自定义类加载器

1.继承ClassLoader父类自定义类加载器
2.自定义类加载器，默认partner为AppClassLoader
初始化自定义类加载器时，调用父类ClassLoader的构造方法

    protected ClassLoader() {
        this(checkCreateClassLoader(), getSystemClassLoader());
    }
    public static ClassLoader getSystemClassLoader() {
        initSystemClassLoader();
        if (scl == null) {
            return null;
        }
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkClassLoaderPermission(scl, Reflection.getCallerClass());
        }
        return scl;
    }
    private static synchronized void initSystemClassLoader() {
        if (!sclSet) {
            if (scl != null)
                throw new IllegalStateException("recursive invocation");
            sun.misc.Launcher l = sun.misc.Launcher.getLauncher();//获取jvm启动器实例
            if (l != null) {
                Throwable oops = null;
                scl = l.getClassLoader();//获取AppClassLoader
                //删除其他逻辑代码
            }
        }
    }

    public class MyClassLoader extends ClassLoader {
        private String classPath;
        public MyClassLoader(String classPath) {
            this.classPath = classPath;
        }
        private byte[] loadByte(String name) throws Exception {
            name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name + ".class");
            int available = fis.available();
            byte[] data = new byte[available];
            fis.read(data);
            fis.close();
            return data;
        }
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                byte[] data = loadByte(name);
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                throw new ClassNotFoundException();
            }
        }
    }