John Chrysostom — as excerpted in the Catena Aurea on Matthew 1:19

“But it should be known, that just here is used to denote one who is in all things virtuous. For there is a particular justice, namely, the being free from covetousness; and another universal virtue, in which sense Scripture generally uses the word justice. Therefore being just, that is kind, merciful, he was minded to put away privily her who according to the Law was liable not only to dismissal, but to death. But Joseph remitted both, as though living above the Law. For as the sun lightens up the world, before he shews his rays, so Christ before He was born caused many wonders to be seen.”