tessl/maven-io-dropwizard-metrics--metrics-jvm

JVM Integration for Metrics - A set of classes which allow you to monitor critical aspects of your Java Virtual Machine using Metrics

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Class Loading Monitoring

Name: tessl/maven-io-dropwizard-metrics--metrics-jvm
Author: tessl

Class loading statistics including total loaded and unloaded class counts, providing insight into application lifecycle and potential class loading issues.

Core Imports

import com.codahale.metrics.jvm.ClassLoadingGaugeSet;
import com.codahale.metrics.MetricRegistry;
import java.lang.management.ClassLoadingMXBean;

Capabilities

ClassLoadingGaugeSet

Monitors JVM class loading behavior through ClassLoadingMXBean integration.

/**
 * A set of gauges for JVM classloader usage.
 */
public class ClassLoadingGaugeSet implements MetricSet {
    /**
     * Creates a new set of class loading gauges using the default ClassLoadingMXBean.
     */
    public ClassLoadingGaugeSet();
    
    /**
     * Creates a new set of class loading gauges using a custom ClassLoadingMXBean.
     * @param mxBean the ClassLoadingMXBean to use for statistics
     */
    public ClassLoadingGaugeSet(ClassLoadingMXBean mxBean);
    
    /**
     * Returns all class loading metrics.
     * @return map of metric names to Metric instances
     */
    public Map<String, Metric> getMetrics();
}

Metrics Provided:

loaded - Total number of classes loaded since JVM start (cumulative count)
unloaded - Total number of classes unloaded since JVM start (cumulative count)

Usage Examples:

import com.codahale.metrics.MetricRegistry;
import com.codahale.metrics.jvm.ClassLoadingGaugeSet;
import java.lang.management.ManagementFactory;

// Basic usage with default ClassLoadingMXBean
MetricRegistry registry = new MetricRegistry();
registry.registerAll(new ClassLoadingGaugeSet());

// Access class loading metrics
Gauge<Long> totalLoaded = registry.getGauges().get("loaded");
Gauge<Long> totalUnloaded = registry.getGauges().get("unloaded");

System.out.println("Total classes loaded: " + totalLoaded.getValue());
System.out.println("Total classes unloaded: " + totalUnloaded.getValue());

// Calculate current loaded classes (approximation)
long currentlyLoaded = totalLoaded.getValue() - totalUnloaded.getValue();
System.out.println("Currently loaded classes: " + currentlyLoaded);

// Custom ClassLoadingMXBean usage
ClassLoadingMXBean customBean = ManagementFactory.getClassLoadingMXBean();
registry.registerAll(new ClassLoadingGaugeSet(customBean));

// Monitor class loading over time
long previousLoaded = totalLoaded.getValue();
long previousUnloaded = totalUnloaded.getValue();

// ... wait some time ...

long newLoaded = totalLoaded.getValue();
long newUnloaded = totalUnloaded.getValue();

long classesLoadedSinceLast = newLoaded - previousLoaded;
long classesUnloadedSinceLast = newUnloaded - previousUnloaded;

System.out.println("Classes loaded in period: " + classesLoadedSinceLast);
System.out.println("Classes unloaded in period: " + classesUnloadedSinceLast);

Monitoring Patterns:

Steady Growth in Loaded Classes: Normal during application startup and dynamic loading
High Unload Activity: May indicate frequent class reloading or memory pressure
No Unloading: Common in production applications with static class sets
Rapid Loading/Unloading Cycles: Possible indication of dynamic code generation or classloader leaks

Performance Implications:

Class loading can be expensive, especially with complex dependency graphs
Class unloading typically occurs during full GC when classes are no longer referenced
Monitoring ratios of loaded to unloaded classes helps identify potential memory leaks
Sudden spikes in class loading may correlate with application events or deployments

Common Use Cases:

Application Server Monitoring: Track deployments and redeployments
Dynamic Framework Monitoring: Monitor Spring, OSGi, or other dynamic loading frameworks
Memory Leak Detection: Identify classloader leaks through increasing loaded class counts
Performance Analysis: Correlate class loading with application startup times

Install with Tessl CLI

npx tessl i tessl/maven-io-dropwizard-metrics--metrics-jvm

docs

buffer-pool-monitoring.md

class-loading-monitoring.md

cpu-time-monitoring.md

file-descriptor-monitoring.md

gc-monitoring.md

index.md

jmx-attribute-access.md

tessl/maven-io-dropwizard-metrics--metrics-jvm