Adam Clarke on Judges 9:8

The trees went forth on a time - This is the oldest, and without exception the best fable or apologue in the world. See the observations at the end of this chapter, Jdg 9:56 (note). It is not to be supposed that a fable, if well formed, requires much illustration; every part of this, a few expressions excepted, illustrates itself, and tells its own meaning.

To anoint a king - Hence it appears that anointing was usual in the installation of kings, long before there was any king in Israel; for there is much evidence that the book of Judges was written before the days of Saul and David.

The olive tree - The olive was the most useful of all the trees in the field or forest, as the bramble was the meanest and the most worthless.