Recently I read android source codes,I find that a pair of methods are always called when doing some binder IPC call,I read the comment but I can't clearly know the root reason.The pair of method is as follow:
final long origId = Binder.clearCallingIdentity();
//other local method.
I don't think I can answer better than the description in the official APIs: http://developer.android.com/reference/android/os/Binder.html
public static final long clearCallingIdentity ()
Reset the identity of the incoming IPC on the current thread. This can be useful if, while handling an incoming call, you will be calling on interfaces of other objects that may be local to your process and need to do permission checks on the calls coming into them (so they will check the permission of your own local process, and not whatever process originally called you).