深入理解 Java 类卸载：避免 90% 的内存泄漏问题

异常君
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生产环境中，一个看似简单的类加载问题可能导致严重的内存泄漏。本文通过实际案例，深入探讨 Java 类在什么情况下会被卸载，以及如何避免类加载器导致的内存问题。

⚠️ 重要：类卸载只在 Full GC 时触发，频繁的类加载/卸载会严重影响性能

类卸载的三个必要条件

Java 类的卸载并不像对象回收那么简单。一个类要被卸载，必须同时满足以下三个条件：

类卸载的三个必要条件.png

类加载器层次结构

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.net.URL;
import java.net.URLClassLoader;

public class ClassLoaderHierarchy {
    private static final Logger logger = LoggerFactory.getLogger(ClassLoaderHierarchy.class);

    public static void printHierarchy() {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        StringBuilder sb = new StringBuilder();

        while (cl != null) {
            sb.append(cl.getClass().getName()).append(": ").append(cl).append("\n");
            if (cl instanceof URLClassLoader) {
                URL[] urls = ((URLClassLoader) cl).getURLs();
                for (URL url : urls) {
                    sb.append("  - ").append(url).append("\n");
                }
            }
            cl = cl.getParent();
        }
        sb.append("Bootstrap ClassLoader (null)");

        logger.info("ClassLoader Hierarchy:\n{}", sb.toString());
    }
}

Metaspace 与类卸载

Java 8+ 使用 Metaspace 替代了 PermGen，类的元数据存储在本地内存中：

import java.lang.management.*;
import java.util.List;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MetaspaceMonitor {
    private static final Logger logger = LoggerFactory.getLogger(MetaspaceMonitor.class);

    public void printMetaspaceInfo() {
        List<MemoryPoolMXBean> pools = ManagementFactory.getMemoryPoolMXBeans();
        for (MemoryPoolMXBean pool : pools) {
            if (pool.getName().contains("Metaspace")) {
                MemoryUsage usage = pool.getUsage();
                logger.info("=== Metaspace 使用情况 ===");
                logger.info("已使用: {}MB", usage.getUsed() / 1024 / 1024);
                logger.info("已提交: {}MB", usage.getCommitted() / 1024 / 1024);
                logger.info("最大值: {}",
                    usage.getMax() == -1 ? "无限制" : usage.getMax() / 1024 / 1024 + "MB");
            }
        }
    }
}

JVM 参数组合建议

public class JVMParameterCombinations {
    private static final Logger logger = LoggerFactory.getLogger(JVMParameterCombinations.class);

    public static void printRecommendedCombinations() {
        logger.info("=== 推荐的 JVM 参数组合 ===");

        logger.info("1. 开发环境（快速启动）：");
        logger.info("   -XX:+TieredCompilation");
        logger.info("   -XX:TieredStopAtLevel=1");
        logger.info("   -XX:MetaspaceSize=64M");
        logger.info("   -XX:MaxMetaspaceSize=256M");

        logger.info("2. 生产环境（稳定性优先）：");
        logger.info("   -XX:+UseG1GC");
        logger.info("   -XX:MaxGCPauseMillis=200");
        logger.info("   -XX:MetaspaceSize=256M");
        logger.info("   -XX:MaxMetaspaceSize=512M");
        logger.info("   -XX:+ParallelRefProcEnabled");

        logger.info("3. 容器环境（资源受限）：");
        logger.info("   -XX:+UseContainerSupport");
        logger.info("   -XX:MaxRAMPercentage=75.0");
        logger.info("   -XX:MaxMetaspaceSize=128M");
    }
}

现代 GC 的类卸载特性

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;

public class ModernGCClassUnloading {
    private static final Logger logger = LoggerFactory.getLogger(ModernGCClassUnloading.class);

    public static void printGCSpecificSettings() {
        String gcName = ManagementFactory.getGarbageCollectorMXBeans()
            .stream()
            .map(GarbageCollectorMXBean::getName)
            .findFirst()
            .orElse("Unknown");

        logger.info("当前 GC: {}", gcName);

        if (gcName.contains("ZGC")) {
            logger.info("ZGC 类卸载建议：");
            logger.info("-XX:+ClassUnloading (默认开启)");
            logger.info("-XX:ZUncommitDelay=300 (5分钟后释放内存)");
        } else if (gcName.contains("Shenandoah")) {
            logger.info("Shenandoah 类卸载建议：");
            logger.info("-XX:+ClassUnloadingWithConcurrentMark");
        }
    }
}

Class Data Sharing (CDS)

public class ClassDataSharing {
    private static final Logger logger = LoggerFactory.getLogger(ClassDataSharing.class);

    public static void explainCDS() {
        logger.info("=== Class Data Sharing (CDS) ===");
        logger.info("CDS 可以减少类加载时间和内存占用");
        logger.info("JDK 12+ 默认开启 AppCDS");

        logger.info("生成共享归档：");
        logger.info("java -XX:ArchiveClassesAtExit=app.jsa -cp app.jar MainClass");

        logger.info("使用共享归档：");
        logger.info("java -XX:SharedArchiveFile=app.jsa -cp app.jar MainClass");
    }
}

性能基准测试数据

场景类数量加载时间卸载时间Metaspace 增长
普通类加载1000245ms89ms12MB
动态代理（未优化）10001823ms456ms156MB
动态代理（优化后）1000312ms95ms18MB
插件系统100567ms234ms45MB
Spring Bean 加载500892ms167ms67MB
Groovy 脚本2001456ms378ms89MB

实战案例：动态代理导致的内存泄漏

import java.net.URL;
import java.net.URLClassLoader;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.Map;

public class DynamicProxyDemo {
    private static final Logger logger = LoggerFactory.getLogger(DynamicProxyDemo.class);

    // 错误示例：类加载器泄漏
    public static class LeakyProxyFactory {
        private static final Map<String, Object> proxyCache = new HashMap<>();

        public static Object createProxy(final Object target) {
            String key = target.getClass().getName();

            return proxyCache.computeIfAbsent(key, k -> {
                // 错误：每次创建新的URLClassLoader
                URLClassLoader loader = new URLClassLoader(
                    new URL[]{target.getClass().getProtectionDomain().getCodeSource().getLocation()},
                    target.getClass().getClassLoader()
                );

                try {
                    Class<?> clazz = loader.loadClass(target.getClass().getName());
                    return Proxy.newProxyInstance(
                        loader,
                        clazz.getInterfaces(),
                        (proxy, method, args) -> {
                            logger.debug("Before: {}", method.getName());
                            Object result = method.invoke(target, args);
                            logger.debug("After: {}", method.getName());
                            return result;
                        }
                    );
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            });
        }
    }
}

