Micrometer.io heap statistics for OpenJ9 VMs

OpenJ9MemoryMetrics.java

import io.micrometer.core.instrument.Gauge;
import io.micrometer.core.instrument.MeterRegistry;
import io.micrometer.core.instrument.binder.BaseUnits;
import io.micrometer.core.instrument.binder.MeterBinder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.util.ReflectionUtils;

import java.lang.reflect.Method;

/**
 * Provides the used heap memory as a percentage so that we can scale based on that metric.
 * Uses OpenJ9-specific class, therefore the code uses reflection so that this will also
 * compile using an OpenJDK.
 *
 * @see <a href="https://www.eclipse.org/openj9/docs/api/jdk11/openj9.jvm/com/ibm/jvm/Stats.html">JavaDoc for Stats</a>
 */
public class OpenJ9MemoryMetrics implements MeterBinder {

    private Object stats;
    private Method getStats;
    private Method getUsedHeap;
    private long maxHeap;

    private Logger logger = LoggerFactory.getLogger(getClass());

    public OpenJ9MemoryMetrics() {
        try {
            Class statsClass = Class.forName("com.ibm.jvm.Stats");
            this.stats = statsClass.getDeclaredConstructor().newInstance();
            this.getStats = statsClass.getMethod("getStats");
            this.getUsedHeap = statsClass.getMethod("getUsedHeap");
            ReflectionUtils.invokeMethod(getStats, stats);
            this.maxHeap = (long) ReflectionUtils.invokeMethod(statsClass.getMethod("getMaxHeap"), stats);
        } catch (Exception e) {
            logger.error("Failed to initialize OpenJ9MemoryMetrics", e);
        }
    }

    @Override
    public void bindTo(MeterRegistry registry) {
        if (this.maxHeap != 0L) {
            try {
                Gauge.builder("jvm.memory.heap-used-percentage", stats, stats -> {
                        ReflectionUtils.invokeMethod(getStats, stats);
                        long usedHeap = (long) ReflectionUtils.invokeMethod(getUsedHeap, stats);
                        return ((double) usedHeap) / maxHeap;
                    })
                    .description("Percentage of heap used by the JVM in the range [0..1], based on used divided by max as reported by OpenJ9's Stats class")
                    .baseUnit(BaseUnits.PERCENT)
                    .register(registry);
            } catch (Exception e) {
                logger.error("Could not create heap percentage gauge", e);
            }
        }
    }
}