JOY: Jesus, Others, Yourself. Seek first the kingdom of God.
Despite limitations, Paul was able to see God given opportunities in every situation to share the gospel. In every event that happens in the life of a believer, there will be unforeseen consequences/results which God can use to further His kingdom (others went out to preach while Paul was imprisoned).
Christians must not be thin-skinned (Paul was still able to rejoice while some were slandering him - because he was spiritually mature and could see that they were still spreading the Good News. God's opinion about him mattered more than other people's).
Paul could see the positive even in being sentenced to death, he would be going to be with Christ (a joy in itself as it is his life's goal), but while he was alive he would focus his energy on spreading the gospel. Being alive meant constant persecution and enduring threats of all kinds; but he was willing to be wherever God willed him to be, serving others for Jesus. Paul would live and help other believers to grow in their faith and joy (a good way to know if we are growing as Christians is to check our level of joy).
To be a people of joy we must not be self-consumed (rather, be God-centered); remember JOY to find your purpose and have meaning in your life; look for opportunities to serve Christ wherever you are.