问题分析：

  1. 静态 Map 持有代理对象引用，导致类加载器无法回收
  2. 每次创建新的 URLClassLoader，造成类元数据重复加载
  3. Metaspace 持续增长，最终导致 OutOfMemoryError

正确的实现方式

import java.lang.ref.WeakReference;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Proxy;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.atomic.AtomicLong;
import com.google.common.util.concurrent.Striped;
import java.text.DecimalFormat;

public class OptimizedProxyFactory {
    private static final Logger logger = LoggerFactory.getLogger(OptimizedProxyFactory.class);

    // 使用 Striped 锁减少锁竞争
    private static final Striped<Lock> locks = Striped.lock(64);
    private static final Map<ClassLoader, Map<Class<?>, WeakReference<Object>>> cache =
        new ConcurrentHashMap<>();

    // 监控指标
    private static final AtomicLong proxyCreationCount = new AtomicLong();
    private static final AtomicLong cacheHitCount = new AtomicLong();
    private static final AtomicLong cacheMissCount = new AtomicLong();

    @SuppressWarnings("unchecked")
    public static <T> T createProxy(Class<T> targetClass, T target, InvocationHandler handler) {
        ClassLoader classLoader = targetClass.getClassLoader();
        Lock lock = locks.get(classLoader);

        // 先尝试无锁读取
        Map<Class<?>, WeakReference<Object>> loaderCache = cache.get(classLoader);
        if (loaderCache != null) {
            WeakReference<Object> ref = loaderCache.get(targetClass);
            if (ref != null) {
                Object proxy = ref.get();
                if (proxy != null) {
                    cacheHitCount.incrementAndGet();
                    return (T) proxy;
                }
            }
        }

        cacheMissCount.incrementAndGet();

        // 需要创建时才加锁
        lock.lock();
        try {
            // 双重检查
            loaderCache = cache.computeIfAbsent(classLoader, k -> new ConcurrentHashMap<>());
            WeakReference<Object> ref = loaderCache.get(targetClass);
            if (ref != null) {
                Object proxy = ref.get();
                if (proxy != null) {
                    return (T) proxy;
                }
            }

            // 创建新代理
            T newProxy = (T) Proxy.newProxyInstance(
                classLoader,
                targetClass.getInterfaces(),
                handler
            );
            loaderCache.put(targetClass, new WeakReference<>(newProxy));
            proxyCreationCount.incrementAndGet();

            return newProxy;
        } finally {
            lock.unlock();
        }
    }

    public static void clearCache(ClassLoader classLoader) {
        Lock lock = locks.get(classLoader);
        lock.lock();
        try {
            cache.remove(classLoader);
        } finally {
            lock.unlock();
        }
    }

    public static void printMetrics() {
        long hits = cacheHitCount.get();
        long misses = cacheMissCount.get();
        double hitRate = (hits + misses) > 0 ?
            (double) hits / (hits + misses) * 100 : 0;

        logger.info("代理创建次数: {}", proxyCreationCount.get());
        logger.info("缓存命中率: {}%", new DecimalFormat("#.##").format(hitRate));
    }
}

批量类加载优化

import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class BatchClassLoading {
    private static final Logger logger = LoggerFactory.getLogger(BatchClassLoading.class);

    // 批量加载类以减少锁竞争
    public static Map<String, Class<?>> loadClasses(
            ClassLoader loader, List<String> classNames) {
        Map<String, Class<?>> result = new ConcurrentHashMap<>();

        // 使用并行流加速加载
        classNames.parallelStream().forEach(className -> {
            try {
                Class<?> clazz = loader.loadClass(className);
                result.put(className, clazz);
            } catch (ClassNotFoundException e) {
                logger.error("Failed to load class: {}", className, e);
            }
        });

        return result;
    }
}

类加载器预热机制

public class ClassLoaderWarmup {
    private static final Logger logger = LoggerFactory.getLogger(ClassLoaderWarmup.class);

    public static void warmupClassLoader(URLClassLoader loader,
                                       List<String> criticalClasses) {
        logger.info("Warming up ClassLoader with {} classes",
            criticalClasses.size());

        long start = System.currentTimeMillis();

        for (String className : criticalClasses) {
            try {
                Class<?> clazz = loader.loadClass(className);
                // 触发类初始化
                clazz.getDeclaredConstructor().newInstance();
            } catch (Exception e) {
                logger.warn("Failed to warmup class: {}", className);
            }
        }

        long elapsed = System.currentTimeMillis() - start;
                logger.info("ClassLoader warmup completed in {} ms", elapsed);
    }
}

类卸载场景

1. Web 容器热部署

import java.io.File;
import java.io.IOException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.*;

public class WebAppLifecycle {
    private static final Logger logger = LoggerFactory.getLogger(WebAppLifecycle.class);

    private URLClassLoader appClassLoader;
    private List<Object> appInstances = new ArrayList<>();
    private List<Thread> appThreads = new ArrayList<>();

    public void deploy(String warPath) throws Exception {
        undeploy();

        File warFile = new File(warPath);
        if (!warFile.exists() || !warFile.getName().endsWith(".war")) {
            throw new IllegalArgumentException("Invalid WAR file: " + warPath);
        }

        URL[] urls = {warFile.toURI().toURL()};
        appClassLoader = new URLClassLoader(urls,
            Thread.currentThread().getContextClassLoader());

        Thread.currentThread().setContextClassLoader(appClassLoader);

        try {
            Class<?> mainClass = appClassLoader.loadClass("com.app.Main");
            Object instance = mainClass.getDeclaredConstructor().newInstance();
            appInstances.add(instance);

            Method init = mainClass.getMethod("init");
            init.invoke(instance);

            logger.info("Application deployed successfully from: {}", warPath);
        } catch (Exception e) {
            undeploy();
            throw new RuntimeException("Failed to deploy application", e);
        }
    }

    public void undeploy() {
        logger.info("Starting application undeploy...");

        stopApplicationThreads();

        for (Object instance : appInstances) {
            try {
                Method destroy = instance.getClass().getMethod("destroy");
                destroy.invoke(instance);
            } catch (Exception e) {
                logger.error("Error destroying instance", e);
            }
        }
        appInstances.clear();

        deregisterJdbcDrivers();

        if (appClassLoader != null) {
            try {
                appClassLoader.close();
            } catch (IOException e) {
                logger.error("Error closing classloader", e);
            }
            appClassLoader = null;
        }

        System.gc();
        logger.info("Application undeployed");
    }

    private void stopApplicationThreads() {
        for (Thread thread : appThreads) {
            if (thread.isAlive()) {
                thread.interrupt();
                try {
                    thread.join(5000);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }
        }
        appThreads.clear();
    }

    private void deregisterJdbcDrivers() {
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        while (drivers.hasMoreElements()) {
            Driver driver = drivers.nextElement();
            if (driver.getClass().getClassLoader() == appClassLoader) {
                try {
                    DriverManager.deregisterDriver(driver);
                    logger.info("Deregistered JDBC driver: {}", driver.getClass().getName());
                } catch (SQLException e) {
                    logger.error("Error deregistering driver", e);
                }
            }
        }
    }
}

