jmx-dev RFR: 8373441: Remove DCmdFactory::_enabled [v2]

[Ioi Lam]

On Mon, 15 Dec 2025 06:11:22 GMT, David Holmes <dholmes at openjdk.org> wrote:

> If we are just changing the implementation then I guess this would be okay. But it seems the API for this allows for specific DCmds to be disabled (how?) - or at least intended for it to be possible. As per the doc:
> 
> > When the set of diagnostic commands currently supported by the Java Virtual Machine is modified, the DiagnosticCommandMBean emits a [Notification](https://docs.oracle.com/javase/8/docs/api/javax/management/Notification.html?is-external=true) with a [type](https://docs.oracle.com/javase/8/docs/api/javax/management/Notification.html?is-external=true#getType--) of[ "jmx.mbean.info.changed"](https://docs.oracle.com/javase/8/docs/api/javax/management/MBeanInfo.html#info-changed) and a [userData](https://docs.oracle.com/javase/8/docs/api/javax/management/Notification.html?is-external=true#getUserData--) that is the new MBeanInfo.

The above paragraph is about new commands being added:


int DCmdFactory::register_DCmdFactory(DCmdFactory* factory) {
  MutexLocker ml(DCmdFactory_lock, Mutex::_no_safepoint_check_flag);
  factory->_next = _DCmdFactoryList;
  _DCmdFactoryList = factory;
  if (_send_jmx_notification && !factory->_hidden
      && (factory->_export_flags & DCmd_Source_MBean)) {
    DCmdFactory::push_jmx_notification_request();
  }
  return 0; // Actually, there's no checks for duplicates
}


Which will eventually cause `DiagnosticCommandImpl::createDiagnosticFrameworkNotification()` to be called by HotSpot to dispatch the `Notification`.

I checked older source code like JDK 8, which has a `DCmdFactory::set_enabled()` method but there are no callers. If there had been a way to disable commands dynamically, that has been lost for a very long time.

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PR Comment: https://git.openjdk.org/jdk/pull/28794#issuecomment-3657448785