Steel Extractor

The Steel Extractor is a Fabric mod written in Kotlin that runs on a Minecraft server and extracts comprehensive game data into JSON files. It is the primary tool used to generate the data files that Steel relies on.

The mod hooks into the server startup lifecycle and automatically runs all extractors once the server is fully loaded. The output is written to the steel_extractor_output/ directory as pretty-printed JSON.

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How to use

At first you need to create the run directory. It’s very easy: build it from the command line (./gradlew runServer), or simply click the Run button (Minecraft server) in your IDE of choice (e.g., IntelliJ). This will start the minecraft server and the extraction will be started automatically, the extraction of the world hashes can take up to 30min.

This will create a steel_extractor_output/ folder in the server working directory. There, you’ll find all generated JSON and binary files, which Steel needs as a reference to vanilla. Not all output files are copied to the same directory in Steel. Check the mapping first before you move something.

To disable a specific extractor, set the corresponding environment variable to true before starting the server. For example, to disable the Blocks extractor, run:

export STEEL_EXTRACTOR_DISABLE_BLOCKS=true

To disable the lengthy chunk extraction, set the following environment variable:

export STEEL_EXTRACTOR_ENABLE_CHUNK_EXTRACTION=false

You can also set the server to shut down once the extractor has run it’s course by setting the following environment variable:

export STEEL_EXTRACTOR_EXIT_ON_COMPLETE=true

How It Works

The extractor registers a SERVER_STARTED event. When the server finishes loading, it iterates through all registered extractors, calls their extract() method, and writes the result to a JSON file.

Every extractor implements the Extractor interface:

interface Extractor {
    fun fileName(): String

    @Throws(Exception::class)
    fun extract(server: MinecraftServer): JsonElement
}

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Extracted Data

The following table lists all current extractors, what data they produce and what environment variable can be used to disable them. Output paths are relative to steel_extractor_output/.