2. 插件系统实现

import java.io.*;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Map;
import java.util.Properties;
import java.util.concurrent.ConcurrentHashMap;

public interface Plugin {
    void onLoad();
    void onEnable();
    void onDisable();
    void onUnload();
}

public class PluginManager {
    private static final Logger logger = LoggerFactory.getLogger(PluginManager.class);

    private final Map<String, PluginContext> plugins = new ConcurrentHashMap<>();

    enum PluginState {
        LOADED, ENABLED, DISABLED, UNLOADED
    }

    static class PluginContext {
        final URLClassLoader classLoader;
        final Plugin pluginInstance;
        final String pluginId;
        volatile PluginState state = PluginState.LOADED;

        PluginContext(String pluginId, URLClassLoader classLoader, Plugin instance) {
            this.pluginId = pluginId;
            this.classLoader = classLoader;
            this.pluginInstance = instance;
        }
    }

    public void loadPlugin(String pluginId, File jarFile) throws Exception {
        if (!jarFile.exists() || !jarFile.getName().endsWith(".jar")) {
            throw new IllegalArgumentException("Invalid plugin file: " + jarFile);
        }

        if (plugins.containsKey(pluginId)) {
            throw new IllegalStateException("Plugin already loaded: " + pluginId);
        }

        URLClassLoader classLoader = null;
        try {
            classLoader = new URLClassLoader(
                new URL[]{jarFile.toURI().toURL()},
                ClassLoader.getSystemClassLoader()
            );

            Properties props = loadPluginProperties(classLoader);
            String mainClass = props.getProperty("plugin.main");

            Class<?> pluginClass = classLoader.loadClass(mainClass);
            if (!Plugin.class.isAssignableFrom(pluginClass)) {
                throw new IllegalArgumentException("Main class must implement Plugin interface");
            }

            Plugin instance = (Plugin) pluginClass.getDeclaredConstructor().newInstance();

            PluginContext context = new PluginContext(pluginId, classLoader, instance);
            plugins.put(pluginId, context);

            instance.onLoad();
            logger.info("Plugin loaded: {}", pluginId);

        } catch (Exception e) {
            if (classLoader != null) {
                classLoader.close();
            }
            throw new RuntimeException("Failed to load plugin: " + pluginId, e);
        }
    }

    public void enablePlugin(String pluginId) {
        PluginContext context = plugins.get(pluginId);
        if (context == null) {
            throw new IllegalArgumentException("Plugin not found: " + pluginId);
        }

        if (context.state == PluginState.ENABLED) {
            return;
        }

        context.pluginInstance.onEnable();
        context.state = PluginState.ENABLED;
        logger.info("Plugin enabled: {}", pluginId);
    }

    public void disablePlugin(String pluginId) {
        PluginContext context = plugins.get(pluginId);
        if (context == null) {
            throw new IllegalArgumentException("Plugin not found: " + pluginId);
        }

        if (context.state != PluginState.ENABLED) {
            return;
        }

        context.pluginInstance.onDisable();
        context.state = PluginState.DISABLED;
        logger.info("Plugin disabled: {}", pluginId);
    }

    public void unloadPlugin(String pluginId) throws IOException {
        PluginContext context = plugins.remove(pluginId);
        if (context == null) {
            return;
        }

        try {
            if (context.state == PluginState.ENABLED) {
                context.pluginInstance.onDisable();
            }
            context.pluginInstance.onUnload();
        } finally {
            context.classLoader.close();
            context.state = PluginState.UNLOADED;
            logger.info("Plugin unloaded: {}", pluginId);
        }
    }

    private Properties loadPluginProperties(URLClassLoader classLoader) throws IOException {
        Properties props = new Properties();
        try (InputStream is = classLoader.getResourceAsStream("plugin.properties")) {
            if (is == null) {
                throw new IllegalArgumentException("plugin.properties not found");
            }
            props.load(is);
        }
        return props;
    }

    public boolean isPluginEnabled(String pluginId) {
        PluginContext context = plugins.get(pluginId);
        return context != null && context.state == PluginState.ENABLED;
    }
}

3. Java Platform Module System (JPMS)

import java.lang.module.Configuration;
import java.lang.module.ModuleFinder;
import java.nio.file.Path;
import java.util.Set;

public class ModuleClassLoading {
    private static final Logger logger = LoggerFactory.getLogger(ModuleClassLoading.class);

    public static void demonstrateModuleLayers() {
        // 创建配置
        ModuleFinder finder = ModuleFinder.of(Path.of("mods"));
        ModuleLayer parent = ModuleLayer.boot();
        Configuration cf = parent.configuration()
            .resolve(finder, ModuleFinder.of(), Set.of("com.example.app"));

        // 创建模块层，每个模块使用独立的类加载器
        ModuleLayer layer = parent.defineModulesWithManyLoaders(cf,
            ClassLoader.getSystemClassLoader());

        // 查找模块的类加载器
        Module module = layer.findModule("com.example.app").orElseThrow();
        ClassLoader moduleLoader = module.getClassLoader();

        try {
            Class<?> clazz = moduleLoader.loadClass("com.example.app.Main");
            Object instance = clazz.getDeclaredConstructor().newInstance();
            logger.info("Successfully loaded class from module: {}", clazz.getName());
        } catch (Exception e) {
            logger.error("Failed to load class from module", e);
        }
    }
}

JDK 17+ 诊断功能

public class JDK17Diagnostics {
    private static final Logger logger = LoggerFactory.getLogger(JDK17Diagnostics.class);

    public static void printProcessInfo() {
        ProcessHandle current = ProcessHandle.current();
        logger.info("PID: {}", current.pid());
        logger.info("Command: {}", current.info().command().orElse("N/A"));
        logger.info("Arguments: {}", String.join(" ",
            current.info().arguments().orElse(new String[0])));
        logger.info("Start time: {}", current.info().startInstant().orElse(null));
    }
}

监控类卸载情况

import java.lang.management.*;
import java.util.concurrent.*;
import java.lang.ref.WeakReference;

public class ClassLoadingMonitor {
    private static final Logger logger = LoggerFactory.getLogger(ClassLoadingMonitor.class);

    private final ClassLoadingMXBean classLoadingBean;
    private final MemoryMXBean memoryBean;

    public ClassLoadingMonitor() {
        this.classLoadingBean = ManagementFactory.getClassLoadingMXBean();
        this.memoryBean = ManagementFactory.getMemoryMXBean();
    }

    public void printDetailedInfo() {
        logger.info("=== 类加载统计 ===");
        logger.info("已加载类总数: {}", classLoadingBean.getTotalLoadedClassCount());
        logger.info("当前加载类数: {}", classLoadingBean.getLoadedClassCount());
        logger.info("已卸载类总数: {}", classLoadingBean.getUnloadedClassCount());

