Monster Spawner

Monster spawners are blocks that can spawn mobs.

Obtaining
Spawners can not be obtained at all in survival mode. The proper tool to mine them with is the pickaxe.

It is also not in Creative mode inventory (except in Pocket Edition) and as such must be obtained with commands.

Natural generation
Spawners can generate naturally in these places, spawning mobs chosen randomly when generated:
 * Dungeons
 * Spawns zombies (50% chance), skeletons (25%), or spiders (25%).


 * Abandoned mine shafts
 * Spawns Cave Spiders. Located in horizontal tunnels, heavily surrounded by cobwebs.
 * In Pocket Edition it spawns regular spiders instead.


 * Strongholds
 * Spawns silverfish. Located in the End Portal room. One per stronghold.


 * Nether Fortresses
 * Spawns Blazes. Located on Nether fortress balconies with stairs leading up to them.

Behavior
A spawner will spawn mobs in the surrounding area, provided suitable spawning locations for the block's mob type can be found in the area. Spawner blocks can spawn some mobs in mid-air, ignoring general rules about spawning on solid ground. The spawned mob can be changed by a spawn egg on the spawner.

A monster spawner is only active when a player is within a radius of 16 blocks from the spawner. While the block is active, it will spawn mobs within an 8×2×8 block volume (8 wide and 2 high and 8 long) centered on the center of the block, effectively meaning mobs can spawn in a 9x9 area, or 3.5 blocks from the spawner. Mobs can spawn anywhere in this range that is suitable, with mobs more likely to spawn closer to the spawner than farther away.

While mobs are spawned at fractional x and z-coordinates (i.e. not aligned to blocks), they are spawned at an integer y-coordinate. Horizontally, a mob can spawn with its center point anywhere within the 8 × 8 range, but vertically, mobs will spawn with their legs at either the same layer as the spawner block, one block above it, or one block below it. Therefore, there are 3 vertical blocks of space in which mobs can spawn even though the spawning volume is only two blocks in height.

For some types of mobs to spawn in the outer planes of the spawning volume, some planes outside the volume may also need to be free of opaque blocks to conform with the mobs' height, width, or other rules governing their individual spawn volumes. For mobs that are two or more blocks tall such as zombies, skeletons, or blazes to spawn in the top y-layer, the layer above that must contain only air.

For all of the volumes listed in the table, the horizontal plane is centered on the center of the spawner block. While the spawning volume for pigs is 8.9×8.9×2.9, the requirement of grass blocks that are necessary for pigs to spawn will reduce the actual volume in which they successfully spawn.

The block will attempt to spawn 4 mobs at randomly chosen points within the spawning area, then wait anywhere from 200 to 799 ticks (10 to 39.95 seconds) before spawning again. As it waits, the mob inside the block will spin faster and faster. Except for spawning on a solid block, all of the usual requirements for spawning must be met (not in a solid block, correct light level, etc.), so the spawner will often produce fewer than 4 mobs. When it does spawn, it will "poof", and more lightless flames will temporarily appear around the Spawn block. If the block fails to spawn any mobs because it did not pick any suitable locations, it will repeat this process every tick until it succeeds. Only when it manages to spawn at least one mob will it start waiting for the next cycle. If, at the time of spawning, 6 or more mobs of the block's type are present within a 9×9×9 area, centered on the spawner block, the spawner "poofs" without creating any mobs and then waits for the next cycle.

In Peaceful difficulty, monster spawner blocks will still appear and work, but any spawned hostile mobs will disappear the instant they spawn. Zombie Pigmen, Magma Cubes, and Ghasts will not spawn at all.

An interesting quirk of monster spawners (and end portal frames and slime blocks) is that they are transparent in their rendering only - unlike all other transparent blocks, they can have torches and redstone placed on them, conduct a redstone current, will suffocate mobs, and more.

Detailed spawning algorithm
This pseudo-code is derived from the de-compiled source of 1.7.10 if (there is a player within the range of activatingRangeFromPlayer) {   if (spawner delay == -1) {       reset spawner delay }

if (spawner delay > 0) {       decrement spawner delay by 1 cancel this spawn }

loop for the number of entities we should spawn {       if (the number of entityTypeName in a 2*spawnRange+1 centered cube > maxNearbyEntities) { reset spawner delay exit loop }

X = Spawner.x + random(spawnRange*2)-spawnRange; Y = Spawner.y + random(3)-1; Z = Spawner.z + random(spawnRange*2)-spawnRange;

if (entity can spawn at (X,Y,Z)) {           Spawn entityTypeName at (X,Y,Z) Play Sound Effect Spawn Particles }   }

if (at least one entity was spawned) {       reset spawner delay } }

Default values: activatingRangeFromPlayer = 16 spawner delay = 20 maxNearbyEntities = 6 spawn range = 4 entityTypeName = Pig

Outcomes (default): A player must be within a 16 block centered sphere There must not be more than 6 pigs within a 9x9x9 centered cube The pig must be able to spawn at the chosen location (must not be inside a block, meet lighting conditions etc.) If all mobs fail to spawn, it will try again on the very next tick.

Custom spawners
Monster spawners are capable of much more than they are used for in survival Minecraft. Using commands, they can be customized in many ways:
 * They can be made to spawn any kind of entity.
 * A single spawner can spawn multiple different entities, chosen at random from a list.
 * Properties can be set on the spawned entities.
 * Various range and timing properties of the spawner can be altered.

Detailed technical information about custom spawners can be found in the chunk format reference.

Trivia

 * Unlike most other solid blocks, the monster spawner supports texture with transparent areas, without showing what's behind.
 * When mining near a monster spawner that has no valid places to spawn, sometimes a monster will spawn immediately after a block is mined. 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 made to spawn a Giant, Ghast, or Iron Golem; the mob inside will be larger than the spawner and will appear to physically extend beyond the confines of the spawner block's sides.
 * The miniature mob inside the monster spawner is an actual mob of that type. Thus, if a spawner is configured to spawn Ender Dragons or Withers, their boss health bar is visible while looking at the spawner.