Scratch.java

class Scratch {
	public interface SampleMBean {
		ObjectName getObjectName();
	}

	static class Sample implements SampleMBean {
		ObjectName objectName;

		static Sample registerBean(MBeanServer mbs) {
			try {
				Sample bean = new Sample();
				bean.objectName = new ObjectName("io.prometheus.jmx.test:name=Sample,uuid=" + UUID.randomUUID());
				mbs.registerMBean(bean, bean.objectName);
				return bean;
			} catch (Exception e) {
				throw new RuntimeException(e);
			}
		}

		void unregister(MBeanServer mbs) {
			try {
				mbs.unregisterMBean(objectName);
			} catch (Exception e) {
				throw new RuntimeException(e);
			}
		}

		@Override
		public ObjectName getObjectName() {
			return objectName;
		}
	}

	public static void main(String... args) throws Exception {
		PrometheusRegistry prometheusRegistry = new PrometheusRegistry();
		JmxCollector jc = new JmxCollector(
				"\n---\nrules:\n- pattern: `^hadoop<service=DataNode, name=DataNodeActivity-ams-hdd001-50010><>replaceBlockOpMinTime:`\n  name: foo"
						.replace('`', '"'))
				.register(prometheusRegistry);

		MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
		while (true) {
			Sample sample = Sample.registerBean(mbs); // register bean before scrape
			prometheusRegistry.scrape(); // during scrape new entry has been added to the exclusion map
			sample.unregister(mbs); // unregister bean after scrape, bean no longer registered in MBeanServer, will be eligible for garbage collection on next loop iteration

			// print MBeanMBeanServer and filter stats
			Field cfg = JmxCollector.class.getDeclaredField("config");
			cfg.setAccessible(true);
			Object config =  cfg.get(jc);
			Field fil = config.getClass().getDeclaredField("objectNameAttributeFilter");
			fil.setAccessible(true);
			Object objectNameAttributeFilter = fil.get(config);
			Field map = objectNameAttributeFilter.getClass().getDeclaredField("dynamicExcludeObjectNameAttributesMap");
			map.setAccessible(true);
			Map excludeObjectNameAttributesMap = (Map) map.get(objectNameAttributeFilter);
			System.out.println("MBeanServer bean count: " + mbs.getMBeanCount());
			System.out.println("Filter size:            " + excludeObjectNameAttributesMap.size());
		}
	}
}