// event listener
@EventHandler
public void onPlayerJoin(final PlayerJoinEvent event) {

    Bukkit.getScheduler().runTaskAsynchronously(plugin, new Runnable() {

        @Override
        public void run() {
            datasource.loadPlayerDataToCache(event.getPlayer());

            // return to main thread
            Bukkit.getScheduler().scheduleSyncDelayedTask(plugin, new Runnable() {

                @Override
                public void run() {
                    onDataLoadComplete(event.getPlayer());
                }
            });
        }
    });
}

public void onDataLoadComplete(Player player) {
    // check sessions, send messages etc.
}

@EventHandler
public void onPlayerQuit(final PlayerQuitEvent event) {

    Bukkit.getScheduler().runTaskAsynchronously(plugin, new Runnable() {

        @Override
        public void run() {

            datasource.savePlayerData(event.getPlayer());
        }
    });
}

// exaple datasource class
public class MysqlDatasource implements Datasource {

    private Map<String, PlayerAuth> cache = new ConcurrentHashMap<>();

    public synchronized void loadPlayerDataToCache(Player player) {

        cache.put(player.getName().toLowerCase(), getAuth(player.getName().toLowerCase());
    }

    public synchronized void savePlayerData(Player player) {

        PlayerAuth auth = cache.remove(player.getName().toLowerCase());
        if ( auth == null ) {
            return;
        }

        // save auth to datasource
    }

    // other methods declared in Datasource
}