[FEATURE REQ] Prevent KeyVault refresh Timer from blocking application termination

[jwillebrands]

Is your feature request related to a problem? Please describe.
We've recently noticed our Spring applications hanging around in a failed state after an unhandled exception tearing down the SpringApplication, without terminating properly. We've traced this back to a change introducing a dependency on azure-spring-boot-starter-keyvault-secrets to sync secrets from Azure KeyVault. Further investigation shows that, when a refreshInterval is configured, the KeyVaultOperation class uses java.util.Timer to set up a task to refresh the secrets.

As stated in the docs, the task execution thread used by Timer does not run as a daemon thread:

By default, the task execution thread does not run as a daemon thread, so it is capable of keeping an application from terminating.

The result is that we see applications being kept alive by this timer thread, despite everything else being torn down. I had considered filing this as a bug, but I'm not sure if this is intentional. We don't see it as a valid use case.

Describe the solution you'd like
Use a daemon thread for the refresh timer tasks. E.g. by creating new Timer objects in KeyVaultOperation setting the daemon flag to true: https://docs.oracle.com/javase/7/docs/api/java/util/Timer.html#Timer(boolean)

Describe alternatives you've considered
There are a couple of options that can be implemented instead:

• If the current behavior is intentional, or changing it is undesirable, the daemon flag could be added to KeyVaultProperties to make it configurable for the consumer.
• Register for one of the available shutdown hooks to manually cancel() the timer.
• Should the non-daemon version be kept unchanged, this should be clearly documented.

Additional context
Related artifact: azure-spring-boot-starter-keyvault-secrets
Observed on version 3.1.0

Information Checklist
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[yiliuTo]

Thanks for your suggestions, we will look into this.

[yiliuTo]

Hi @jwillebrands , the pr to make Timer task run as a daemon thread has been merged. You can wait for our next release of 3.4.0 for the fix.

[jwillebrands]

Will keep an eye on the PR and we'll look out for the next release. Thanks for picking this up so quickly.

[backwind1233]

#20014

[yiliuTo]

Hi @jwillebrands , now 3.4.0 has been released and I will close this issue.