The only director I can think of who is as secretive as J.J. Abrams is Christopher Nolan, and even then I don't think Nolan goes to the extremes that Abrams is rumored to go to. Information about the new Star Wars trilogy leaked before Abrams involvement, but since then it seems like things have been tightened up.
According to the Hollywood Reporter:
It took nine months of meet-and-greets, script readings and screen tests, all conducted with more secrecy than a raid on the Death Star, for director J.J. Abrams, Lucasfilm and Disney to assemble the cast of Star Wars: Episode VII: John Boyega, 22; Adam Driver, 30; Domhnall Gleeson, 30; Oscar Isaac, 35; Daisy Ridley, 22; Andy Serkis, 50; and Max von Sydow, 85, who were unveiled April 29, will join original stars Mark Hamill, 62, Carrie Fisher, 57, and Harrison Ford, 71. Abrams cast a wide net, with actors asked to read material that was not from the top secret script. They couldn't tell anyone for which parts they were reading, and their agents and publicists had to sign strict nondisclosure agreements.
Despite such efforts, sites like Latino-Review, HitFix, and Variety were able get around these security measures to break news on some of the above listed actors and production details. A few things we posted as rumors that are now confirmed Adam Driver, Millennium Falcon, and PineWood Studios.
But casting rumors for Andy Serkis, Oscar Isaac, and Daisy Ridley didn't beak until right before the official announcement. So at the moment, I think Abrams and crew are holding up better than expected given how easy it is for information to leak.
Star Wars: Episode VII's release date of December 18th, 2015 is a long way away, and this battle for secret information is just warming up.