Currently Throwable.backtrace is treated by Class.getDeclaredField like an injected field; it is not reported back to the caller. The reason it is hidden is because a crash that was reported (JDK-4496456) when a user tries to get the contents of the backtrace Object array.

Without knowing the history here JDK-4496456 was very likely because of the specific representation behind Throwable.backtrace back then. This changed. The Object[] looks now like:

Object[] backtrace = { short[] method_ids, int[] bcis, Object[] mirrors };

and it's perfectly sane to do:

        // Retrieve the class of throwable.backtrace[0][0].
        Class class2 = ((Object[]) ((Object[]) backtrace)[0])[0].getClass();

(although it will bail out with a ClassCastException).

To make sure this doesn't break again we could take the test case of JDK-4496456 and make a regression test.