Java类加载机制

BootstrapClassLoader、AppClassLoader和ExtClassLoader，自定义类加载器

BootstrapClassLoader没有继承classLoader,由JVM自定义实现

自定义类加载器，须实现classLoader的findclass方法

/**
 * Load class from network
 */
public class NetworkClassLoader extends ClassLoader {
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData = downloadClassData(name); // 从远程下载
        if (classData == null) {
            super.findClass(name); // 未找到，抛异常
        } else {
            return defineClass(name, classData, 0, classData.length); // convert class byte data to Class<?> object
        }
        return null;
    }
    private byte[] downloadClassData(String name) {
        // 从 localhost 下载 .class 文件
        String path = "http://localhost" + File.separatorChar + "java" + File.separatorChar + name.replace('.', File.separatorChar) + ".class"; 
        try {
            URL url = new URL(path);
            InputStream ins = url.openStream();
            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); // 把下载的二进制数据存入 ByteArrayOutputStream
            }
            return baos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
    public String getName() {
        System.out.printf("Real NetworkClassLoader\n");
        return "networkClassLoader";
    }
}