Happiness is a state of mind, while joy is a mind-set.
Happiness comes and goes, while joy can be constant.
Happiness is dependent, while joy is independent.
Happiness is conditional, while joy is unconditional.
The apostle Paul had learned the secret to the joy found in Jesus:
I am not saying this because I am in need, for I have learned to be content whatever the circumstances. I know what it is to be in need, and I know what it is to have plenty. I have learned the secret of being content in any and every situation, whether well fed or hungry, whether living in plenty or in want. I can do all this through Him who gives me strength. — Philippians 4:11-13