8263311: Watch registry changes for remote printers update instead of polling #2915
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| ERROR_SUCCESS == RegNotifyChangeKeyValue(hKey, TRUE, | ||
| REG_NOTIFY_CHANGE_NAME, | ||
| NULL, | ||
| FALSE); |
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RegNotifyChangeKeyValue notifies the caller about changes to the attributes or contents of a specified registry key.
• hKey: A handle to HKEY_CURRENT_USER\Printers\Connections key which is opened above.
• bWatchSubtree = TRUE: The function reports changes in the specified key and its subkeys.
• dwNotifyFilter = REG_NOTIFY_CHANGE_NAME: Notify the caller if a subkey is added or deleted.
• hEvent = NULL: If fAsynchronous is FALSE, hEvent is ignored.
• fAsynchronous = FALSE: The function does not return until a change has occurred.
When a new remote printer is added, a new key is created under HKCU\Printers\Connections; when an existing remote printer is removed, the key below Connections is removed; no values are added or removed in Connections key, thus REG_NOTIFY_CHANGE_LAST_SET filter is not needed.
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Is this only about addition/removal? What about printer name change? Shouldn't we get notified in that case as trying to print on printer with old name will not find the printer!!
If yes, in that regard I guess REG_NOTIFY_CHANGE_LAST_SET is the one to go for as it states
"Notify the caller of changes to a value of the key. This can include adding or deleting a value, or changing an existing value. "
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Is this only about addition/removal? What about printer name change?
You cannot change the name of a remote printer.
Opening Printer Properties dialog from the printer context menu on the local host and editing its name changes the name of the printer on the remote host which shares it. Windows warns, “This is a shared printer. If you rename a shared printer, existing connections to this printer from other computers will break and will have to be created again.”
Nothing has been updated on the local system after renaming the printer. As the warning said, the printer does not work any more because it refers to a printer that does not exist.
To fix this, one has to add a new remote printer which refers to the new name of the printer on the remote host.
Shouldn't we get notified in that case as trying to print on printer with old name will not find the printer!!
I'm afraid there's nothing Java can do to mitigate renaming the printer.
If yes, in that regard I guess
REG_NOTIFY_CHANGE_LAST_SETis the one to go for as it states
"Notify the caller of changes to a value of the key. This can include adding or deleting a value, or changing an existing value. "
Since no values are created, removed, or changed when a remote printer is added or removed under Connections key, adding REG_NOTIFY_CHANGE_LAST_SET to dwNotifyFilter is not required. There are some values stored in the printer key but Java does not use them directly; if any value is changed there, getting notifications about such an event only creates noise.
So, as far as Java is concerned, getting notifications about new keys being created or removed under Connections is enough. This is what REG_NOTIFY_CHANGE_NAME filter does.
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I can understand that as a user, you cannot /shouldn't change the name of a remote network printer but a network admin can change the name, so shouldn't we get notification on that name change when this method gets called after the name change? or would it be notified as a new printer in that case? Then what will happen to the old stale printer in the registry?
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You can't. Try it yourself. The printer connection is per-user, after all it's stored under HKCU.
Windows displays the name of remote printers as “Generic / Text Only on 192.168.1.18”: the name of the printer and the remote host.
If you open the Printer Properties dialog of a remote printer and edit its name, you change the name of the printer on the remote host which shares it. It changes absolutely nothing on the local system.
shouldn't we get notification on that name change when this method gets called after the name change?
Yes, we should. But we cannot.
For Windows, nothing has changed on the local system, it's the remote system that has been changed.
or would it be notified as a new printer in that case?
No, it wouldn't because nothing has changed on the local system.
Then what will happen to the old stale printer in the registry?
Nothing, again. It just stays there but Windows reports an error if you try to open its Printer Properties, Windows reports an error if you try to print to it. I tried printing with Java and Notepad: the behaviour is the same. The difference is that Notepad displays the error message whereas Java throws an exception (it depends on the Java app, I guess).
The behaviour aligns to the one seen when the printer is unreachable because of, let's say, network issues.
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@aivanov-jdk can you please check what that old comment was about: 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
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?. The link to this article is in the description of JDK-8153732. The article discusses the subject with WMI and WQL. This is what it says:
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:
This statement is a bit confusing. It says they cannot use I interpret the following statement this way: Registry notifications do not provide information on what changed under 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 to watch for registry updates and to get notified as soon as a change under |
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@aivanov-jdk Since your change was applied there have been 128 commits pushed to the
Your commit was automatically rebased without conflicts. Pushed as commit a85dc55. |
JDK-8153732 implemented polling for remote printers.
That bug description also mentions watching the registry for changes and links to the article which describes the method yet it does so in terms of WMI. Using WMI is not necessary to watch for the registry updates.
It is possible to replace polling mechanism with registry change notifications. If the registry at
HKCU\Printers\Connectionsis updated, refresh the list of print services.It works perfectly well in my own testing with sharing a Generic / Text Only printer from another laptop. The notification comes as soon as the printer is installed, it results in a new key created under
Connections. If a remote printer is removed, the notification is also triggered as the key corresponding to that printer is removed from the registry.I updated the steps in the manual test:
RemotePrinterStatusRefresh.java.Progress
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$ git fetch https://git.openjdk.java.net/jdk pull/2915/head:pull/2915$ git checkout pull/2915