Justice exists in the human realm. When plant succession leads to one species-type replacing another no plant cries "foul." When a snake comes upon a mouse hole and enters and eats its inhabitants there is no picketing for justice by other mice! However a sentimental human may see these occurrences and try to superimpose a belief and come up with an interpretation that an injustice has occurred. Without the existence of humans it can be said that there is no concept of justice.
Only if humans were not really human - all exactly equal - would justice be a non-concept. And so in reality justice is an omnipresent concept that is interwoven throughout all of human action. Of course human beliefs again come into play and so interpretations of what is just and what is unjust are innumerable and varied. One of the consequences of living in the Dark Ages of economics has to do with a very poor understanding of human action and this is significantly aggravated by not understanding that the appropriate scientific methodology for the study of human sciences is subjectivism. With these biases in place it is safe to say that justice is a slippery concept.
However justice can be known by its fruits. The drive in socialism and communism to create equality is ironically the cause of great injustices because it runs contrary to natural law. First of all it is unjust to forcibly apply an unrealistic nature on humans. The fruits of this kind of 'justice' are rotten. The political class that tries to impose equality on the masses enjoy the great inequality of being the ordained ego-driven interventionist and ego-driven interpreter, with all of its perks. This rotten fruit of injustice cannot be considered justice. The squelching of the creative expressions of naturally subjective human beings requires force and so many of the 'equals' will have to be either killed or incarcerated. This is another rotten fruit of this kind of injustice. Those who meekly abide by the will of the overlords have no chance of developing any ethics as they are simply told what to do and how to behave and what to believe. This supposedly creates a condition of 'equality' by forcing humans to be slave-like but anyone with any brains or any morals know instinctively and intuitively that this is a rotten fruit.
Justice is that which enhances voluntary social cooperation. By this definition humans are given the freedom to express themselves and to contribute to an ever-advancing civilization.