And my soul shall be joyful in the LORD: it shall rejoice in his salvation.
My soul shall be joyful. In anticipation of the gracious answer to his prayer, and in order to give additional weight to his entreaty, he vows a soul-felt thanksgiving to the Lord.
It shall rejoice in his salvation - whenever Yahweh shall have said to my soul, in accordance with my prayer (Psalms 35:3), "I am thy salvation."