I think we could probably all answer this question without studying, but I wanted to study it anyway. Joy comes from God. But how do we access that joy?
In John 16:17-24 and 33, Jesus is trying to tell His disciples that He is leaving (referring to His death). But He promised them that their sorrow would be turned to joy, and fullness of joy. They would see Him again after His resurrection, and then they would receive His indwelling Holy Spirit.
Joy is this passage is a comfort. But because the disciples were trying to conform His words to their expectations of a physical kingdom set up on earth, their disappointment blinded them to truth. Jesus told them that “in that day” (when they would see Him again) they would ask Him nothing. Why? Because they would understand what He had meant.
But then He instructed them to ask of the Father in His name, and they would receive it. Jesus even went on to explain that the asking and receiving through prayer would lead to fullness of joy. We are directed to ask for all that we need and all that He has promised. And we are assured to receive it. The blessed effect of the prayer of faith is that it fills us with joy.
“Would we have our joy full, as full as it is capable of being in this world, we must be much in prayer. When we are told to rejoice evermore, it follows immediately, pray without ceasing” – Matthew Henry (referencing 1 Thess. 5:16 ff)
God’s gifts, through Christ, fill the treasures of the soul, they fill its joy. So what is the source of joy? The presence of the Savior, and God’s spiritual blessings accessed in prayer. Nothing physical, for the disciples were sorrowful and the world would be trying to steal joy away from them. But Jesus promised that their joy would remain full as they remained connected to the Father in prayer.