John Gill Commentary 1 Samuel 23:17

"And he said unto him, Fear not; for the hand of Saul my father shall not find thee; and thou shalt be king over Israel, and I shall be next unto thee; and that also Saul my father knoweth." — 1 Samuel 23:17 (ASV)

And he said unto him, fear not
Distrust not the power, providence, and promises of God, nor dread the wrath of Saul, or fear falling into his hands:

for the hand of Saul my father shall not find thee ;
to lay hold on him, seize him, and do him any hurt:

and thou shalt be king over Israel ;
meaning after the death of his father; which he knew either by some special revelation made to him; or rather by being informed he had been anointed by Samuel, and which he had either from Samuel or from David himself; and this he most firmly believed, though David was now in so low a condition:

and I shall be next unto thee ;
not succeed him in the kingdom; but if living when he came to the throne, he should be the second man in civil affairs, as he now was, and that he should be content with:

and that also Saul my father knoweth ;
having knowledge of the anointing of David by Samuel; or he might, and did conclude this from various circumstances, that David was his neighbour Samuel had told him of, God had given his kingdom to, (1 Samuel 15:28) .