Monster Spawner

Monster Spawners are blocks with a transparent blue lattice structure found in dungeons as a guardian to the treasure found within. Inside the block there are flames that do not give off any light, and a miniature spinning version of the mob that is spawned, which slowly increases speed until the next mob is spawned. They are also found in Abandoned Mine Shaft surrounded by Cobweb. They always spawn a Cave Spider, explaining the cobweb.

There are many contraptions designed to kill whatever drops from it, usually a drowning trap, allowing the player to collect spoils in great numbers with little risk (see dungeon mob farming.)



Spawning Behavior
A spawner will create mobs provided there is space for its mob type to be generated and the light level within the spawning radius remains below seven. Mobs are created within a 8 x 8 x 3 high area centered on the spawner block's lower left corner. Note that mobs can spawn in any dark space within range, even if the spawner is fully lit The spawner will only spawn mobs if the player is within 16 blocks of it.

Note: Placing torches around a monster spawner used to deactivate it so that it could not spawn monsters, but that has been changed and no longer works. If there is no valid place dark enough for monsters to spawn, however, no monsters will be spawned.

The monster spawner will wait 200 + x ticks between spawns where x is a random number selected between 0 and 599 on each spawn, and a tick is 1/10 of a second. This means that on average, it will attempt to spawn a mob every 24.975 seconds. If 6 or more monsters of its type are present within a 17 x 17 x 9 area centered on the spawner block, the spawner "poofs" without creating a monster and then waits for the next interval. Otherwise, it tries 4 times to place the mob at a random position within a 8 x 8 x 3 high area centered on the spawner block's lower left corner. If the 4 places that it tries are all invalid, it waits for the next spawning to try again.

Unedited Monster Spawners will spawn Spiders, Skeletons, Zombies, or Cave Spiders. Cave Spider spawners appear only in Abandoned Mine Shafts; the other three kinds appear only in Dungeons, with Zombies chosen at 50% probability and Skeletons or Spiders chosen at 25% probability each. Spawners do not drop resources when destroyed, and a placed spawner will spawn only pigs. The spawner type can be changed through Hacking. Spawners can be given unusual entity IDs like Minecart or Boat to make a decorative block with the named entity spinning inside the cage.

In the Nether, Monster Spawners can spawn Blazes, and can only be found in Nether Strongholds. Unlike the overworld, however, the amount of light around a spawner will not stop it from spawning Blazes.

In Peaceful difficulty, Monster Spawner blocks will still appear, but any spawned mobs will disappear the instant they spawn. This makes it easy to make the aforementioned trap without any trouble.

Bugs

 * As of Beta 1.4, on multiplayer, a surface dungeon's spawner will be a pig spawner.
 * As of Beta 1.4, on multiplayer, monster spawners are extremely rarely found inside of dungeon chests.
 * Rarely, a monster spawner "spawns" no visible enemy, though sounds can be heard. When you move closer the mob will sometimes appear.
 * In multiplayer, all monster spawners show a pig model inside, despite what it actually spawns.
 * If a mob spawner is destroyed in multiplayer, sometimes the block itself will disappear and will stop spawning, but the flames inside will stay. If you relog, the flames will go away.
 * When a monster spawner spawns a mob in multiplayer, no smoke/sound is made.

Trivia

 * There is no way to legitimately get a monster spawner to place.
 * If you are not within range, the doll inside the spawner will stop spinning, and the mob spawner will stop emitting fire particles.
 * When killed, a zombie will drop 0, 1, or 2 rotten flesh, for an average of 1 rotten flesh/zombie. This means that the ideal production rate of a zombie spawner trap is (60 seconds/minute) / (24.975 seconds/zombie) * (1 rotten flesh/zombie) = 800/333 rotten flesh/minute ~= 2.40 rotten flesh/minute. Analogously, ideal spider traps will produce 2.40 strings/minute, and ideal skeleton traps will produce both 2.40 bones/minute and 2.40 arrows/minute. At this rate, the expected time for a stack of 64 items is 26 minutes and 38 seconds.
 * If the player removes the ceiling of the dungeon and exposes the Monster Spawner and surrounding area to direct sunlight, it prevents monster spawns during the day.
 * A convenient strategy for designing spawner traps is to make the ceiling of the room out of glass and build a dark room on top which can be filled with torches to disable the spawner if the trap needs to be modified or repaired.
 * When mining near a monster spawner that has no valid places to spawn, sometimes a monster will spawn immediately after a block is mined. The player should take caution for this reason while collecting Moss Stone from a dungeon but hasn't destroyed the monster spawner.  It's speculated this might happen because the spawner gets a chance to spawn before the lighting calculation for the space the block occupied.
 * If a spawner is hacked to create Ghasts, Giants, Squid, Slimes, or Endermen (Giants don't spawn naturally), the model inside will be miniaturized, but not enough to fit the cage properly, because the scaling factor used is the same as with other mobs.
 * The miniature mob model displayed inside the monster spawner was enabled again in Beta 1.2 after missing for several versions.
 * The model inside the mob spawner will spin faster when it is about to spawn monsters. There are set minimum and maximum speeds for which the model can rotate. If the model slowly builds up speed, it will take longer to spawn a mob. If the model builds up speed very quickly, mobs will spawn very quickly.
 * Occasionally, a monster spawner may spawn two monsters (maximum to four) at once. It is likely that another monster will spawn sooner than normal if this occurs.
 * If the fog key is rapidly pressed, everything in the map will become invisible except particles (such as the fire particle inside a monster spawner). This can make finding dungeons easy. This relies on client-side lag, so a faster computer will not be subject to this exploit. This no longer works as of Beta 1.8.
 * When hacked into the inventory, given with server commands, or in Infdev, placed spawners will only spawn pigs. These player placed spawners will only create pigs if passive mob spawning conditions are met.
 * Monster spawners can be edited with a map editor or client mod to spawn any mob (Most mobs have certain conditions that must be met to spawn).
 * Monster spawners can spawn monsters in a nearby cave, even when the monster spawner is fully lit, due to its 8 x 8 x 3 spawn radius.
 * The model inside the monster spawner has an ID number like all other mobs, though it is only visible at certain angles and cannot be seen in Beta 1.8.
 * When monsters spawn on peaceful there is a glitch that allows the mob it would create to play its sound file before it despawns.
 * Since monster spawners are stored as tile entities, they cannot be pushed by pistons.
 * It is possible, although very rare, for a monster spawner to occur near the surface and completely buried in sand, with monsters spawning in small gaps underneath sandstone.
 * Unlike most other transparent blocks, it is possible to place torches on a monster spawner.