        MemoryUsage heapUsage = memoryBean.getHeapMemoryUsage();
        logger.info("堆内存使用: {} / {}",
            formatBytes(heapUsage.getUsed()),
            formatBytes(heapUsage.getMax()));
    }

    public void enableVerboseClassLoading() {
        classLoadingBean.setVerbose(true);
        logger.info("已启用详细类加载日志");
    }

    private String formatBytes(long bytes) {
        if (bytes < 1024) return bytes + " B";
        if (bytes < 1024 * 1024) return (bytes / 1024) + " KB";
        return (bytes / 1024 / 1024) + " MB";
    }
}

// 类卸载检测工具
public class ClassUnloadingDetector {
    private static final Logger logger = LoggerFactory.getLogger(ClassUnloadingDetector.class);

    private final Map<String, WeakReference<ClassLoader>> trackedLoaders =
        new ConcurrentHashMap<>();
    private final ScheduledExecutorService scheduler =
        Executors.newSingleThreadScheduledExecutor(r -> {
            Thread t = new Thread(r, "class-unloading-detector");
            t.setDaemon(true);
            return t;
        });

    public void startMonitoring() {
        scheduler.scheduleAtFixedRate(this::checkUnloadedClasses,
            0, 30, TimeUnit.SECONDS);
        logger.info("Started class unloading monitoring");
    }

    public void trackClassLoader(String name, ClassLoader loader) {
        trackedLoaders.put(name, new WeakReference<>(loader));
        logger.debug("Tracking ClassLoader: {}", name);
    }

    private void checkUnloadedClasses() {
        System.gc(); // 提示 GC

        trackedLoaders.entrySet().removeIf(entry -> {
            if (entry.getValue().get() == null) {
                logger.info("ClassLoader 已卸载: {}", entry.getKey());
                return true;
            }
            return false;
        });
    }

    public void shutdown() {
        scheduler.shutdown();
        try {
            if (!scheduler.awaitTermination(5, TimeUnit.SECONDS)) {
                scheduler.shutdownNow();
            }
        } catch (InterruptedException e) {
            scheduler.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
}

内存泄漏报告生成

import java.nio.file.*;
import java.io.*;
import java.time.Instant;
import java.util.*;

public class MemoryLeakReporter {
    private static final Logger logger = LoggerFactory.getLogger(MemoryLeakReporter.class);

    public static class LeakReport {
        private final String timestamp;
        private final long totalClasses;
        private final long unloadedClasses;
        private final Map<String, Integer> classLoaderCounts;
        private final List<String> suspiciousLoaders;

        public LeakReport(String timestamp, long totalClasses, long unloadedClasses,
                         Map<String, Integer> classLoaderCounts, List<String> suspiciousLoaders) {
            this.timestamp = timestamp;
            this.totalClasses = totalClasses;
            this.unloadedClasses = unloadedClasses;
            this.classLoaderCounts = classLoaderCounts;
            this.suspiciousLoaders = suspiciousLoaders;
        }

        // getters
        public String getTimestamp() { return timestamp; }
        public long getTotalClasses() { return totalClasses; }
        public long getUnloadedClasses() { return unloadedClasses; }
        public Map<String, Integer> getClassLoaderCounts() { return classLoaderCounts; }
        public List<String> getSuspiciousLoaders() { return suspiciousLoaders; }
    }

    public static LeakReport generateReport() {
                ClassLoadingMXBean bean = ManagementFactory.getClassLoadingMXBean();

        Map<String, Integer> loaderCounts = new HashMap<>();
        List<String> suspicious = new ArrayList<>();

        for (ClassLoader cl : DiagnosticHelper.getAllClassLoaders()) {
            String name = cl.getClass().getName();
            loaderCounts.merge(name, 1, Integer::sum);

            // 检测可疑的类加载器
            if (name.contains("RestartClassLoader") ||
                name.contains("GroovyClassLoader")) {
                suspicious.add(cl.toString());
            }
        }

        return new LeakReport(
            Instant.now().toString(),
            bean.getTotalLoadedClassCount(),
            bean.getUnloadedClassCount(),
            loaderCounts,
            suspicious
        );
    }

    public static void saveReport(LeakReport report, Path outputPath) {
        try (PrintWriter writer = new PrintWriter(
                Files.newBufferedWriter(outputPath))) {
            writer.println("=== Memory Leak Report ===");
            writer.println("Timestamp: " + report.getTimestamp());
            writer.println("Total Classes Loaded: " + report.getTotalClasses());
            writer.println("Classes Unloaded: " + report.getUnloadedClasses());
            writer.println("\nClassLoader Distribution:");
            report.getClassLoaderCounts().forEach((k, v) ->
                writer.println("  " + k + ": " + v));

            if (!report.getSuspiciousLoaders().isEmpty()) {
                writer.println("\n ⚠ Suspicious ClassLoaders:");
                report.getSuspiciousLoaders().forEach(cl ->
                    writer.println("  - " + cl));
            }

            logger.info("Report saved to: {}", outputPath);
        } catch (IOException e) {
            logger.error("Failed to save report", e);
        }
    }
}

常见的类卸载问题

1. ThreadLocal 导致的泄漏

import java.util.UUID;

public class ThreadLocalLeak {
    private static final Logger logger = LoggerFactory.getLogger(ThreadLocalLeak.class);

    private static final ThreadLocal<CustomObject> threadLocal = new ThreadLocal<>();

    static class CustomObject {
        private byte[] data = new byte[1024 * 1024]; // 1MB
        private final String id = UUID.randomUUID().toString();
    }

    public void correctUsage() {
        try {
            threadLocal.set(new CustomObject());
            // 使用ThreadLocal中的数据
            doSomething(threadLocal.get());
        } finally {
            threadLocal.remove(); // 必须清理
        }
    }

    // 更安全的封装
    public static class ThreadLocalResource implements AutoCloseable {
        private final ThreadLocal<CustomObject> tl = new ThreadLocal<>();

        public ThreadLocalResource() {
            tl.set(new CustomObject());
        }

        public CustomObject get() {
            return tl.get();
        }

        @Override
        public void close() {
            tl.remove();
            logger.debug("ThreadLocal resource cleaned");
        }
    }

    // 使用 try-with-resources 自动清理
    public void safeUsage() {
        try (ThreadLocalResource resource = new ThreadLocalResource()) {
            CustomObject obj = resource.get();
            doSomething(obj);
        }
    }

    private void doSomething(CustomObject obj) {
        logger.debug("Processing object: {}", obj.id);
    }
}

