根据stackoverflow上面的大婶的回答：http://stackoverflow.com/questions/6475727/why-does-static-nsstring-leak
This message shows up when you autorelease an object on a thread that does not have any release pools in its stack. By default, there is always an autorelease pool on the main thread. It's created and managed within the UIApplicationMain() function
that is usually called by your app's main() function. However, additional threads you create (with performSelectorInBackground: orNSThread)
do not have an autorelease pool in place unless you specifically put one there, so any autoreleased objects on that background thread have no pool to release them later, and will just leak.
If you're kicking something off to a background thread, the first thing you should do is create an autorelease pool. Under ARC, use the new @autoreleasepool construct
to do that.
当你要在没有autoreleasepool的线程里面autorelease一些对象的时候，这个消息就会爆出。默认情况下，主线程会有一个autoreleasepool。这个autoreleasepool由uiapplicationmain（）函数创建并且管理的，就是我们经常说的app的主函数。然而，一些你另外创建的线程（通过performSelectorInBackground:或者NSThread）并没有autoreleasepool，除非你专门搞一个。所以后台线程中所有自动释放的对象并没有autoreleasepool来释放他们，然后就尼玛的泄露了。