A spawn jail is a structure implemented on multiplayer servers that traps newly spawned players. It can be used as a security check for new arrivals, or as a server lock for when there are no OPs on the server.
In versions of Minecraft previous to Beta, a spawnjail takes the form of a box just big enough inside to contain a player, built over the spawnpoint, out of bedrock. There may be a gap in the wall at head level so that the player can be looked at from outside the box, or so the player can see what is outside the box.
In Beta, the spawnpoint in multiplayer servers spans a large region of space (possibly 17x17) and so building spawnjails is more difficult. It can still be done by determining the spawn area and building a floor and walls out of bedrock.
Rarely, some servers create a closed box that has no windows/gaps so they cannot see what's going on. Sometimes they spawn in a dark room with mobs.
New arrivals spawn inside the box and cannot escape until allowed out by an op. If there is no op or admin in the room, the spawn jail effectively closes the server, meaning no-one can enter when the room is unpoliced. This is an anti-griefing measure: each new arrival feels scrutinized, may be followed once allowed out, and knows that the room is policed, purportedly making them less likely to grief. The viewing window can yield whether or not the player is a premium owner of Minecraft - a demographic who are not expected to be griefers due to their monetary commitment to the game.
Certain hacks allow players to escape spawnjails by changing their respawn point to an area outside of the accessible area.
Sometimes, a spawnjail is more of a "spawnmaze" where a catacomb built of bedrock starts at the spawnpoint and acts as an intelligence test for new arrivals; they must find the end of the maze in order to access the room proper. The forums contain several maze generator mods.
Some servers may have an entire section fenced off by bedrock. This is so the admins can let players on the server when no admins are online. It may also provide more selective server entrance policies, only letting those with good building talent or a premium membership in.
Some players consider this a rude practice which defeats the point of a public or freebuild server. However, many admins support it, because it keeps griefers out at minimal cost to them. It also helps if the person is not a griefer, an admin can let them in. The effectiveness of this for griefing reduction is debatable.
In Classic, if spawnjails are larger than one square block, then it is possible to break out of it. This does not work in later versions of Minecraft. Simply by pressing enter, moving aside, then placing a block at the place you pressed enter, will get you out of the spawnjail (by pressing r to respawn). Fortunately, most servers ban people who build in spawnjails. Using hacks can also allow you to break out of a spawnjail, where this is also frowned upon.
This problem can be fixed by creating a net of 1x1x2 rooms that the players move up into when they try to escape. Since a room like this would have no viewing window it would leave people who try to escape stuck and would need to rejoin.
If there is a room with bedrock as a ceiling, it would also be better to put the spawnjail at the top. When the person respawns and there is a block on top of the spawning point, he will be transported over the top of the map, trapping him, and will have to rejoin. Unfortunately, if the person places the block at the spawn block, anyone else who joins will be stuck. Also, this would only work with cave maps or closed underwater maps. The jail buildings with iron bars are optional.
If you use noclip hacks, it may be possible to walk out of the map and set the spawn point there.
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