2. 注册但未注销的监听器

import java.lang.annotation.*;
import java.lang.reflect.Method;
import java.util.Set;
import java.lang.ref.WeakReference;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface Subscribe {}

public class ListenerLeak {
    private static final Logger logger = LoggerFactory.getLogger(ListenerLeak.class);

    // 使用弱引用的事件总线
    public static class WeakEventBus {
        private final Map<Class<?>, Set<WeakReference<Object>>> listeners =
            new ConcurrentHashMap<>();

        public void register(Object listener) {
            Class<?> clazz = listener.getClass();
            listeners.computeIfAbsent(clazz, k -> ConcurrentHashMap.newKeySet())
                    .add(new WeakReference<>(listener));
            logger.debug("Registered listener: {}", clazz.getSimpleName());
        }

        public void post(Object event) {
            Class<?> eventType = event.getClass();
            Set<WeakReference<Object>> refs = listeners.get(eventType);
            if (refs != null) {
                // 清理已被回收的引用
                refs.removeIf(ref -> ref.get() == null);

                // 分发事件
                for (WeakReference<Object> ref : refs) {
                    Object listener = ref.get();
                    if (listener != null) {
                        invokeListener(listener, event);
                    }
                }
            }
        }

        private void invokeListener(Object listener, Object event) {
            try {
                Method[] methods = listener.getClass().getDeclaredMethods();
                for (Method method : methods) {
                    Subscribe annotation = method.getAnnotation(Subscribe.class);
                    if (annotation != null &&
                        method.getParameterCount() == 1 &&
                        method.getParameterTypes()[0].isAssignableFrom(event.getClass())) {
                        method.setAccessible(true);
                        method.invoke(listener, event);
                        logger.debug("Invoked listener method: {}", method.getName());
                    }
                }
            } catch (Exception e) {
                logger.error("Failed to invoke listener", e);
            }
        }
    }
}

3. 反射缓存导致的泄漏

import com.github.benmanes.caffeine.cache.Cache;
import com.github.benmanes.caffeine.cache.Caffeine;
import com.github.benmanes.caffeine.cache.CacheStats;
import java.time.Duration;
import java.util.Objects;

public class ReflectionCacheLeak {
    private static final Logger logger = LoggerFactory.getLogger(ReflectionCacheLeak.class);

    // 使用 Caffeine 缓存库
    public static class SafeReflectionCache {
        private static final Cache<CacheKey, Method> methodCache = Caffeine.newBuilder()
            .maximumSize(1000)
            .weakKeys()
            .weakValues()
            .expireAfterAccess(Duration.ofMinutes(10))
            .recordStats()
            .build();

        static class CacheKey {
            private final WeakReference<Class<?>> classRef;
            private final String methodName;
            private final int hashCode;

            CacheKey(Class<?> clazz, String methodName) {
                this.classRef = new WeakReference<>(clazz);
                this.methodName = methodName;
                this.hashCode = Objects.hash(clazz, methodName);
            }

            @Override
            public boolean equals(Object o) {
                if (this == o) return true;
                if (!(o instanceof CacheKey)) return false;
                CacheKey that = (CacheKey) o;
                Class<?> thisClass = classRef.get();
                Class<?> thatClass = that.classRef.get();
                return thisClass != null && thisClass.equals(thatClass)
                    && methodName.equals(that.methodName);
            }

            @Override
            public int hashCode() {
                return hashCode;
            }
        }

        public static Method getMethod(Class<?> clazz, String methodName)
                throws NoSuchMethodException {
            CacheKey key = new CacheKey(clazz, methodName);
            return methodCache.get(key, k -> {
                try {
                    return clazz.getMethod(methodName);
                } catch (NoSuchMethodException e) {
                    throw new RuntimeException(e);
                }
            });
        }

        public static void printCacheStats() {
            CacheStats stats = methodCache.stats();
            logger.info("缓存命中率: {}%", String.format("%.2f", stats.hitRate() * 100));
            logger.info("缓存大小: {}", methodCache.estimatedSize());
        }
    }
}

问题诊断与工具使用

1. 使用 jmap 分析类加载器

# 查看类加载器统计
jmap -clstats <pid>

# 生成堆转储文件
jmap -dump:format=b,file=heap.hprof <pid>

# 查看类加载器层次结构
jcmd <pid> VM.classloader_stats

# JDK 17+ 新增命令
jcmd <pid> VM.class_hierarchy

2. 程序化生成堆转储

import javax.management.*;
import java.lang.reflect.Field;
import java.util.*;

public class DiagnosticHelper {
    private static final Logger logger = LoggerFactory.getLogger(DiagnosticHelper.class);

    public static void dumpHeap(String filePath) {
        try {
            MBeanServer server = ManagementFactory.getPlatformMBeanServer();
            ObjectName mbeanName = new ObjectName("com.sun.management:type=HotSpotDiagnostic");
            server.invoke(mbeanName, "dumpHeap",
                new Object[]{filePath, Boolean.TRUE},
                new String[]{"java.lang.String", "boolean"});
            logger.info("Heap dump created: {}", filePath);
        } catch (Exception e) {
            throw new RuntimeException("Failed to dump heap", e);
        }
    }

    public static Set<ClassLoader> getAllClassLoaders() {
        Set<ClassLoader> classLoaders = new HashSet<>();

        // 通过线程获取类加载器
        for (Thread thread : Thread.getAllStackTraces().keySet()) {
            ClassLoader cl = thread.getContextClassLoader();
            if (cl != null) {
                classLoaders.add(cl);
            }
        }

        // 添加系统类加载器
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        classLoaders.add(systemClassLoader);
        ClassLoader parent = systemClassLoader.getParent();
        if (parent != null) {
            classLoaders.add(parent);
        }

        return classLoaders;
    }

    public static void detectClassLoaderLeaks() {
        logger.info("=== 检测类加载器泄漏 ===");

        // 强制 Full GC
        System.gc();
        System.runFinalization();
        System.gc();

        try {
            Thread.sleep(1000); // 等待GC完成
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }

        // 获取所有类加载器
        Set<ClassLoader> classLoaders = getAllClassLoaders();

        for (ClassLoader cl : classLoaders) {
            if (cl instanceof URLClassLoader) {
                URLClassLoader urlCl = (URLClassLoader) cl;
                logger.info("URLClassLoader: {}", cl);
                logger.info("  URLs: {}", Arrays.toString(urlCl.getURLs()));
                logger.info("  Parent: {}", cl.getParent());

