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Named Binary Tag (NBT)

NBT is a format introduced in the earliest days of Minecraft, written by Notch himself. It is widely used throughout the Minecraft codebase for data storage.

Specification​

The NBT spec is similar to the JSON spec, with a few differences:

  • Distinct types for bytes, shorts, longs and floats exist, suffixed by b, s, l and f, respectively, similar to how they would be represented in Java code.
    • Doubles may also be suffixed with d, but this is not required, similar to Java code. The optional i suffix available in Java for integers is not permitted.
    • The suffixes are not case-sensitive. So for example, 64b is the same as 64B, and 0.5F is the same as 0.5f.
  • Booleans do not exist, they are instead represented by bytes. true becomes 1b, false becomes 0b.
    • The current implementation treats all non-zero values as true, so 2b would be treated as true as well.
  • There is no null equivalent in NBT.
  • Quotes around keys are optional. So a JSON property "duration": 20 can become both duration: 20 and "duration": 20 in NBT.
  • What is known in JSON as a sub-object is known in NBT as a compound tag (or just compound).
  • NBT lists cannot mix and match types, unlike in JSON. The list type is determined by the first element, or defined in code.
    • However, lists of lists can mix and match different list types. So a list of two lists, where the first one is a list of strings and the second one is a list of bytes, is allowed.
  • There are special array types that are different from lists, but follow their scheme of containing elements in square brackets. There are three array types:
    • Byte arrays, denoted by a B; at the beginning of the array. Example: [B;0b,30b]
    • Integer arrays, denoted by a I; at the beginning of the array. Example: [I;0,-300]
    • Long arrays, denoted by an L; at the beginning of the array. Example: [L;0l,240l]
  • Trailing commas in lists, arrays and compound tags are allowed.

NBT Files​

Minecraft uses .nbt files extensively, for example for structure files in datapacks. Region files (.mca) that contain the contents of a region (i.e. a collection of chunks), as well as the various .dat files used in different places by the game, are NBT files as well.

NBT files are typically compressed with GZip. As such, they are binary files and cannot be edited directly.

NBT in Code​

Like in JSON, all NBT objects are children of an enclosing object. So let's create one:

CompoundTag tag = new CompoundTag();

We can now put our data into that tag:

tag.putInt("Color", 0xffffff);
tag.putString("Level", "minecraft:overworld");
tag.putDouble("IAmRunningOutOfIdeasForNamesHere", 1d);

Several helpers exist here, for example, putIntArray also has a convenience method that takes a List<Integer> in addition to the standard variant that takes an int[].

Of course, we can also get values from that tag:

int color = tag.getInt("Color");
String level = tag.getString("Level");
double d = tag.getDouble("IAmRunningOutOfIdeasForNamesHere");

Number types will return 0 if absent. Strings will return "" if absent. More complex types (lists, arrays, compounds) will throw an exception if absent.

As such, we want to safeguard by checking if a tag element exists:

boolean hasColor = tag.contains("Color");
boolean hasColorMoreExplicitly = tag.contains("Color", Tag.TAG_INT);

The TAG_INT constant is defined in Tag, which is the super interface for all tag types. Most tag types besides CompoundTag are mostly internal, for example ByteTag or StringTag, though the direct CompoundTag#get and #put methods can work with them if you ever stumble across some.

There is one obvious exception, though: ListTags. Working with these is special because when getting a list tag through CompoundTag#getList, you must also specify the list type. So getting a list of strings, for example, would work like this:

ListTag list = tag.getList("SomeListHere", Tag.TAG_STRING);

Similarly, when creating a ListTag, you must also specify the list type during creation:

ListTag list = new ListTag(List.of("Value1", "Value2"), Tag.TAG_STRING);

Finally, working with CompoundTags inside other CompoundTags directly utilizes CompoundTag#get and #put:

tag.put("Tag", new CompoundTag());
tag.get("Tag");

Usages of NBT​

NBT is used in a lot of places in Minecraft. Some of the most common examples include ItemStacks, BlockEntitys and Entitys.

See Also​