JSPrismarine

Data-driven content ​

JSPrismarine has moved from hardcoded block, item and entity classes to a server whose content is built at runtime from data. Blocks, items, entities, recipes, loot, trading, spawning, biomes and world generation come from packs and generated tables, assembled through a small set of composable parts on every server that starts. The design is laid out in the data-driven vanilla plan; these pages explain the system for people who have not read the plan or the research behind it.

The format decision in one paragraph ​

The runtime model and every identifier are Bedrock-shaped: Bedrock names, Bedrock block states, the Bedrock palette, and the runtime ids this server already writes. Java Edition is the gameplay data source wherever Mojang publishes it as data (loot, recipes, advancements, tags, enchantments, item components, world generation) and, since Java 26.1 ships unobfuscated, also the source for the block properties and entity stats no Mojang report carries. Both pack formats load: Bedrock behavior packs define content a Bedrock client can render, and Java datapacks configure it. A generated mapping table, built from GeyserMC's MIT tables, joins the two editions in one place.

The parity claim ​

The project does not claim 1:1 Java parity. The adopted wording, from the edition differences research note (section 14):

JSPrismarine implements Java Edition's gameplay rules for every behavior a server decides, verified against Java Edition's own implementation, and presents them through the Bedrock client's model of the world. Where a Java rule has no Bedrock presentation, or where the Bedrock client decides the outcome locally, the server implements the behavior the client expects.

The note classifies 278 behavior differences between the editions and enumerates the excluded categories; parity.md explains how the claim is tested.

Reading map ​

  • architecture.md: the pack stack, definitions and registries, the Java to Bedrock seam, where each subsystem lives, and what exists today versus what is planned.
  • data-pipeline.md: the five stages that turn pinned Mojang and GeyserMC publications into committed goldens and generated tables, with the real commands.
  • using-packs.md: loading Bedrock packs and Java datapacks with @jsprismarine/packs, precedence rules, and diagnostics.
  • molang.md: the @jsprismarine/molang expression engine that Bedrock pack definitions use, with its versioned precedence rules.
  • mapping.md: the @jsprismarine/mapping seam between the editions, its four lookup functions, the generated tables and the reviewed loss list.
  • parity.md: the @jsprismarine/parity test suite and dashboard that measure the claim above against committed reference data.
  • gameplay-data.md: the loot, recipes, advancements, enchantments, predicates and tags the server runs out of Java's data, how a datapack replaces any of them, and what will not take effect.
  • entities.md: how a Bedrock entity definition becomes a running mob, what its components, events, filters, goals and brains do here, and what a custom mob will and will not manage.
  • block-properties.md: the per-state block property table dumped from the Java 26.3 jar and how the server reads it.
  • structures.md: which of Java's 52 structures the overworld generates, how placement and piece writing are split, and where the generation differs from Java.
  • dimensions.md: dimension types, level stems and world presets, the worlds.<name> config shape and its migration, the generator kinds, custom dimensions, portals and the client's dimension change, with the questions a real client has yet to answer.
  • end-fight.md: End dragon combat, the exit portal and credits, four-crystal respawn, and the server-side light values used by gameplay rules.
  • engine-systems.md: the systems with no data form in either edition, which are redstone, fluids, fire, explosions, light and random ticking, with what each one simulates, what a tick of it costs, and where it stops.
  • commands.md: which commands the server registers and which it deliberately leaves out, what Bedrock's flat command tree costs /execute, the scoreboard, and running .mcfunction files from a datapack.