Testing Java Extensions

MBD2 21.1.1

Test a capability and trait together. A codec-only test cannot detect duplication, simulation mutation, wrong IO direction, or a handler that is never discovered by recipe logic.

Minimum test matrix

Area	Required cases
Registration	Registry contains the exact stable name; optional modID guard behaves correctly
Serialization	Definition, condition, and content codec round-trip without losing fields
Trait persistence	Stored value survives block-entity save/load and clamps invalid values
Simulation	simulate=true returns the correct remainder and changes no state
Commit	simulate=false changes state once and returns the same remainder as simulation predicted
IO	IN consumes machine storage; OUT fills it; incompatible handler IO returns content unchanged
Slot routing	Named content reaches only handlers advertising that slot name
Multiple handlers	Remainder passes to the next handler; isDistinct produces the intended behavior
UI/XEI	Template binds without class casts or missing IDs; per-tick and chance annotations display
Lifecycle	Caches/listeners are released on unload and removal

Direct handler test pattern

var trait = getHeatTrait(machine);
trait.setStored(500);
var handler = trait.getRecipeHandlerTraits().getFirst();

var simulatedLeft = handler.handleRecipe(
    IO.IN, recipe, List.of(400), null, true);
assertNull(simulatedLeft);
assertEquals(500, trait.getStored());

var committedLeft = handler.handleRecipe(
    IO.IN, recipe, List.of(400), null, false);
assertNull(committedLeft);
assertEquals(100, trait.getStored());

Also test partial handling: request 700 from a store of 500, expect [200] and a final store of zero on commit.

Game tests

Use NeoForge game tests for block capability exposure, multiblock proxying, automatic IO, world-dependent conditions, and full recipe-logic transitions. Soft-dependency test classes must only be loaded inside ModList.isLoaded(...) guards; annotation discovery can otherwise load absent-mod classes eagerly.