                // 检查是否应该被回收但仍然存活
                if (shouldBeGarbageCollected(cl)) {
                    logger.warn("  ⚠ 潜在泄漏：此类加载器应该已被回收");
                }
            }
        }
    }

    private static boolean shouldBeGarbageCollected(ClassLoader cl) {
        // 检查是否是应该被回收的类加载器（如已关闭的Web应用）
        String className = cl.getClass().getName();
        return className.contains("WebappClassLoader") ||
               className.contains("PluginClassLoader");
    }
}

3. Spring 框架中的类加载器问题

import org.springframework.beans.factory.DisposableBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.context.ApplicationListener;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.event.ContextClosedEvent;
import org.springframework.stereotype.Component;
import java.lang.reflect.Field;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

@Component
public class SpringClassLoaderIssues {
    private static final Logger logger = LoggerFactory.getLogger(SpringClassLoaderIssues.class);

    @Configuration
    @ConditionalOnClass(name = "org.springframework.boot.devtools.restart.Restarter")
    public static class DevToolsConfiguration {

        @Bean
        public ApplicationListener<ContextClosedEvent> classLoaderCleanupListener() {
            return event -> {
                logger.info("Cleaning up RestartClassLoader resources");
                cleanupRestartClassLoader();
            };
        }

        private void cleanupRestartClassLoader() {
            try {
                // 清理 Spring DevTools 的缓存
                Field cacheField = Class.forName(
                    "org.springframework.boot.devtools.restart.classloader.RestartClassLoader")
                    .getDeclaredField("cache");
                cacheField.setAccessible(true);

                // 获取所有 RestartClassLoader 实例并清理
                for (ClassLoader cl : DiagnosticHelper.getAllClassLoaders()) {
                    if (cl.getClass().getName().contains("RestartClassLoader")) {
                        Object cache = cacheField.get(cl);
                        if (cache instanceof Map) {
                            ((Map<?, ?>) cache).clear();
                            logger.debug("Cleared RestartClassLoader cache");
                        }
                    }
                }
            } catch (Exception e) {
                logger.debug("DevTools not in use or cache clearing failed", e);
            }
        }
    }

    @Component
    public static class ScheduledTaskManager implements DisposableBean {
        private final ScheduledExecutorService executor =
            Executors.newScheduledThreadPool(5, r -> {
                Thread t = new Thread(r);
                t.setDaemon(true);
                t.setName("scheduled-task-" + t.getId());
                return t;
            });

        @Override
        public void destroy() {
            logger.info("Shutting down scheduled task executor");
            executor.shutdown();
            try {
                if (!executor.awaitTermination(10, TimeUnit.SECONDS)) {
                    executor.shutdownNow();
                    logger.warn("Forced shutdown of scheduled task executor");
                }
            } catch (InterruptedException e) {
                executor.shutdownNow();
                Thread.currentThread().interrupt();
            }
        }
    }
}

异常处理和自动修复建议

public class ClassLoaderExceptionHandler {
    private static final Logger logger = LoggerFactory.getLogger(ClassLoaderExceptionHandler.class);

        public static void handleClassLoaderException(Exception e) {
        if (e instanceof LinkageError) {
            logger.error("类链接错误，可能存在版本冲突");
            logger.error("建议检查：");
            logger.error("1. 是否有重复的JAR包");
            logger.error("2. 类路径中是否有不同版本的相同库");
            logger.error("3. 使用 mvn dependency:tree 分析依赖冲突");
        } else if (e instanceof ClassCircularityError) {
            logger.error("类循环依赖错误");
            logger.error("建议检查类的继承和实现关系");
        } else if (e instanceof NoClassDefFoundError) {
            logger.error("类定义未找到，检查类路径");
            printClassPath();
        } else if (e instanceof OutOfMemoryError && e.getMessage().contains("Metaspace")) {
            logger.error("Metaspace 内存溢出");
            handleMetaspaceOOM();
        }
    }

    private static void printClassPath() {
        String classPath = System.getProperty("java.class.path");
        logger.info("当前类路径：");
        for (String path : classPath.split(File.pathSeparator)) {
            logger.info("  - {}", path);
        }
    }

    private static void handleMetaspaceOOM() {
        logger.error("处理建议：");
        logger.error("1. 增加 -XX:MaxMetaspaceSize 参数");
        logger.error("2. 检查是否有类加载器泄漏");
        logger.error("3. 运行 jmap -clstats <pid> 查看详情");

        // 尝试清理
        System.gc();
        logger.info("已触发 Full GC 尝试释放 Metaspace");
    }
}

public class AutoFixSuggestions {
    private static final Logger logger = LoggerFactory.getLogger(AutoFixSuggestions.class);

    public static List<String> analyzeProblem(Throwable error) {
        List<String> suggestions = new ArrayList<>();

        if (error instanceof OutOfMemoryError) {
            String message = error.getMessage();
            if (message != null && message.contains("Metaspace")) {
                suggestions.add("增加 -XX:MaxMetaspaceSize 参数");
                suggestions.add("检查是否有类加载器泄漏");
                suggestions.add("运行 jmap -clstats <pid> 查看详情");
            }
        } else if (error instanceof ClassNotFoundException) {
            suggestions.add("检查类路径配置");
            suggestions.add("确认 JAR 文件完整性");
            suggestions.add("检查模块依赖关系");
        }

        return suggestions;
    }

    public static void printSuggestions(Throwable error) {
        List<String> suggestions = analyzeProblem(error);
        if (!suggestions.isEmpty()) {
            logger.info("=== 修复建议 ===");
            suggestions.forEach(s -> logger.info("- {}", s));
        }
    }
}

性能影响分析

类卸载对性能的影响

public class ClassUnloadingPerformance {
    private static final Logger logger = LoggerFactory.getLogger(ClassUnloadingPerformance.class);

    public static void measureClassUnloadingImpact() {
        logger.info("=== 类卸载性能测试 ===");

        long initialClasses = getLoadedClassCount();
        List<Long> gcTimes = new ArrayList<>();

        for (int i = 0; i < 10; i++) {
            // 加载类
            List<ClassLoader> loaders = createClassLoaders(100);

            // 清除引用
            loaders.clear();

            // 测量GC时间
            long gcTime = measureGCTime();
            gcTimes.add(gcTime);

            logger.info("第 {} 次迭代 - GC时间: {} ms, 当前类数: {}",
                i + 1, gcTime, getLoadedClassCount());
        }

        double avgGCTime = gcTimes.stream()
            .mapToLong(Long::longValue)
            .average()
            .orElse(0);

        logger.info("平均GC时间: {} ms", String.format("%.2f", avgGCTime));
        logger.info("类卸载数量: {}",
            initialClasses + 1000 - getLoadedClassCount());
    }

    private static List<ClassLoader> createClassLoaders(int count) {
        List<ClassLoader> loaders = new ArrayList<>();

        for (int i = 0; i < count; i++) {
            URLClassLoader loader = new URLClassLoader(new URL[0]);
            loaders.add(loader);
        }

        return loaders;
    }

    private static long measureGCTime() {
        long startTime = System.currentTimeMillis();
        System.gc();
        return System.currentTimeMillis() - startTime;
    }

    private static long getLoadedClassCount() {
        return ManagementFactory.getClassLoadingMXBean().getLoadedClassCount();
    }
}

