Java Edition Classic level format

Template:Outdated Template:Exclusive The Classic level format refers to the order information is saved in level files from Minecraft Classic and Pre-Classic. There were three separate formats used during these versions. In all formats, they were compressed with gzip. The first format lasted from rd-132211 to Classic 0.0.12a_03 and only contained the blocks in the world. The second format lasted from Classic 0.0.13a to Classic 0.0.13a_03 and contained a short header followed by the blocks. The third format lasted from Classic 0.0.14a to Classic 0.30 and contained a short header followed by serialized Java objects with lots of different data. Single-player levels had the extension ".dat" or ".mine". Levels used by the Classic servers were named "server_level.dat".

The Classic level format can be read by the ClassicExplorer program. For the last format that used Java's Serializable interface, the easiest way to work with it was through the Classic server itself, minecraft-server.jar. Sample code was provided to show how to build an editor on top of minecraft-server.jar.

File Format

First Format

Data	Length (Bytes)	Description
Block Array	4194304 (2^22)	An array of blocks in the world in the order: layers of the world from y = 0 to build height, in each layer have strips from z = 0 to the edge of the world, in each strip have blocks from x = 0 to the edge of the world. Each byte represents a single block using their ID.

Second Format

Data	Length (Bytes)	Description
Magic Number	4	A number used to identify the file. Always will be 27 1B B7 88.
Version	1	A version identifier byte 01.
World Name	Variable	First two bytes will be a short for the length of the string then that length bytes will be ASCII characters for the string. This string will always be either "A Nice World" or "--" depending on the start version.
Creator Player Name	Variable	First two bytes will be a short for the length of the string then that length bytes will be ASCII characters for the string. In Classic 0.0.13a-dev this will always be "noname" but Classic 0.0.13a_03 will be the actual user's name.
Time Created	8	A long value for the time the level was created in the Unix epoch.
Width	2	The width of the world (x direction).
Height	2	The height of the world (z direction).
Depth	2	The depth of the world (y direction).
Block Array	4194304 (2^22)	An array of blocks in the world in the following order: layers of the world from y = 0 to build height, in each layer have strips from z = 0 to the edge of the world, in each strip have blocks from x = 0 to the edge of the world. Each byte represents a single block using their ID.

Third Format

Position	Size (bytes)	Name	Description
0	4	Magic ID	A magic ID is a constant number used to identify the Minecraft file format. The current value is 0x271bb788.
4	1	Version Number	The version number represents the current format used to save the level. The current value is 2.
5	Variable	Serialized Java com.mojang.minecraft.level.Level Class	More information about the serialization format used by Java is available in the manual, however, the easiest way to edit the file is to use the classes provided here with the official minecraft-server.jar file.

Accessing the array of bytes

The most interesting part of a level was the block array. Each byte in this array defined a block type at a corresponding location in the world. One generally had two options for accessing the byte array of blocks:

The player could deserialize the compressed .dat file directly back into an instance of a Level object inside of Java, thus having access to the instance of the Level object in exactly the same way the Minecraft Server does. This would allow the player to set the blocks, dimensions, spawn point and other aspects of the map directly by calling the methods on the instantiated Level object. Manual decompression is not needed before loading, because Java can compress and decompress gzipped files on the fly. To load the datafile back into an instance of the Level class, the player would need the class definition for the Level class. This was included with the minecraft-server.jar file. An example of this could be seen in the Development resources/Example Minecraft Classic Level Editing Class.

Others have read and modified the map's data by simply accessing the raw byte array in the datafile file. To do this, the player would decompress it, make changes to the bytes where the byte array is stored, and then compress it again. The 256x256x64 bytes (default dimensions) labeled blocks in each format were where the byte array was stored. The formula 65536 * y + 256 * z + x = Byte # will convert Minecraft coordinates to the byte number in the array.