类加载器

类加载器

1. 启动加载器（BootstrapClassLoader）
2. 扩展类加载器（ExtClassLoader）
3. 应用类加载器（AppClassLoader）

特点

1. 双亲委托
2. 负责依赖
3. 缓存加载

自定义类加载器（代码）

package demo;

import java.util.Base64;

public class MarbleClassLoader extends ClassLoader {
    public static void main(String[] args) {
        try {
            new MarbleClassLoader().findClass("jvm.Hello").newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        String helloBase64 = "yv66vgAAADQAHAoABgAOCQAPABAIABEKABIAEwcAFAcAFQEABjxpbml0PgEAAygpVgEABENvZGUB" +
                "AA9MaW5lTnVtYmVyVGFibGUBAAg8Y2xpbml0PgEAClNvdXJjZUZpbGUBAApIZWxsby5qYXZhDAAH" +
                "AAgHABYMABcAGAEALkhlbGxvIENsYXNzIEluaXRpYWxpemVkIHdpdGggTWFyYmxlQ2xhc3Nsb2Fk" +
                "ZXIHABkMABoAGwEACWp2bS9IZWxsbwEAEGphdmEvbGFuZy9PYmplY3QBABBqYXZhL2xhbmcvU3lz" +
                "dGVtAQADb3V0AQAVTGphdmEvaW8vUHJpbnRTdHJlYW07AQATamF2YS9pby9QcmludFN0cmVhbQEA" +
                "B3ByaW50bG4BABUoTGphdmEvbGFuZy9TdHJpbmc7KVYAIQAFAAYAAAAAAAIAAQAHAAgAAQAJAAAA" +
                "HQABAAEAAAAFKrcAAbEAAAABAAoAAAAGAAEAAAADAAgACwAIAAEACQAAACUAAgAAAAAACbIAAhID" +
                "tgAEsQAAAAEACgAAAAoAAgAAAAUACAAGAAEADAAAAAIADQ==";
        byte[] bytes = decode(helloBase64);
        return defineClass(name, bytes, 0, bytes.length);
    }

    public byte[] decode(String base64) {
        return Base64.getDecoder().decode(base64);
    }
}