Wrap fileDescriptorRatioGauge which breaks in java9

[jhaals]

Java9 fails with InaccessibleObjectException which causes all metric
reporting to fail. This is a new exception in java9 so we can't catch it
in Java8, simply return 0 if exception is thrown. This is tested in
java8/java9

java.lang.reflect.InaccessibleObjectException: Unable to make public long com.sun.management.internal.OperatingSystemImpl.getOpenFileDescriptorCount() accessible: module jdk.management does not "opens com.sun.management.internal" to unnamed module @385c9627

[udoprog]

Hey,

Any way this can be detected before we are polling the Gauge, so that we don't have to register the gauge at all if it's not supported?

Maybe try to poll it once during registration?

[jhaals]

Hi,
Yes that's a good points, we created a quickfix in apollo for this spotify/apollo#231 but we can patch it to not register at all in java9

[udoprog]

@jhaals proper fix would be appreciated if you have the time :)

[jhaals]

@udoprog I can wrap the whole gauges.put() in a try catch which would return an empty HashMap on every call to getMetrics, would that be sufficient or do you have anything else in mind?
An alternative could be to throw an exception in the constructor and fail early so that the service using the library can handle it themselves.

[jhaals]

This was merged into dropwizard/metrics 12 days ago dropwizard/metrics#1236
It won't be out until 4.0 but I guess we can copy their code if you prefer that.

[udoprog]

Hm, that's an interesting approach. I'd think it would be preferably not to register anything if the operation is not supported.

Apparently we can do an instanceof check instead, that seem to be cleaner than a try-catch?

[mattnworb]

I upgraded the metrics library for exactly this issue in #30, sorry I missed this PR when making that

[jhaals]

No worries @mattnworb, your PR is preferred 👍