How to run Android system app without root permission?

Question

Is there any way to run Android system app without root permission? I can execute system app via adb such as:

adb shell /system/bin/screencap -p /sdca

Answer 1

Is there any way to run android system app without root permission?

It have to be NO, but some times, some functions which are not for public use still can be used. I've seen examples using java reflection.

I can execute system app via adb such as: ...

In my own application, I wanna run a shell command like that without "su" command. Is there any way? How does android prevent user apps to execute system app?

I think, no.

The thing is adb shell and user app have different security levels based on User and Group IDs (UID and GID).

More info: http://android-dls.com/wiki/index.php?title=Android_UIDs_and_GIDs

Besides, there are limitations via app permissions and hiden & internal classes, procedures, etc which made for internal use.

P.S.: About screenshots. On android market (google play) there are few apps which provide screenshots without ROOT access. So, it's possible. Although, since Android 4.0 screenshots are available "from box".