Bukkit/CraftBukkit — pull request #655 by oloflarsson (2012-01-25 19:35:59+00:00)
Title: Converting allowMonsters/allowAnimals from boolean into Set<SpawnReason>...

.... With this you can for example have worlds where no mobs spawn naturally but DO spawn from mobspawners. If this where to be done using bukkit events only it would cause heavy CPU usage since the algorithm that spawns mobs naturally would run all the time.