Metaspace 配置建议

public class MetaspaceOptimization {
    private static final Logger logger = LoggerFactory.getLogger(MetaspaceOptimization.class);

    public static void recommendSettings() {
        Runtime runtime = Runtime.getRuntime();
        long maxMemory = runtime.maxMemory();

        logger.info("=== Metaspace 配置建议 ===");

        long recommendedMetaspaceSize = maxMemory / 8;
        long recommendedMaxMetaspaceSize = maxMemory / 4;

        logger.info("推荐 JVM 参数：");
        logger.info("-XX:MetaspaceSize={}M", recommendedMetaspaceSize / 1024 / 1024);
        logger.info("-XX:MaxMetaspaceSize={}M", recommendedMaxMetaspaceSize / 1024 / 1024);

        logger.info("特定场景建议：");
        logger.info("1. 微服务应用：");
        logger.info("   -XX:MaxMetaspaceSize=128M");
        logger.info("   -XX:CompressedClassSpaceSize=64M");

        logger.info("2. 应用服务器（Tomcat/Jetty）：");
        logger.info("   -XX:MaxMetaspaceSize=512M");
        logger.info("   -XX:+UseStringDeduplication");

        logger.info("3. 使用动态语言（Groovy/JRuby）：");
        logger.info("   -XX:MaxMetaspaceSize=1G");
        logger.info("   -XX:+UnlockExperimentalVMOptions");
        logger.info("   -XX:+UseG1GC");

        logger.info("监控参数：");
        logger.info("   -XX:+TraceClassLoading");
        logger.info("   -XX:+TraceClassUnloading");
        logger.info("   -Xlog:class+unload=info (JDK 9+)");
    }
}

配置文件示例

application.yml 配置

# application.yml 示例
jvm:
  metaspace:
    initial-size: 128M
    max-size: 512M
  class-loading:
    trace-loading: true
    trace-unloading: true
  gc:
    type: G1
    max-pause-millis: 200

# 监控配置
monitoring:
  class-loader:
    check-interval: 30s
    leak-detection: true
    report-path: /var/log/app/classloader-reports

Docker 环境配置

FROM openjdk:17-jdk-slim

# 设置 JVM 参数
ENV JAVA_OPTS="-XX:MaxMetaspaceSize=256m \
               -XX:MetaspaceSize=128m \
               -XX:+TraceClassLoading \
               -XX:+TraceClassUnloading \
               -XX:+UseG1GC \
               -XX:MaxGCPauseMillis=200"

# 添加监控脚本
COPY scripts/monitor-metaspace.sh /usr/local/bin/
RUN chmod +x /usr/local/bin/monitor-metaspace.sh

# 健康检查
HEALTHCHECK --interval=30s --timeout=3s \
  CMD jcmd 1 VM.metaspace || exit 1

COPY target/app.jar /app.jar
ENTRYPOINT ["sh", "-c", "java $JAVA_OPTS -jar /app.jar"]

安全性考虑

import java.security.CodeSource;
import java.security.PermissionCollection;
import java.security.Permissions;

public class SecureClassLoading {
    private static final Logger logger = LoggerFactory.getLogger(SecureClassLoading.class);

    public static URLClassLoader createSecureClassLoader(URL[] urls) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkCreateClassLoader();
        }

        return new URLClassLoader(urls) {
            @Override
            protected PermissionCollection getPermissions(CodeSource codesource) {
                PermissionCollection perms = super.getPermissions(codesource);
                perms.add(new RuntimePermission("accessDeclaredMembers"));
                return perms;
            }
        };
    }
}

测试验证

import static org.junit.Assert.*;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;
import java.io.File;

@RunWith(JUnit4.class)
public class ClassUnloadingTest {
    private static final Logger logger = LoggerFactory.getLogger(ClassUnloadingTest.class);

    @Test
    public void testClassUnloading() throws Exception {
        WeakReference<Class<?>> classRef = loadClassInIsolation();

        // 触发 GC
        System.gc();
        Thread.sleep(100);
        System.gc();

        // 验证类已被卸载
        assertNull("Class should be unloaded", classRef.get());
        logger.info("Class unloading test passed");
    }

    private WeakReference<Class<?>> loadClassInIsolation() throws Exception {
        URLClassLoader loader = new URLClassLoader(
            new URL[]{new File("test-classes").toURI().toURL()});
        Class<?> clazz = loader.loadClass("TestClass");
        loader.close();
        return new WeakReference<>(clazz);
    }

    @Test
    public void testPluginLifecycle() throws Exception {
        PluginManager manager = new PluginManager();

        // 创建测试插件JAR
        File pluginJar = createTestPluginJar();

        // 加载插件
        manager.loadPlugin("test-plugin", pluginJar);

        // 启用插件
        manager.enablePlugin("test-plugin");

        // 验证插件状态
        assertTrue(manager.isPluginEnabled("test-plugin"));

        // 卸载插件
        manager.unloadPlugin("test-plugin");

        // 验证类已卸载
        System.gc();
        Thread.sleep(100);

        // 验证插件已被移除
        assertFalse(manager.isPluginEnabled("test-plugin"));
    }

    private File createTestPluginJar() throws Exception {
        // 创建临时目录
        Path tempDir = Files.createTempDirectory("test-plugin");

        // 编译 TestPlugin.java
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        compiler.run(null, null, null,
            "src/test/java/com/example/TestPlugin.java",
            "-d", tempDir.toString());

        // 创建 plugin.properties
        Path propsFile = tempDir.resolve("plugin.properties");
        Files.write(propsFile, Arrays.asList(
            "plugin.main=com.example.TestPlugin",
            "plugin.name=Test Plugin",
            "plugin.version=1.0.0"
        ));

        // 打包成 JAR
        File jarFile = File.createTempFile("test-plugin", ".jar");
        // ... JAR 打包逻辑

        return jarFile;
    }
}

快速参考

紧急情况处理

  1. OutOfMemoryError: Metaspace

    # 临时增加 Metaspace
-XX:MaxMetaspaceSize=1G

# 查看类加载情况
jcmd <pid> VM.classloader_stats

  2. 类加载器泄漏检测

    # 生成堆转储
jmap -dump:format=b,file=heap.hprof <pid>

# 使用 MAT 分析
# 查找路径：ClassLoader -> GC Roots

性能调优

  • ✓ 设置合理的 Metaspace 大小
  • ✓ 启用类卸载日志监控
  • ✓ 定期检查类加载器数量
  • ✓ 实现资源清理钩子
  • ✓ 使用弱引用缓存

