How to define and use a system property in Android Instrumentation test?

Question

I am trying to use some arguments for an Instrumentation test. I noticed that I can read system properties with System.getProperty() function. So I use setprop command to set a system property. For example: adb shell setprop AP 123. Inside my Test code I try to read this AP property with :

tmp = System.getProperty("AP"); 
Log.d("MyTest","AP Value = " + tmp);

Then I use logcat to view this debug message but I get a null value for this property. Any ideas on what could be wrong? Note that I can still read the system property with adb shell getprop AP command.