搞个双亲委派然后再破坏，图啥？

public class ClassLoaderDemo1 {

    public static void main(String[] args){
        // null
        System.out.println(String.class.getClassLoader());
        ClassLoader loader = ClassLoaderDemo1.class.getClassLoader();
        while (loader != null) {
            // sun.misc.Launcher$AppClassLoader@58644d46
            // sun.misc.Launcher$ExtClassLoader@7ea987ac
            System.out.println(loader);
            loader = loader.getParent();
        }
    }
}

Class.forName("com.mysql.jdbc.Driver");
Connection conn = DriverManager.getConnection("jdbc:mysql://myhost/test?useUnicode=true&characterEncoding=utf-8&useSSL=false", "test", "test");

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    public Driver() throws SQLException {
    }

    static {
        try {
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
            throw new RuntimeException("Can't register driver!");
        }
    }
}

Connection conn = DriverManager.getConnection("jdbc:mysql://myhost/test?useUnicode=true&characterEncoding=utf-8&useSSL=false", "test", "test");

public class DriverManager {

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

    private static void loadInitialDrivers(){

        // 省略部分代码
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run(){

                // 根据配置文件加载驱动实现类
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();

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

        // 省略部分代码
    }

}

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

public class Launcher {

    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);
        }

        // 设置线程上下文类加载器为AppClassLoader
        Thread.currentThread().setContextClassLoader(this.loader);

        // 省略部分代码

    }
}    

// 从这方法开始加载
public Class<?> loadClass(String name) throws ClassNotFoundException {
 return loadClass(name, false);
}

protected Class<?> loadClass(String name, boolean resolve)
 throws ClassNotFoundException
{
 synchronized (getClassLoadingLock(name)) {
  // First, check if the class has already been loaded
  // 先从缓存查找该class对象，找到就不用重新加载
  Class<?> c = findLoadedClass(name);
  if (c == null) {
   long t0 = System.nanoTime();
   try {
    if (parent != null) {
                    // 如果加载不到，委托父类去加载 
                    // 这里体现了自底向上检查类是否已经加载     
     c = parent.loadClass(name, false);
    } else {
        // 如果没有父类，委托启动加载器去加载
     c = findBootstrapClassOrNull(name);
    }
   } catch (ClassNotFoundException e) {
    // ClassNotFoundException thrown if class not found
    // 这里体现了自顶向下尝试加载类，当父类加载加载不到时
    // 会抛出ClassNotFoundException
    // 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();
    // 如果都没有找到，通过自己的实现的findClass去加载
    // findClass方法没有找到会抛出ClassNotFoundException
    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();
   }
  }
  // 是否需要在加载时进行解析
  if (resolve) {
   resolveClass(c);
  }
  return c;
 }
}

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

public class DemoObj {

    public String toString(){
        return "I am DemoObj";
    }

}

public class FileClassLoader extends ClassLoader {

    // class文件的目录
    private String rootDir;

    public FileClassLoader(String rootDir){
        this.rootDir = rootDir;
    }

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

    private byte[] getClassData(String className) {

        String path = rootDir + File.separatorChar + className.replace('.', File.separatorChar) + ".class";
        try {
            InputStream ins = new FileInputStream(path);
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            int bufferSize = 4096;
            byte[] buffer = new byte[bufferSize];
            int bytesNumRead = 0;
            while ((bytesNumRead = ins.read(buffer)) != -1) {
                baos.write(buffer, 0, bytesNumRead);
            }
            return baos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    public static void main(String[] args){

        String rootDir = "/Users/peng/study-code/java-learning/src/main/java";
        FileClassLoader loader = new FileClassLoader(rootDir);

        try {
            // 传入class文件的全限定名
            Class<?> clazz = loader.loadClass("com.javashitang.classloader.DemoObj");
            // com.javashitang.classloader.FileClassLoader@1b28cdfa
            System.out.println(clazz.getClassLoader());
            // I am DemoObj
            System.out.println(clazz.newInstance().toString());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

String rootDir = "/Users/peng/study-code/java-learning/src/main/java";
FileClassLoader loader1 = new FileClassLoader(rootDir);
FileClassLoader loader2 = new FileClassLoader(rootDir);

Class class1 = loader1.findClass("com.javashitang.classloader.DemoObj");
Class class2 = loader2.findClass("com.javashitang.classloader.DemoObj");

// false
System.out.println(class1 == class2);