Extractor	Output File	Description	Enviroment Variable
Blocks	steel-registry/build_assets/blocks.json	All blocks with behavior properties, block states, default values, collision and outline shapes	STEEL_EXTRACTOR_DISABLE_BLOCKS
BlockEntities	steel-registry/build_assets/block_entities.json	Registry keys of all block entity types	STEEL_EXTRACTOR_DISABLE_BLOCK_ENTITIES
Items	steel-registry/build_assets/items.json	All items with components, block references, and class names	STEEL_EXTRACTOR_DISABLE_ITEMS
ParticleTypeRegistryExtractor	steel-registry/build_assets/particle_types.json	Particle type registry keys	STEEL_EXTRACTOR_DISABLE_PARTICLE_TYPES
VillagerTypeRegistryExtractor	steel-registry/build_assets/villager_types.json	Villager type registry keys	STEEL_EXTRACTOR_DISABLE_VILLAGER_TYPES
VillagerProfessionRegistryExtractor	steel-registry/build_assets/villager_professions.json	Villager profession registry keys	STEEL_EXTRACTOR_DISABLE_VILLAGER_PROFESSIONS
Packets	steel-registry/build_assets/packets.json	All serverbound and clientbound packets grouped by protocol phase	STEEL_EXTRACTOR_DISABLE_PACKETS
MenuTypes	steel-registry/build_assets/menutypes.json	All menu/GUI types, such as crafting table and furnace	STEEL_EXTRACTOR_DISABLE_MENU_TYPES
Entities	steel-registry/build_assets/entities.json	Entities with dimensions, synched data, attributes, and behavior flags	STEEL_EXTRACTOR_DISABLE_ENTITIES
EntityEvents	steel-utils/build_assets/entity_events.json	Entity event constants	STEEL_EXTRACTOR_DISABLE_ENTITY_EVENTS
Fluids	steel-registry/build_assets/fluids.json	All fluids with behavior properties and state data	STEEL_EXTRACTOR_DISABLE_FLUIDS
GameRulesExtractor	steel-registry/build_assets/game_rules.json	All game rules with types, defaults, and bounds	STEEL_EXTRACTOR_DISABLE_GAME_RULES
Classes	steel-core/build/classes.json	Java class names for all blocks and items, plus extra per-entry metadata when available	STEEL_EXTRACTOR_DISABLE_CLASSES
Attributes	steel-registry/build_assets/attributes.json	Entity attributes with defaults, ranges, and sync info	STEEL_EXTRACTOR_DISABLE_ATTRIBUTES
MobEffects	steel-registry/build_assets/mob_effects.json	Status effects with categories and colors	STEEL_EXTRACTOR_DISABLE_MOB_EFFECTS
Potions	steel-registry/build_assets/potions.json	Potions with their effects, durations, and amplifiers	STEEL_EXTRACTOR_DISABLE_POTIONS
SoundTypes	steel-registry/build_assets/sound_types.json	Block sound types with volume, pitch, and sound event references	STEEL_EXTRACTOR_DISABLE_SOUND_TYPES
SoundEvents	steel-registry/build_assets/sound_events.json	Mapping of all sound event paths to registry IDs	STEEL_EXTRACTOR_DISABLE_SOUND_EVENTS
MultiNoiseBiomeParameters	steel-registry/build_assets/multi_noise_biome_source_parameters.json	Multi-noise biome source parameter lists	STEEL_EXTRACTOR_DISABLE_MULTI_NOISE_BIOME_PARAMETERS
BiomeHashes	steel-core/test_assets/biome_hashes.json	Deterministic biome hash fixtures used by Steel tests	STEEL_EXTRACTOR_DISABLE_BIOME_HASHES
LevelEvents	steel-registry/build_assets/level_events.json	All level event constants, including particles and sounds	STEEL_EXTRACTOR_DISABLE_LEVEL_EVENTS
Tags	steel-registry/build_assets/tags.json	Block and item tags excluding the minecraft namespace	STEEL_EXTRACTOR_DISABLE_TAGS
StructureStarts	steel-core/test_assets/structure_starts.json	Structure start fixtures used by Steel tests	STEEL_EXTRACTOR_DISABLE_STRUCTURE_STARTS
Strippables	steel-core/build/strippables.json	Block mappings for axe stripping behavior	STEEL_EXTRACTOR_DISABLE_STRIPPABLES
Weathering	steel-core/build/weathering.json	Block mappings for copper weathering behavior	STEEL_EXTRACTOR_DISABLE_WEATHERING
CandleCakes	steel-core/build/candle_cakes.json	Candle-to-candle-cake block mappings	STEEL_EXTRACTOR_DISABLE_CANDLE_CAKES
Waxables	steel-core/build/waxables.json	Block mappings for waxed variants	STEEL_EXTRACTOR_DISABLE_WAXABLES
PoiTypesExtractor	steel-registry/build_assets/poi_types.json	Point of interest type registry data	STEEL_EXTRACTOR_DISABLE_POI_TYPES
GameEvents	steel-registry/build_assets/game_events.json	Game event registry keys	STEEL_EXTRACTOR_DISABLE_GAME_EVENTS
ChunkStageHashes	steel-core/test_assets/chunk_stage_hashes.json and steel-core/test_assets/chunk_stage_*_blocks.bin.gz	Chunk generation stage hashes and binary block dumps for sampled chunks	STEEL_EXTRACTOR_DISABLE_CHUNK_STAGE_HASHES

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Writing a Simple Extractor

Here is a minimal example of how to create a new extractor. This extractor outputs all entity attributes with their default values:

package com.steelextractor.extractors

import com.google.gson.JsonArray
import com.google.gson.JsonElement
import com.google.gson.JsonObject
import com.steelextractor.SteelExtractor
import net.minecraft.core.registries.BuiltInRegistries
import net.minecraft.server.MinecraftServer

class Attributes : SteelExtractor.Extractor {

    override fun fileName(): String {
        return "attributes.json"
    }

    override fun extract(server: MinecraftServer): JsonElement {
        val attributesArray = JsonArray()

        for (attribute in BuiltInRegistries.ATTRIBUTE) {
            val key = BuiltInRegistries.ATTRIBUTE.getKey(attribute)
            val name = key?.path ?: "unknown"

            val attributeJson = JsonObject()
            attributeJson.addProperty("id", BuiltInRegistries.ATTRIBUTE.getId(attribute))
            attributeJson.addProperty("name", name)
            attributeJson.addProperty("default_value", attribute.defaultValue)

            attributesArray.add(attributeJson)
        }

        return attributesArray
    }
}

To register your new extractor, add it to the immediateExtractors array in SteelExtractor.kt:

val immediateExtractors = arrayOf(
    Blocks(),
    // ... other extractors ...
    Attributes(),
    MyNewExtractor()  // Add your extractor here
)

After starting the server, the output will appear in steel_extractor_output/attributes.json.

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Other useful resources

• Reflection in Extractors - How to use Java reflection to access private Minecraft internals