When writing BL code onPreFetch(), I found request.username works fine, but request.requestContext is null? 

This contradicts the reference: 

http://devcenter.kinvey.com/rest/reference/business-logic/reference.html#request

by the way, is there any way to get the userId instead of username ? 

Yonghui,

RequestContext is invoked differently.  The documentation for it is over here:  http://devcenter.kinvey.com/rest/reference/business-logic/reference.html#requestcontext-module

Thanks,