面经手册 · 第25篇《JVM内存模型总结，有各版本JDK对比、有元空间OOM监控案例、有Java版虚拟机，综合学习更容易！》

public static float circumference(float r){
        float pi = 3.14f;
        float area = 2 * pi * r;
        return area;
}

运行时数据区
├── heap
│   ├── constantpool
│   ├── methodarea
│   │   ├── Class.java
│   │   ├── ClassMember.java
│   │   ├── Field.java
│   │   ├── Method.java
│   │   ├── MethodDescriptor.java
│   │   ├── MethodDescriptorParser.java
│   │   ├── MethodLookup.java
│   │   ├── Object.java
│   │   ├── Slots.java
│   │   └── StringPool.java
│   └── ClassLoader.java
├── Frame.java
├── JvmStack.java
├── LocalVars.java
├── OperandStack.java
├── Slot.java
└── Thread.java

public class OperandStack {

    private int size = 0;
    private Slot[] slots;

    public OperandStack(int maxStack) {
        if (maxStack > 0) {
            slots = new Slot[maxStack];
            for (int i = 0; i < maxStack; i++) {
                slots[i] = new Slot();
            }
        }
    }
    //...
}

public class JvmStack {

    private int maxSize;
    private int size;
    private Frame _top;
    
    //...
}

public class Frame {

    //stack is implemented as linked list
    Frame lower;

    //局部变量表
    private LocalVars localVars;

    //操作数栈
    private OperandStack operandStack;

    private Thread thread;

    private Method method;

    private int nextPC;
 
    //...
}

public static void main(String[] args) throws InterruptedException {
    
    Thread.sleep(5000);
    
    ClassLoadingMXBean loadingBean = ManagementFactory.getClassLoadingMXBean();
    while (true) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(MetaSpaceOomMock.class);
        enhancer.setCallbackTypes(new Class[]{Dispatcher.class, MethodInterceptor.class});
        enhancer.setCallbackFilter(new CallbackFilter() {
            @Override
            public int accept(Method method) {
                return 1;
            }
            @Override
            public boolean equals(Object obj) {
                return super.equals(obj);
            }
        });
        System.out.println(enhancer.createClass().getName() + loadingBean.getTotalLoadedClassCount() + loadingBean.getLoadedClassCount() + loadingBean.getUnloadedClassCount());
    }
}

-XX:MetaspaceSize=8m
-XX:MaxMetaspaceSize=80m

-Djava.rmi.server.hostname=127.0.0.1
-Dcom.sun.management.jmxremote
-Dcom.sun.management.jmxremote.port=7397
-Dcom.sun.management.jmxremote.ssl=false
-Dcom.sun.management.jmxremote.authenticate=false

org.itstack.interview.MetaSpaceOomMock$$EnhancerByCGLIB$$bd2bb16e999099900
org.itstack.interview.MetaSpaceOomMock$$EnhancerByCGLIB$$9c774e64999199910
org.itstack.interview.MetaSpaceOomMock$$EnhancerByCGLIB$$cac97732999299920
org.itstack.interview.MetaSpaceOomMock$$EnhancerByCGLIB$$91c6a15a999399930
Exception in thread "main" java.lang.IllegalStateException: Unable to load cache item
	at net.sf.cglib.core.internal.LoadingCache.createEntry(LoadingCache.java:79)
	at net.sf.cglib.core.internal.LoadingCache.get(LoadingCache.java:34)
	at net.sf.cglib.core.AbstractClassGenerator$ClassLoaderData.get(AbstractClassGenerator.java:119)
	at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:294)
	at net.sf.cglib.proxy.Enhancer.createHelper(Enhancer.java:480)
	at net.sf.cglib.proxy.Enhancer.createClass(Enhancer.java:337)
	at org.itstack.interview.MetaSpaceOomMock.main(MetaSpaceOomMock.java:34)
Caused by: java.lang.OutOfMemoryError: Metaspace
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:348)
	at net.sf.cglib.core.ReflectUtils.defineClass(ReflectUtils.java:467)
	at net.sf.cglib.core.AbstractClassGenerator.generate(AbstractClassGenerator.java:339)
	at net.sf.cglib.proxy.Enhancer.generate(Enhancer.java:492)
	at net.sf.cglib.core.AbstractClassGenerator$ClassLoaderData$3.apply(AbstractClassGenerator.java:96)
	at net.sf.cglib.core.AbstractClassGenerator$ClassLoaderData$3.apply(AbstractClassGenerator.java:94)
	at net.sf.cglib.core.internal.LoadingCache$2.call(LoadingCache.java:54)
	at java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:266)
	at java.util.concurrent.FutureTask.run(FutureTask.java)
	at net.sf.cglib.core.internal.LoadingCache.createEntry(LoadingCache.java:61)
	... 6 more