Joy is listed as a spiritual fruit, Gal. 5. Joy results from believing truth, the wisdom of God. Joy is Godly, of spirit, deep. Happiness is a flighty feeling, we have when we believe in the treasures of this life. Like a child believing the lie of Santa clause. Happiness to some degree is the 5 senses counterfeit for the Joy. Believing the Wisdom of this world can result in temporary happiness, but exercising faith, believing the Wisdom of God results in Joy.
I like my wife’s understanding for happiness. It is the feeling you get when you like the outcome of when something happens. For me Joy is the amalgam of peace, contentment, and happiness. For joy and happiness to be separate, a person would have to be able to feel all three at the same time. If a person is not able to, then joy and happiness become synonymous until they can know the difference.
I felt these when I fully comprehended my salvation. I have the peace that the punishment of all my sins and past guilt have been thrown into deepest sea and will be remembered no more. I have the contentment that God knows my future and will guide me so that all things will work for what is best for me. Also God is sovereign and He keeps His promises. And I have happiness from the moments that God meets my needs and uses me to meet other’s needs.
Even though I may not be happy about what is currently happening, I can have joy because I still remember past happy moments and God’s faithfulness.
I think of Joyful generally in regards to holidays, or that sound you make in church if you don't know how to sing.
I see happy as more of a state of mind, and when someone says they are happy, it is an attitude that stays with you for a period of time.
Ie. Joyful could be a superficial happy, and Happy a geniune happy, from the bottom of my heart.
Joyful - Euphoric. Usually transitory, often triggered by some event. Perhaps best thought of as a spontaneous, outwardly visible expression of emotion. Synonymous with 'happy' to be sure. The opposite of 'joy' is probably 'sad'.
Happy - A state of being or set of circumstances. 'Happy' connotes a sense of well-being. To me, 'happy' can certainly be associated with emotion (like joy, delight), but I view 'happy' as more of a virtue; a way of living. I agree with those who believe that happiness is a moral obligation. The opposite of 'happy' is probably 'miserable'.