被问到JVM类加载机制中双亲委派模型是什么，三次被破坏指什么？

package com.manong.jvm;
public class Math {
    public static final int initData = 666;
    public static User user = new User();

    public int compute() {  
    //一个方法对应一块栈帧内存区域
        int a = 1;
        int b = 2;
        int c = (a + b) * 10;
        return c;
    }

    public static void main(String[] args) {
        Math math = new Math();
        math.compute();
    }

//此源码只有相关的部分，并且省略了异常捕获相关的代码
public class Launcher {

  private static Launcher launcher = new Launcher();
  private ClassLoader loader;
  
  public static Launcher getLauncher() {
      return launcher;
  }
  
  public Launcher() {
   Launcher.ExtClassLoader var1;    
   var1=Launcher.ExtClassLoader.getExtClassLoader();
   this.loader=Launcher.AppClassLoader.getAppClassLoader(var1);   
   Thread.currentThread().setContextClassLoader(this.loader);  
   }
   public ClassLoader getClassLoader() {
        return this.loader;
    }
}

public class TestJDKClassLoader {

    public static void main(String[] args) {
        System.out.println(String.class.getClassLoader());
        System.out.println(com.sun.crypto.provider.DESKeyFactory.class.getClassLoader().getClass().getName());
        System.out.println(TestJDKClassLoader.class.getClassLoader().getClass().getName());

        System.out.println();
        ClassLoader appClassLoader = ClassLoader.getSystemClassLoader();
        ClassLoader extClassloader = appClassLoader.getParent();
        ClassLoader bootstrapLoader = extClassloader.getParent();
        System.out.println("the bootstrapLoader : " + bootstrapLoader);
        System.out.println("the extClassloader : " + extClassloader);
        System.out.println("the appClassLoader : " + appClassLoader);

        System.out.println();
        System.out.println("bootstrapLoader加载以下文件：");
        URL[] urls = Launcher.getBootstrapClassPath().getURLs();
        for (int i = 0; i < urls.length; i++) {
            System.out.println(urls[i]);
        }

        System.out.println();
        System.out.println("extClassloader加载以下文件：");
        System.out.println(System.getProperty("java.ext.dirs"));

        System.out.println();
        System.out.println("appClassLoader加载以下文件：");
        System.out.println(System.getProperty("java.class.path"));

    }
}

运行结果：
null
sun.misc.Launcher$ExtClassLoader
sun.misc.Launcher$AppClassLoader

the bootstrapLoader : null
the extClassloader : sun.misc.Launcher$ExtClassLoader@3764951d
the appClassLoader : sun.misc.Launcher$AppClassLoader@14dad5dc

bootstrapLoader加载以下文件：
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/resources.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/rt.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/sunrsasign.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/jsse.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/jce.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/charsets.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/lib/jfr.jar
file:/D:/dev/Java/jdk1.8.0_45/jre/classes

extClassloader加载以下文件：
D:\dev\Java\jdk1.8.0_45\jre\lib\ext;C:\Windows\Sun\Java\lib\ext

appClassLoader加载以下文件：
D:\dev\Java\jdk1.8.0_45\jre\lib\charsets.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\deploy.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\access-bridge-64.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\cldrdata.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\dnsns.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\jaccess.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\jfxrt.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\localedata.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\nashorn.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\sunec.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\sunjce_provider.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\sunmscapi.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\sunpkcs11.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\ext\zipfs.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\javaws.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\jce.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\jfr.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\jfxswt.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\jsse.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\management-agent.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\plugin.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\resources.jar;D:\dev\Java\jdk1.8.0_45\jre\lib\rt.jar;D:\ideaProjects\project-all\target\classes;C:\Users\zhuge\.m2\repository\org\apache\zookeeper\zookeeper\3.4.12\zookeeper-3.4.12.jar;C:\Users\zhuge\.m2\repository\org\slf4j\slf4j-api\1.7.25\slf4j-api-1.7.25.jar;C:\Users\zhuge\.m2\repository\org\slf4j\slf4j-log4j12\1.7.25\slf4j-log4j12-1.7.25.jar;C:\Users\zhuge\.m2\repository\log4j\log4j\1.2.17\log4j-1.2.17.jar;C:\Users\zhuge\.m2\repository\jline\jline\0.9.94\jline-0.9.94.jar;C:\Users\zhuge\.m2\repository\org\apache\yetus\audience-annotations\0.5.0\audience-annotations-0.5.0.jar;C:\Users\zhuge\.m2\repository\io\netty\netty\3.10.6.Final\netty-3.10.6.Final.jar;C:\Users\zhuge\.m2\repository\com\google\guava\guava\22.0\guava-22.0.jar;C:\Users\zhuge\.m2\repository\com\google\code\findbugs\jsr305\1.3.9\jsr305-1.3.9.jar;C:\Users\zhuge\.m2\repository\com\google\errorprone\error_prone_annotations\2.0.18\error_prone_annotations-2.0.18.jar;C:\Users\zhuge\.m2\repository\com\google\j2objc\j2objc-annotations\1.1\j2objc-annotations-1.1.jar;C:\Users\zhuge\.m2\repository\org\codehaus\mojo\animal-sniffer-annotations\1.14\animal-sniffer-annotations-1.14.jar;D:\dev\IntelliJ IDEA 2018.3.2\lib\idea_rt.jar

