In the time of Harper Lee's novel To Kill a Mockingbird, justice for a black person in the deep south was a rare thing. Tom Robinson was obviously an innocent man, but he really had no chance of being found not guilty of the accusation against him by a white man and by an all-white jury. Instead, Harper Lee's version of justice in the novel comes in two ways.
First, Atticus Finch is made a heroic figure seeking justice for Tom Robinson. Justice in this instance is that Tom Robinson had a good, albeit white, man on his side who was willing to stand up to most of the town in court and planned on carrying the fight for justice to a higher court on appeal.
Second, the sword of justice falls on Bob Ewell when Boo Radley kills him defending Scout and Jem from Ewell's attack on the children at the end of the story. Although Boo did not kill Ewell directly because of the verdict against Tom Robinson, Ewell's attack on Scout and Jem was a direct result of Atticus's defense of Tom Robinson.