故障排查

1. 检查类加载器引用

  • 静态变量是否持有类实例
  • ThreadLocal 是否已清理
  • 监听器是否已注销

2. 监控 Metaspace

  • 使用 -XX:+TraceClassUnloading 查看卸载日志
  • 定期检查 Metaspace 使用率
  • 设置合理的 MaxMetaspaceSize

3. 分析堆转储

  • 使用 MAT 查找 ClassLoader 泄漏
  • 检查 GC Roots 路径
  • 分析大对象和重复类

4. 代码审查重点

  • 动态代理和反射使用
  • 自定义类加载器实现
  • 资源清理逻辑

常见错误信息处理

1. java.lang.OutOfMemoryError: Metaspace

// 解决方案：增加 Metaspace 大小或查找类加载器泄漏
// -XX:MaxMetaspaceSize=512M
// 使用 jmap -clstats <pid> 查看类加载器统计

2. java.lang.ClassNotFoundException 在热部署时

// 确保正确设置线程上下文类加载器
Thread.currentThread().setContextClassLoader(newClassLoader);

最佳方案总结

场景问题解决方案监控方法
动态代理静态缓存持有代理对象使用 WeakReference 和 Striped 锁监控 Metaspace 使用率
热部署ClassLoader 未正确释放清理所有引用并调用 close()jmap -clstats
插件系统跨 ClassLoader 引用使用接口隔离MAT 分析 GC Roots
ThreadLocal线程池环境下内存泄漏使用后调用 remove()线程转储分析
事件监听注册后未注销使用弱引用监控引用数量
Spring 应用线程池未关闭实现 DisposableBeanSpring Actuator

💡 关键提醒：

  1. 类卸载只在 Full GC 时发生
  2. Metaspace 不会自动收缩，需要合理设置最大值
  3. 生产环境建议开启类卸载日志进行监控
  4. GraalVM Native Image 不存在运行时类加载和卸载，但可通过特定配置实现有限的动态特性

相关工具资源

问题诊断流程图

问题诊断流程图.png

监控脚本示例

monitor-metaspace.sh

#!/bin/bash
# Metaspace 监控脚本

PID=$1
if [ -z "$PID" ]; then
    echo "Usage: $0 <pid>"
    exit 1
fi

while true; do
    echo "=== Metaspace Monitor - $(date) ==="

    # 获取 Metaspace 使用情况
    jcmd $PID VM.metaspace | grep -E "Used:|Committed:|Reserved:"

    # 获取类加载统计
    echo ""
    echo "Class Loading Stats:"
    jcmd $PID VM.classloader_stats | head -20

    # 检查是否有异常
    METASPACE_USED=$(jcmd $PID VM.metaspace | grep "Used:" | awk '{print $2}' | sed 's/K//')
    METASPACE_MAX=$(jcmd $PID VM.flags | grep MaxMetaspaceSize | awk '{print $3}')

    if [ ! -z "$METASPACE_MAX" ] && [ "$METASPACE_MAX" != "0" ]; then
        USAGE_PERCENT=$((METASPACE_USED * 100 / (METASPACE_MAX / 1024)))
        if [ $USAGE_PERCENT -gt 80 ]; then
            echo "WARNING: Metaspace usage is at ${USAGE_PERCENT}%"
        fi
    fi

    echo "----------------------------------------"
    sleep 30
done

完整的测试插件示例

TestPlugin.java

// 用于测试的插件实现
public class TestPlugin implements Plugin {
    private static final Logger logger = LoggerFactory.getLogger(TestPlugin.class);

    private volatile boolean running = false;
    private Thread workerThread;

    @Override
    public void onLoad() {
        logger.info("TestPlugin loaded");
    }

    @Override
    public void onEnable() {
        logger.info("TestPlugin enabled");
        running = true;

        workerThread = new Thread(() -> {
            while (running) {
                try {
                    logger.debug("TestPlugin is working...");
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }
        });
        workerThread.setName("TestPlugin-Worker");
        workerThread.start();
    }

    @Override
    public void onDisable() {
        logger.info("TestPlugin disabled");
        running = false;
        if (workerThread != null) {
            workerThread.interrupt();
            try {
                workerThread.join(1000);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }

    @Override
    public void onUnload() {
        logger.info("TestPlugin unloaded");
        // 清理资源
    }
}

plugin.properties

# 插件配置文件
plugin.main=com.example.TestPlugin
plugin.name=Test Plugin
plugin.version=1.0.0
plugin.author=Your Name
plugin.description=A test plugin for demonstrating class loading

生产环境监控配置

Prometheus 监控指标

import io.micrometer.core.instrument.MeterRegistry;
import io.micrometer.core.instrument.Gauge;

@Component
public class ClassLoaderMetrics {
    private final ClassLoadingMXBean classLoadingBean;

    public ClassLoaderMetrics(MeterRegistry registry) {
        this.classLoadingBean = ManagementFactory.getClassLoadingMXBean();

        // 注册监控指标
        Gauge.builder("jvm.classes.loaded", classLoadingBean,
            ClassLoadingMXBean::getLoadedClassCount)
            .description("Number of classes currently loaded")
            .register(registry);

        Gauge.builder("jvm.classes.unloaded", classLoadingBean,
            ClassLoadingMXBean::getUnloadedClassCount)
            .description("Total number of classes unloaded")
            .register(registry);

        // Metaspace 监控
        List<MemoryPoolMXBean> pools = ManagementFactory.getMemoryPoolMXBeans();
        for (MemoryPoolMXBean pool : pools) {
            if (pool.getName().contains("Metaspace")) {
                Gauge.builder("jvm.metaspace.used", pool,
                    p -> p.getUsage().getUsed())
                    .description("Metaspace used memory")
                    .baseUnit("bytes")
                    .register(registry);

                Gauge.builder("jvm.metaspace.committed", pool,
                    p -> p.getUsage().getCommitted())
                    .description("Metaspace committed memory")
                    .baseUnit("bytes")
                    .register(registry);
            }
        }
    }
}

Grafana Dashboard 配置

{
  "dashboard": {
    "title": "JVM Class Loading Monitor",
    "panels": [
      {
        "title": "Classes Loaded",
        "targets": [
          {
            "expr": "jvm_classes_loaded"
          }
        ]
      },
      {
        "title": "Metaspace Usage",
        "targets": [
          {
            "expr": "jvm_metaspace_used / jvm_metaspace_committed * 100"
          }
        ]
      },
      {
        "title": "Class Unload Rate",
        "targets": [
          {
            "expr": "rate(jvm_classes_unloaded[5m])"
          }
        ]
      }
    ]
  }
}
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