java-tutorials/spring-boot-deployment/src/main/java/com/baeldung/compare/StartupEventHandler.java

package com.baeldung.compare;

import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.function.BiFunction;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.context.event.ApplicationReadyEvent;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

import io.micrometer.core.instrument.Meter;
import io.micrometer.core.instrument.MeterRegistry;
import io.micrometer.core.instrument.Statistic;

@Component
public class StartupEventHandler {

    // logger, constructor
    private static Logger logger = LoggerFactory.getLogger(StartupEventHandler.class);
    
    public StartupEventHandler(MeterRegistry registry) {
        this.meterRegistry = registry;
    }
    
    private String[] METRICS = {
      "jvm.memory.used", 
      "jvm.classes.loaded", 
      "jvm.threads.live"};

    private String METRIC_MSG_FORMAT = "Startup Metric >> {}={}";
    
    private MeterRegistry meterRegistry;

    @EventListener
    public void getAndLogStartupMetrics(
      ApplicationReadyEvent event) {
        Arrays.asList(METRICS)
          .forEach(this::getAndLogActuatorMetric);
    }

    private void getAndLogActuatorMetric(String metric) {
        Meter meter = meterRegistry.find(metric).meter();
        Map<Statistic, Double> stats = getSamples(meter);
 
        logger.info(METRIC_MSG_FORMAT, metric, stats.get(Statistic.VALUE).longValue());
    }

    private Map<Statistic, Double> getSamples(Meter meter) {
        Map<Statistic, Double> samples = new LinkedHashMap<>();
        mergeMeasurements(samples, meter);
        return samples;
    }

    private void mergeMeasurements(Map<Statistic, Double> samples, Meter meter) {
        meter.measure().forEach((measurement) -> samples.merge(measurement.getStatistic(),
                measurement.getValue(), mergeFunction(measurement.getStatistic())));
    }

    private BiFunction<Double, Double, Double> mergeFunction(Statistic statistic) {
        return Statistic.MAX.equals(statistic) ? Double::max : Double::sum;
    }    
}