While I'm thinking of it, let me strike a blow for Virtue-Based Morality by taking up a point made by Herbert McCabe in his posthumously published The Good Life (and touched on in a comment below by JohnMcG).
To repeat myself, what makes an act of giving to neighbor his due virtuous is that the act is chosen as an act of giving to neighbor his due. The wicked judge gave the widow her due so she would stop bugging him. He chose his act, not as a just act, but as an act that would shut her up. Even though he did "the right," he did not act with justice.
In addition to virtue and self-interest, there is a third way in which one can choose "the right," which is to choose the act as in accordance with rules. A different judge, for example, might give the widow what she asked for, not because it is the right thing to do, nor because it will get her out of his chambers, but because that's what the law tells him to do. Should the law change, his decision would change.
In effect, such a rule-based justice doesn't recognize natural rights; the positive law defines the set of positive rights, and those are the only rights that matter. From this perspective, "unjust law" is a meaningless expression, and anything that is not contrary to an explicit law cannot be unjust.
Of course it's true that a virtue-based concept of justice follows the positive laws -- at least those that are not "a perversion of law," i.e., those that do not violate natural law -- but it does so because following a just positive law is just. That may sound like a meaningless distinction, but it preserves both concepts of "unjust law" and "unjust but legal."