在java中，所有的类都是通过类加载器所加载的，除根类加载器（bootstrap）外，扩展类加载器和应用类加载器都是ClassLoader的子类。下面是一个自定义ClassLoader类，从网络中加载Class文件的一个简单的例子

import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.net.URL;

public class MyClassLoader extends ClassLoader {

    private String url;

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

    public MyClassLoader(ClassLoader parent, String url) {
        super(parent);
        this.url = url;
    }

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

    private byte[] loadClassData(String name) throws ClassNotFoundException {
        String uri = url + name;
        return loadDataFromUrl(uri);
    }

    private byte[] loadDataFromUrl(String uri) throws ClassNotFoundException {
        try {
            URL url = new URL(uri);
            InputStream inputStream = url.openStream();
            byte[] b = new byte[1024];
            int len;
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            while ((len = inputStream.read(b)) != -1) {
                bos.write(b, 0, len);
            }
            byte[] classData = bos.toByteArray();
            return classData;
        } catch (Exception ex) {
            throw new ClassNotFoundException();
        }
    }
}

import java.lang.reflect.Field;

public class Main {
    public static void main(String[] args) {
        String url = "https://raw.githubusercontent.com/zjbshk/zjbshk.github.io/master/";
        MyClassLoader myClassLoader = new MyClassLoader(url);
        try {
            Class<?> aClass = myClassLoader.loadClass("Student");
            Object o = aClass.newInstance();
            Field[] declaredFields = aClass.getDeclaredFields();
            for (Field declaredField : declaredFields) {
                declaredField.setAccessible(true);
                Object o1 = declaredField.get(o);
                System.out.println(o1);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        }
    }
}

public class Student {
    private int age = 21;
    private String name = "zjb";
    private boolean sex = true;

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isSex() {
        return sex;
    }

    public void setSex(boolean sex) {
        this.sex = sex;
    }
}