John Gill, on Lam 3:1

I am the man that hath seen affliction,.... Had a much experience of it, especially ever since he had been a prophet; being reproached and ill used by his own people, and suffering with them in their calamities; particularly, as Jarchi observes, his affliction was greater than the other prophets, who indeed prophesied of the destruction of the city and temple, but did not see it; whereas he lived to see it: he was not indeed the only man that endured affliction, but he was remarkable for his afflictions; he had a large share of them, and was herein a type of Christ, who was a man of sorrows, and acquainted with griefs: by the rod of his wrath; that is, by the rod of the wrath of God, for he is understood; it is a relative without an antecedent, as in Sol 1:1; unless the words are to be considered in connection Lam 2:22. The Targum is, "by the rod of him that chastiseth in his anger;'' so Jarchi; but God's chastisements of his own people are in love, though thought sometimes by them to be in wrath and hot displeasure; so the prophet imagined, but it was not so; perhaps some regard may be had to the instrument of Jerusalem's destruction, the king of Babylon, called the rod of the Lord's anger, Isa 10:5; all this was true of Christ, as the surety of his people, and as sustaining their persons, and standing in their room.