good idea to actively look out for possible macroscopic misbehavior if handlers/listeners are not removed properly. SkinCleanupTest has examples.

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PR: https://git.openjdk.org/jfx/pull/906