On Sat, 13 Mar 2021 00:18:25 GMT, Sergey Bylokhov <s...@openjdk.org> wrote:
> Any idea why RegistryValueChange was rejected as a solution? I've no idea. I read that comment, it's exactly the comment that was in the code above `RemotePrinterChangeListener` class in `PrintServiceLookupProvider.java`. > RegistryValueChange cannot be used in combination with WMI to get registry > value change notification because of an error that may be generated because > the scope of the query would be too big to handle(at times). It's very vague. I don't know what it really means. Neither do I know if a prototype was even tried. If fact, this comment cites portions of the article [How to listen for Printer Connections?](https://docs.microsoft.com/en-gb/archive/blogs/hmahrt/how-to-listen-for-printer-connections). The link to this article is in the description of [JDK-8153732](https://bugs.openjdk.java.net/browse/JDK-8153732). The article discusses the subject with WMI and WQL. This is what it says: > There is no equivalent to `FindFirstPrinterChangeNotification`, which listens > for new/changed Printer Connections – and a polling mechanism was not an > option. However, there is another way, using WMI. Note: _polling mechanism was not an option_. Yet it is what was implemented in Java. Then the articles describes the steps needed to monitor for printer changes. Close to the end, there's the quoted sentence: > Unfortunately, we cannot use the `RegistryValueChangeEvent` (because the > scope of the query would be too big (we’d receive an error message), so we > can only know **when** something changed below the `Printers\Connections` > key, but not **what**. This is why we have to rely on **`EnumPrinters`**. This statement is a bit confusing. It says they cannot use `RegistryValueChangeEvent` (it's not a Windows API function, it's another WMI class) but the WQL query above _uses it_. I interpret the following statement this way: Registry notifications do not provide information on **what** changed under `Printers\Connections`, only that a change occurred. So `EnumPrinters` is used to enumerate the remote printers and update the stored list of printers after the change to the registry occurs. I used this article as the inspiration and the implementation I'm proposing in this PR is purely based on this article. Yet I'm using Windows API function [RegNotifyChangeKeyValue](https://docs.microsoft.com/en-us/windows/win32/api/winreg/nf-winreg-regnotifychangekeyvalue) to watch for registry updates and to get notified as soon as a change under `HKCU\Printers\Connections` occurs. Then the list of printers is updated by the `refreshServices` upcall into Java which refreshes both _local_ and _remote_ printers. ------------- PR: https://git.openjdk.java.net/jdk/pull/2915
