see more from https://github.com/openzipkin/brave/issues/867, also see https://github.com/apache/incubator-dubbo/issues/3576
removeAttachment(RETURN_KEY);
        }
    }
    /**
     * @return
     * @throws IllegalStateException
     */
    @SuppressWarnings("unchecked")
    public static AsyncContext startAsync() throws IllegalStateException {
        RpcContext currentContext = getContext();
        if (currentContext.asyncContext == null) {
            currentContext.asyncContext = new AsyncContextImpl();
        }
        currentContext.asyncContext.start();
        return currentContext.asyncContext;
    }
    public boolean isAsyncStarted() {
        if (this.asyncContext == null) {
            return false;
        }
        return asyncContext.isAsyncStarted();
    }
    public boolean stopAsync() {
        return asyncContext.stop();
    }
    public AsyncContext getAsyncContext() {
        return asyncContext;
    }
}