Now the first lesson social justice teaches us -- which governments nowadays have certainly not learnt nor seem to want to learn -- is that civil government with its acts and ordinances must never transgress the natural bounds of its authority, which cannot be defined without prior definition of the type of institution proper to civil government. Unless and until the sovereign rule of justice is accepted, there are no limits a government will not transgress. Utility alone, such a vague and empty word, cannot prescribe any definite limits to it because it depends on the probable evaluation of circumstances. Utility which is of its nature variable, depends on the judgment of the person who carries out the evaluation.
[Antonio Rosmini, Introduction to Philosophy, Volume 1: About the Author's Studies, Murphy, tr. Rosmini House (Durham: 2004) p. 27.] Rosmini actually talks quite a bit about social justice, la giustizia sociale -- he is arguably the person most responsible for the existence of the phrase, for although he did not invent it, he regularized it, gave it philosophical background, and popularized it. I haven't read The Constitution Under Social Justice, though, nor the full Philosophy of Right, so I can't say all that much on his particular use of it, but I do know that the two main features of his own account of how to establish and maintain social justice are strong property rights (which he argues is one of the things necessary to protect people's lives -- private property is a sphere of security from intrusion and coercion) and sharp limitation of coercive powers (including those of the government) in light of the dignity of human reason.