John Calvin Commentary Psalms 35:9

"And my soul shall be joyful in Jehovah: It shall rejoice in his salvation." — Psalms 35:9 (ASV)

And my soul is joyful in Jehovah. Others read this in the optative mood: May my soul rejoice in Jehovah, and may it be glad in his salvation. But instead of continuing to express his desires, David, in my opinion, rather promises in this verse that he will be grateful to God.

This is still more evident from the following verse, in which, extolling very highly God's goodness, he says that he will celebrate its remembrance with every member of his body.

Therefore, while some ascribe to fortune, and others to their own skill, the praise for their deliverance from danger, and few, if any, yield all the praise to God, David here declares that he will not forget the favor God had bestowed on him. My soul, he says, shall rejoice, not in a deliverance whose source it does not know, but in God's salvation.

To put the matter even more strongly, he assigns to his very bones the office of declaring the divine glory. As if not content for his tongue to be employed in this, he applies all the members of his body to the work of setting forth God's praises.

The style of speaking he employs is hyperbolical, but in this way he shows unfeignedly that his love for God was so strong that he desired to use his very sinews and bones in declaring the reality and truth of his devotion.