//ClassLoader的loadClass方法，里面实现了双亲委派机制
protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // 检查当前类加载器是否已经加载了该类
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {  //如果当前加载器父加载器不为空则委托父加载器加载该类
                    c = parent.loadClass(name, false);
                } else {  //如果当前加载器父加载器为空则委托引导类加载器加载该类
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                //都会调用URLClassLoader的findClass方法在加载器的类路径里查找并加载该类
                c = findClass(name);

                // this is the defining class loader; record the stats
                sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                sun.misc.PerfCounter.getFindClasses().increment();
            }
        }
        if (resolve) {  //不会执行
            resolveClass(c);
        }
        return c;
    }

package java.lang;

public class String {
    public static void main(String[] args) {
        System.out.println("**************My String Class**************");
    }
}

运行结果：
错误: 在类 java.lang.String 中找不到 main 方法, 请将 main 方法定义为......

public class MyClassLoaderTest {
    static class MyClassLoader extends ClassLoader {
        private String classPath;

        public MyClassLoader(String classPath) {
            this.classPath = classPath;
        }

        private byte[] loadByte(String name) throws Exception {
            name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name
                    + ".class");
            int len = fis.available();
            byte[] data = new byte[len];
            fis.read(data);
            fis.close();
            return data;
        }

        protected Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                byte[] data = loadByte(name);
                //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节数组。
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }
        }

    }

    public static void main(String args[]) throws Exception {
        //初始化自定义类加载器，会先初始化父类ClassLoader，其中会把自定义类加载器的父加载器设置为应用程序类加载器AppClassLoader
        MyClassLoader classLoader = new MyClassLoader("D:/test");
        //D盘创建 test/com/tuling/jvm 几级目录，将User类的复制类User1.class丢入该目录
        Class clazz = classLoader.loadClass("com.tuling.jvm.User1");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("sout", null);
        method.invoke(obj, null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }
}

运行结果：
=======自己的加载器加载类调用方法=======

public class MyClassLoaderTest {
    static class MyClassLoader extends ClassLoader {
        private String classPath;

        public MyClassLoader(String classPath) {
            this.classPath = classPath;
        }

        private byte[] loadByte(String name) throws Exception {
            name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name
                    + ".class");
            int len = fis.available();
            byte[] data = new byte[len];
            fis.read(data);
            fis.close();
            return data;

        }

        protected Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                byte[] data = loadByte(name);
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }
        }

        /**
         * 重写类加载方法，实现自己的加载逻辑，不委派给双亲加载
         * @param name
         * @param resolve
         * @return
         * @throws ClassNotFoundException
         */
        protected Class<?> loadClass(String name, boolean resolve)
                throws ClassNotFoundException {
            synchronized (getClassLoadingLock(name)) {
                // First, check if the class has already been loaded
                Class<?> c = findLoadedClass(name);

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
                if (resolve) {
                    resolveClass(c);
                }
                return c;
            }
        }
    }

    public static void main(String args[]) throws Exception {
        MyClassLoader classLoader = new MyClassLoader("D:/test");
        //尝试用自己改写类加载机制去加载自己写的java.lang.String.class
        Class clazz = classLoader.loadClass("java.lang.String");
        Object obj = clazz.newInstance();
        Method method= clazz.getDeclaredMethod("sout", null);
        method.invoke(obj, null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }
}

运行结果：
java.lang.SecurityException: Prohibited package name: java.lang
 at java.lang.ClassLoader.preDefineClass(ClassLoader.java:659)

public static int value = 123;

public static final int value = 123;

public class Test { 
  static { 
  i = 0;
  // 给变量复制可以正常编译通过 
  System.out.print(i); 
  // 这句编译器会提示“非法向前引用” 
  }
  static int i = 1; 
  }

static class Parent { 
  public static int A = 1; 
    static { 
      A = 2;
    } 
  }
  static class Sub extends Parent { 
    public static int B = A; 
  }
  
  public static void main(String[] args) { 
    System.out.println(Sub.B); 
  }

static class DeadLoopClass {

        static {
            // 如果不加上这个if语句，编译器将提示“Initializer does not complete normally” 并拒绝编译 
            if (true) {
                System.out.println(Thread.currentThread() + "init DeadLoopClass");
                while (true) {
                }
            }
        }
    }
    public static void main(String[] args) {
        Runnable script = new Runnable() {
            public void run() {
                System.out.println(Thread.currentThread() + "start");
                DeadLoopClass dlc = new DeadLoopClass();
                System.out.println(Thread.currentThread() + " run over");
            }
        };
        Thread thread1 = new Thread(script);
        Thread thread2 = new Thread(script);
        thread1.start();
        thread2.start();
    }

Thread[Thread-0,5,main]start 
Thread[Thread-1,5,main]start 
Thread[Thread-0,5,main]init DeadLoopClass