public class SuperClazz {
    static  {
        System.out.println("SuperClass init！");
    }
    public static int value=123;
    public static final String HELLOWORLD="hello world";
    public static final int WHAT = value;
}

public class SubClaszz extends SuperClazz {
    static{
        System.out.println("SubClass init！");
    }

}

public class Initialization {
    public static void main(String[]args){
        Initialization initialization = new Initialization();
        initialization.M1();
    }

    public void M1(){
        System.out.println(SubClaszz.value);
    }
}

SuperClass init！
123

0: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
3: getstatic     #6                  // Field ex7/init/SubClaszz.value:I
6: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
9: return

public void M2(){
     SubClaszz[]sca = new SubClaszz[10];
}

public void M3(){
    System.out.println(SuperClazz.HELLOWORLD);
}

public void M4(){
      System.out.println(SubClaszz.WHAT);
}

public class ClassLoader {
    public static void main(String[] args) {
        System.out.println(String.class.getClassLoader()); //启动类加载器
        System.out.println(sun.net.spi.nameservice.dns.DNSNameService.class.getClassLoader());//拓展类加载器
        System.out.println(ClassLoader.class.getClassLoader());//应用程序类加载器
    }
}

null
sun.misc.Launcher$ExtClassLoader@4b67cf4d
sun.misc.Launcher$AppClassLoader@14dad5dc

 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) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        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();
                    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) {
                resolveClass(c);
            }
            return c;
        }
    }

    protected Class findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }

    private static void loadInitialDrivers() {
        String drivers;
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction() {
                public String run() {
                    return System.getProperty("jdbc.drivers");
                }
            });
        } catch (Exception ex) {
            drivers = null;
        }

        AccessController.doPrivileged(new PrivilegedAction() {
            public Void run() {

                ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator driversIterator = loadedDrivers.iterator();

                try{
                    while(driversIterator.hasNext()) {
                        driversIterator.next();
                    }
                } catch(Throwable t) {
                // Do nothing
                }
                return null;
            }
        });

        println("DriverManager.initialize: jdbc.drivers = " + drivers);

        if (drivers == null || drivers.equals("")) {
            return;
        }
        String[] driversList = drivers.split(":");
        println("number of Drivers:" + driversList.length);
        for (String aDriver : driversList) {
            try {
                println("DriverManager.Initialize: loading " + aDriver);
                Class.forName(aDriver, true,
                        ClassLoader.getSystemClassLoader());
            } catch (Exception ex) {
                println("DriverManager.Initialize: load failed: " + ex);
            }
        }
    }

public static  ServiceLoader load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

public Launcher() {
        Launcher.ExtClassLoader var1;
        try {
            var1 = Launcher.ExtClassLoader.getExtClassLoader();
        } catch (IOException var10) {
            throw new InternalError("Could not create extension class loader", var10);
        }

        try {
            this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
        } catch (IOException var9) {
            throw new InternalError("Could not create application class loader", var9);
        }

        Thread.currentThread().setContextClassLoader(this.loader);
        String var2 = System.getProperty("java.security.manager");
        if (var2 != null) {
            SecurityManager var3 = null;
            if (!"".equals(var2) && !"default".equals(var2)) {
                try {
                    var3 = (SecurityManager)this.loader.loadClass(var2).newInstance();
                } catch (IllegalAccessException var5) {
                } catch (InstantiationException var6) {
                } catch (ClassNotFoundException var7) {
                } catch (ClassCastException var8) {
                }
            } else {
                var3 = new SecurityManager();
            }

            if (var3 == null) {
                throw new InternalError("Could not create SecurityManager: " + var2);
            }

            System.setSecurityManager(var3);
        }

    }

source: https://www.cnblogs.com/yewy/p/13414144.html