NBT

A java implementation of the NBT protocol, including a way to implement custom tags.

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Specification

According to the specification, there are currently 13 different types of tags:

Tag class	Superclass	ID	Payload
EndTag	Tag	0	None
ByteTag	NumberTag	1	1 byte / 8 bits, signed
ShortTag	NumberTag	2	2 bytes / 16 bits, signed, big endian
IntTag	NumberTag	3	4 bytes / 32 bits, signed, big endian
LongTag	NumberTag	4	8 bytes / 64 bits, signed, big endian
FloatTag	NumberTag	5	4 bytes / 32 bits, signed, big endian, IEEE 754-2008, binary32
DoubleTag	NumberTag	6	8 bytes / 64 bits, signed, big endian, IEEE 754-2008, binary64
ByteArrayTag	ArrayTag	7	IntTag payload size, then size ByteTag payloads
StringTag	Tag	8	ShortTag payload length, then a UTF-8 string with size length
ListTag	Tag	9	ByteTag payload tagId, then IntTag payload size, then size tags' payloads, all of type tagId
CompoundTag	Tag	10	Fully formed tags, followed by an EndTag
IntArrayTag	ArrayTag	11	IntTag payload size, then size IntTag payloads
LongArrayTag	ArrayTag	12	IntTag payload size, then size LongTag payloads

• The EndTag is only used to mark the end of a CompoundTag in its serialized state or an empty ListTag.

• The maximum depth of the NBT structure is 512. If the depth exceeds this restriction during serialization, deserialization or String conversion, a MaxDepthReachedException is thrown. This usually happens when a circular reference exists in the NBT structure. The NBT specification does not allow circular references, as there is no tag to represent this.

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Example usage:

The following code snippet shows how to create a CompoundTag:

CompoundTag ct = new CompoundTag();

ct.put("byte", new ByteTag((byte) 1));
ct.put("double", new DoubleTag(1.234));
ct.putString("string", "stringValue");

An example how to use a ListTag:

ListTag<FloatTag> fl = new ListTag<>(FloatTag.class);

fl.add(new FloatTag(1.234f);
fl.addFloat(5.678f);

Nesting

All methods serializing instances or deserializing data track the nesting levels to prevent circular references or malicious data which could, when deserialized, result in thousands of instances causing a denial of service.

These methods have a parameter for the maximum nesting depth they are allowed to traverse. A value of 0 means that only the object itself, but no nested object may be processed.

If an instance is nested further than allowed, a MaxDepthReachedException will be thrown. A negative maximum depth will cause an IllegalArgumentException.

Some methods do not provide a parameter to specify the maximum depth, but instead use Tag.DEFAULT_MAX_DEPTH (512) which is also the maximum used in Minecraft.

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Utility

There are several utility methods to make your life easier if you use this library.

NBTUtil

NBTUtil.writeTag() lets you write a Tag into a gzip compressed or uncompressed file in one line (not counting exception handling). Files are gzip compressed by default.

Example usage:

NBTUtil.writeTag(tag, "filename.dat");

NBTUtil.readTag() reads any file containing NBT data. No worry about compression, it will automatically uncompress gzip compressed files.

Example usage:

Tag<?> tag = NBTUtil.readTag("filename.dat");

Playing Minecraft?

Each tag can be converted into a JSON-like NBT String used in Minecraft commands.

Example usage:

CompoundTag c = new CompoundTag();
c.putByte("blah", (byte) 5);
c.putString("foo", "bär");
System.out.println(c.toTagString()); // {blah:5b,foo:"bär"}

ListTag<StringTag> s = new ListTag<>(StringTag.class);
s.addString("test");
s.add(new StringTag("text"));
c.add("list", s);
System.out.println(c.toTagString()); // {blah:5b,foo:"bär",list:[test,text]}

There is also a tool to read, change and write MCA files.

Here are some examples:

// This changes the InhabitedTime field of the chunk at x=68, z=81 to 0
MCAFile mcaFile = MCAUtil.readMCAFile("r.2.2.mca");
Chunk chunk = mcaFile.getChunk(68, 81);
chunk.setInhabitedTime(0);
MCAUtil.writeMCAFile("r.2.2.mca", mcaFile);

There is also an optimized api to retrieve and set block information (BlockStates) in MCA files.

Example:

// Retrieves block information from the MCA file
CompoundTag blockState = mcaFile.getBlockStateAt(1090, 25, 1301);

// Retrieves block information from a single chunk
CompoundTag blockState = chunk.getBlockStateAt(2, 25, 5);

// Set block information
CompoundTag stone = new CompoundTag();
stone.putString("Name", "minecraft:stone");
mcaFile.setBlockStateAt(1090, 25, 1301, stone, false);

To ensure good performance even when setting a lot of blocks and / or editing sections with a huge palette of block states, the size of the BlockStates array is only updated when the size of the palette requires it. This means there might be blocks in the palette that are not actually used in the BlockStates array. You can trigger a cleanup process by calling one of the following three methods, depending on the desired depth:

mcaFile.cleanupPalettesAndBlockStates();
chunk.cleanupPalettesAndBlockStates();
section.cleanupPaletteAndBlockStates();

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Custom tags

Interested in more advanced features, and the default NBT protocol just isn't enough? Simply create your own tags! There are 4 example classes in net.querz.nbt.custom that show how to implement custom tags:

Class	ID	Description
ObjectTag	90	A wrapper tag that serializes and deserializes any object using the default java serialization.
ShortArrayTag	100	In addition to the already existing ByteArrayTag, IntArrayTag and LongArrayTag.
CharTag	110	Character (char) tag.
StructTag	120	Similar to the ListTag, but with the ability to store multiple types.

To be able to use a custom tag with deserialization, a Supplier and the custom tag class must be registered at runtime alongside its id with TagFactory.registerCustomTag(). The Supplier can be anything that returns a new instance of this custom tag. Here is an example using the custom tags no-args constructor:

TagFactory.registerCustomTag(90, ObjectTag::new, ObjectTag.class);

Nesting

As mentioned before, serialization and deserialization methods are provided with a parameter indicating the maximum processing depth of the structure. This is not guaranteed when using custom tags, it is the responsibility of the creator of that custom tag to call Tag#decrementMaxDepth(int) to correctly update the nesting depth.

It is also highly encouraged to document the custom tag behaviour when it does so to make users aware of the possible exceptions thrown by Tag#decrementMaxDepth(int).