Bukkit/CraftBukkit — comment on commit 710f63b83f29bbd6ee9089d049152e2cba1e43d7
File: src/main/java/net/minecraft/server/World.java line 886
By Moo0:

Or even if !(spawnReason == SpawnReason.CUSTOM || spawnReason == SpawnReason.SPAWNER_EGG || spawnReason = etc) to specify the ones you don't want it affecting, rather than the long list of ones you do want it affecting.