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docs: document license-check Java prefs workaround #5585

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27 changes: 27 additions & 0 deletions articles/flow/kb/index.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -148,4 +148,31 @@
See <<{articles}/flow/configuration/licenses#offline-license-key,Offline License Key>> for details on which key type to use for your Vaadin version.
====

[[license-prefs-backing-store]]
.+++<h3>Repeated "Could Not Lock User Prefs" Warnings During License Validation</h3>+++

Check failure on line 152 in articles/flow/kb/index.adoc

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[vale] reported by reviewdog 🐶
[Vale.Spelling] Did you really mean 'Prefs'?

Raw Output:
{"message": "[Vale.Spelling] Did you really mean 'Prefs'?", "location": {"path": "articles/flow/kb/index.adoc", "range": {"start": {"line": 152, "column": 39}}}, "severity": "ERROR"}
[%collapsible]
====
When the JVM user has no writable home directory -- for example, a hardened Tomcat deployment, a CI agent, or any other process running as a locked-down user -- the JVM may repeatedly log warnings like the following while a Vaadin tool performs license validation:

[source]
----
WARNING: Could not lock User prefs. Unix error code 2.
WARNING: Couldn't flush user prefs: java.util.prefs.BackingStoreException: Couldn't get file lock.
----

The Vaadin license checker uses the Java `Preferences` API as part of license validation. On Linux and other Unix-like systems, the JVM stores those preferences as files under `~/.java/.userPrefs/` and starts a background thread that periodically flushes them. If that directory is not writable, every flush attempt produces a warning.

Point the Java Preferences store to a directory the JVM user can write to by setting the `java.util.prefs.userRoot` system property on the JVM running the affected tool -- the application server, the Maven or Gradle build, the TestBench test runner, and so on. For example:

[source,terminal]
----
-Djava.util.prefs.userRoot=/path/to/writable/dir
----

The directory is created on first use if it doesn't exist. Any location the JVM user can write to works -- somewhere under the server's instance directory, `/var/tmp`, or similar.

[NOTE]
This affects only Linux and other Unix-like operating systems. Windows and macOS use different mechanisms that don't write to the user's home directory, so they aren't affected by this issue, and the `java.util.prefs.userRoot` property has no effect there.
====

// end::licenses[]
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