Any use cases which require an HttpContext attribute to be set aren't covered in this implementation, and the extensions required make the whole thing so messy as to make using the Rest Template more of a burden than a benefit. Here is the particular case that I've been fussing with today to little avail, setting preemptive authentication (i.e. send basic authentication headers by default rather than performing 2 requests to authenticate):
The problem being that the RestTemplate implementation as it is has no support for setting HttpContext parameters (the template calls the Http.execute(HttpUriRequest) which uses only the non-accessible default HttpContext), there aren't any good ways to set the default context parameters on the HttpClient in 4.x Apache HTTP Components. You end up needing to extend HttpClient to provide for default HttpContext params on the HttpClient object and the whole thing just becomes a spaghetti mess from there making the template more pain than pleasure.