HDDS-2413. Set configuration variables from annotated java objects

[adoroszlai]

What changes were proposed in this pull request?

Allow setting Configuration from strongly-typed config objects (ie. ones annotated with @Config). This is the opposite direction of existing ConfigurationSource.getObject().

One caveat is that the config object needs to be created via getObject() for default values from annotations to be properly set.

https://issues.apache.org/jira/browse/HDDS-2413

How was this patch tested?

Added unit test. Also updated a single part of "real" code where this change makes string constants unnecessary (DUFactory.Conf and TestDUFactory).

https://github.com/adoroszlai/hadoop-ozone/runs/795413474

[elek]

I really don't like the approach when the current port number (after using :0) is added to the configuration. It's easier if the configuration is immutable after loading.

I understand that the immutable is not possible with this injection and I accept that it's necessary.

One possible approach is to introduce two interfaces: ConfigSource (read-only) and ConfigDestination (write only). OzoneConfiguration can implement both. But there could be a specific ConfigSource (eg. EnvironmentVariableConfigSource) which would be read only.

But I am fine with committing this patch as is, but interested about opinion.

Also: it seems to be useful for testing to introduce an OzoneConfiguration.fromAnnotatedObject() method.

[adoroszlai]

One possible approach is to introduce two interfaces: ConfigSource (read-only) and ConfigDestination (write only). OzoneConfiguration can implement both.

Thanks @elek for the suggestion. I was considering this, but needed this nudge to implement it.

[elek]

I was considering this, but needed this nudge to implement it.

Oh, I see why. It makes the patch bigger because we use this dirty hack to store the port in the config.

[elek]

+1 thanks the update, I didn't think it's such a big change (sorry, commenting is cheap and easy ;-) )

[adoroszlai]

Thanks @elek for reviewing and committing it.