Why can't it find the class?

Question

Java

In general, I want to generate a derived class using javassist

ClassPool pool = ClassPool.getDefault();
        pool.appendClassPath(new LoaderClassPath(new UrlClassLoader( .... )));

        CtClass cc = pool.makeClass("com.example.AppInfoServiceImpl", pool.getCtClass("com.example.AppInfoProto$AppInfoService"));
        //..... generate methods
        cc.toClass();

as a result on cc.toClass(); throws java.lang.NoClassDefFoundError. wherein

new UrlClassLoader( .... ).loadClass("com.example.AppInfoProto$AppInfoService")

works fine