protected static FastMethod make(final Method method) {
        Class<?> klass = method.getDeclaringClass();
        String descriptor = Type.getMethodDescriptor(method);
        String key = "$FM$" + method.getName() + "$" + Lang.md5(descriptor);
        String className = klass.getName() + key;
        if (klass.getName().startsWith("java"))
            className = FastMethod.class.getPackage().getName() + ".fast." + className;
        FastMethod fm = cache.get(className);
        if (fm != null)
            return fm;
        try {
            fm = (FastMethod) klass.getClassLoader().loadClass(className).newInstance();
            cache.put(className, fm);
            return fm;
        }
        catch (Throwable e) {}
        try {
            byte[] buf = _make(klass,
                               method.getModifiers(),
                               method.getParameterTypes(),
                               _Method(method),
                               method.getReturnType(),
                               className,
                               descriptor);
            Class<?> t = DefaultClassDefiner.defaultOne().define(className,
                                                                 buf,
                                                                 klass.getClassLoader());
            fm = (FastMethod) t.newInstance();
        }

进入到这个方法里了，fm为null了。这个是什么原因呢？