Monitoring wrapped ThreadPoolExecutor returned from Executors

[Tommy Ludwig]

Hello,

In the Micrometer project, we provide metrics instrumentation of `ExectorService`s. For `ThreadPoolExecutor`s, we track the number of completed tasks, active tasks, thread pool sizes, task queue size and remaining capacity via methods from `ThreadPoolExecutor`. We are currently using a brittle reflection hack[1] to do this for the wrapped `ThreadPoolExecutor` returned from `Executors` methods `newSingleThreadExecutor` and `newSingleThreadScheduledExecutor`. With the introduction of JEP-396 in JDK 16, our reflection hack throws an InaccessibleObjectException by default.

I am not seeing a proper way to get at the methods we use for the metrics (e.g. `ThreadPoolExecutor::getCompletedTaskCount`) in this case. Is there a way that I am missing?

>From the JavaDocs, the intention of the wrapping is to prevent changing the ThreadPool configuration. Our use case does not call for changing the thread pool configuration - we want to observe the ExecutorService (e.g. thread pool state) via getter methods for monitoring purposes.

Thanks,
Tommy

[1] https://github.com/micrometer-metrics/micrometer/blob/25f120833f8b73e4bd7cf604831dfddf0112fe9c/micrometer-core/src/main/java/io/micrometer/core/instrument/binder/jvm/ExecutorServiceMetrics.java#L272-L301