public static void main(String[] args) {
        // App ClassLoader
        System.out.println(new User().getClass().getClassLoader());
        // Ext ClassLoader
        System.out.println(new User().getClass().getClassLoader().getParent());
        // Bootstrap ClassLoader
        System.out.println(new User().getClass().getClassLoader().getParent().getParent());
        // Bootstrap ClassLoader
        System.out.println(new String().getClass().getClassLoader());
    }

sun.misc.Launcher$AppClassLoader@18b4aac2
sun.misc.Launcher$ExtClassLoader@5fdef03a
null
null

 protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            // 檢查類是否已經(jīng)加載
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                	// 父加載器不為空，調(diào)用父加載器loadClass()方法處理
                    if (parent != null) {
                    	// 讓上一層加載器進(jìn)行加載
                        c = parent.loadClass(name, false);
                    } else {
                    	// 父加載器為空，使用啟動類加載器 BootstrapClassLoader 加載
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
               		 // 拋出異常說明父類加載器無法完成加載請求
                    // 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();
                    // 調(diào)用此類加載器所實(shí)現(xiàn)的findClass方法進(jìn)行加載
                    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方法是當(dāng)字節(jié)碼加載到內(nèi)存后進(jìn)行鏈接操作，對文件格式和字節(jié)碼驗(yàn)證，并為 static 字段分配空間并初始化，符號引用轉(zhuǎn)為直接引用，訪問控制，方法覆蓋等
                resolveClass(c);
            }
            return c;
        }
    }

import cn.ybzy.demo.Test;

import java.io.*;

public class MyClassLoader extends ClassLoader {

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData;
        try {
            classData = loadClassData(name);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        if (classData == null) {
            throw new ClassNotFoundException();
        } else {
            return defineClass(name, classData, 0, classData.length);
        }
    }

    private byte[] loadClassData(String className) throws IOException {
        String replace = className.replace('.', File.separatorChar);
        String path = ClassLoader.getSystemResource("").getPath() + replace + ".class";
        InputStream inputStream = null;
        ByteArrayOutputStream byteArrayOutputStream = null;
        try {
            inputStream = new FileInputStream(path);
            byteArrayOutputStream = new ByteArrayOutputStream();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int length = 0;
            while ((length = inputStream.read(buffer)) != -1) {
                byteArrayOutputStream.write(buffer, 0, length);
            }
            return byteArrayOutputStream.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (byteArrayOutputStream != null) {
                byteArrayOutputStream.close();
            }
            if (inputStream != null) {
                inputStream.close();
            }
        }

        return null;
    }

    @Override
    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 {
                    // 修改classloader的原雙親委派邏輯，從而打破雙親委派
                    if (name.startsWith("cn.ybzy.demo")) {
                        c = findClass(name);
                    } else {
                        c = this.getParent().loadClass(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;
        }
    }
}

    public static void main(String[] args) throws ClassNotFoundException {
        MyClassLoader classLoader = new MyClassLoader();
        Class<?> aClass = classLoader.loadClass(Test.class.getName());
        System.out.println(aClass.getClassLoader());
    }

cn.ybzy.demo.MyClassLoader@2f410acf

package cn.ybzy.demo;

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

import java.io.*;

public class MyClassLoader extends ClassLoader {
    private String root;

    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData;
        try {
            classData = loadClassData(name);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        if (classData == null) {
            throw new ClassNotFoundException();
        } else {
            return defineClass(name, classData, 0, classData.length);
        }
    }

    private byte[] loadClassData(String className) throws IOException {
        String fileName = root + File.separatorChar + className.replace('.', File.separatorChar) + ".class";
        InputStream inputStream = null;
        ByteArrayOutputStream byteArrayOutputStream = null;
        try {
            inputStream = new FileInputStream(fileName);
            byteArrayOutputStream = new ByteArrayOutputStream();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int length = 0;
            while ((length = inputStream.read(buffer)) != -1) {
                byteArrayOutputStream.write(buffer, 0, length);
            }
            return byteArrayOutputStream.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (byteArrayOutputStream != null) {
                byteArrayOutputStream.close();
            }
            if (inputStream != null) {
                inputStream.close();
            }
        }

        return null;
    }

    public String getRoot() {
        return root;
    }

    public void setRoot(String root) {
        this.root = root;
    }
}

    public static void main(String[] args) {
        MyClassLoader classLoader = new MyClassLoader();
        classLoader.setRoot("D:\\Temp");
        Class<?> testClass = null;
        try {
            testClass = classLoader.loadClass("cn.ybzy.demo.Test");
            Object object = testClass.newInstance();
            System.out.println(object.getClass().getClassLoader());
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

cn.ybzy.demo.MyClassLoader@5679c6c6

sun.misc.Launcher$AppClassLoader@18b4aac2