Make fabric.mod.json path configurable at configuration stage.

src/main/java/net/fabricmc/loom/api/LoomGradleExtensionAPI.java

@@ -66,6 +66,8 @@ public interface LoomGradleExtensionAPI {
 
 	RegularFileProperty getAccessWidenerPath();
 
+	RegularFileProperty getFabricModJsonPath();
+
 	NamedDomainObjectContainer<DecompilerOptions> getDecompilerOptions();
 
 	void decompilers(Action<NamedDomainObjectContainer<DecompilerOptions>> action);

src/main/java/net/fabricmc/loom/api/fabricapi/DataGenerationSettings.java

@@ -24,9 +24,14 @@
 
 package net.fabricmc.loom.api.fabricapi;
 
+import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
 import org.gradle.api.file.RegularFileProperty;
 import org.gradle.api.provider.Property;
 
+import java.io.File;
+import java.io.IOException;
+import java.util.Objects;
+
 /**
  * Represents the settings for data generation.
  */
@@ -67,4 +72,15 @@ public interface DataGenerationSettings {
 	 * Contains a boolean property indicating whether data generation will be compiled and ran with the client.
 	 */
 	Property<Boolean> getClient();
+
+	/**
+	 * Sets {@link #getModId()} property based on the {@code id} field defined in the provided file.
+	 */
+	default void modId(File fabricModJsonFile) {
+        try {
+            getModId().set(Objects.requireNonNull(FabricModJsonFactory.createFromOverrideNullable(fabricModJsonFile)).getId());
+        } catch (IOException | NullPointerException e) {
+            throw new RuntimeException("Failed to set mod id from %s".formatted(fabricModJsonFile), e);
+        }
+    }
 }

src/main/java/net/fabricmc/loom/api/fabricapi/GameTestSettings.java

@@ -24,10 +24,15 @@
 
 package net.fabricmc.loom.api.fabricapi;
 
+import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
 import org.gradle.api.provider.Property;
 import org.gradle.api.tasks.Optional;
 import org.jetbrains.annotations.ApiStatus;
 
+import java.io.File;
+import java.io.IOException;
+import java.util.Objects;
+
 /**
  * Represents the settings for game and/or client tests.
  */
@@ -89,4 +94,15 @@ public interface GameTestSettings {
 	 */
 	@Optional
 	Property<String> getUsername();
+
+	/**
+	 * Sets {@link #getModId()} property based on the {@code id} field defined in the provided file.
+	 */
+	default void modId(File fabricModJsonFile) {
+		try {
+			getModId().set(Objects.requireNonNull(FabricModJsonFactory.createFromOverrideNullable(fabricModJsonFile)).getId());
+		} catch (IOException | NullPointerException e) {
+			throw new RuntimeException("Failed to set mod id from %s".formatted(fabricModJsonFile), e);
+		}
+	}
 }

src/main/java/net/fabricmc/loom/configuration/fabricapi/FabricApiAbstractSourceSet.java

@@ -24,10 +24,11 @@
 
 package net.fabricmc.loom.configuration.fabricapi;
 
-import java.io.IOException;
+import java.util.List;
 
 import javax.inject.Inject;
 
+import net.fabricmc.loom.util.fmj.FabricModJsonHelpers;
 import org.gradle.api.Project;
 import org.gradle.api.artifacts.ConfigurationContainer;
 import org.gradle.api.provider.Property;
@@ -37,7 +38,6 @@
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.configuration.providers.minecraft.MinecraftSourceSets;
 import net.fabricmc.loom.util.fmj.FabricModJson;
-import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
 
 abstract class FabricApiAbstractSourceSet {
@@ -64,17 +64,14 @@ protected SourceSet configureSourceSet(Property<String> modId, boolean isClient)
 		});
 
 		modId.convention(getProject().provider(() -> {
-			try {
-				final FabricModJson fabricModJson = FabricModJsonFactory.createFromSourceSetsNullable(getProject(), sourceSet);
+			List<FabricModJson> fabricModJsons = FabricModJsonHelpers
+				.getModsInProject(getProject(), null, sourceSet);
 
-				if (fabricModJson == null) {
-					throw new RuntimeException("Could not find a fabric.mod.json file in the data source set or a value for DataGenerationSettings.getModId()");
-				}
-
-				return fabricModJson.getId();
-			} catch (IOException e) {
-				throw new org.gradle.api.UncheckedIOException("Failed to read mod id from the datagen source set.", e);
+			if (fabricModJsons.isEmpty()) {
+				return null;
 			}
+
+			return fabricModJsons.getFirst().getId();
 		}));
 
 		extension.getMods().create(modId.get(), mod -> {

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionApiImpl.java

@@ -30,11 +30,11 @@
 import java.util.Map;
 import java.util.Set;
 
+import net.fabricmc.loom.util.fmj.FabricModJsonHelpers;
 import org.gradle.api.Action;
 import org.gradle.api.NamedDomainObjectContainer;
 import org.gradle.api.NamedDomainObjectList;
 import org.gradle.api.Project;
-import org.gradle.api.UncheckedIOException;
 import org.gradle.api.artifacts.Dependency;
 import org.gradle.api.file.ConfigurableFileCollection;
 import org.gradle.api.file.FileCollection;
@@ -73,8 +73,6 @@
 import net.fabricmc.loom.task.GenerateSourcesTask;
 import net.fabricmc.loom.util.DeprecationHelper;
 import net.fabricmc.loom.util.MirrorUtil;
-import net.fabricmc.loom.util.fmj.FabricModJson;
-import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
 
 /**
@@ -86,6 +84,7 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 	protected final ListProperty<JarProcessor> jarProcessors;
 	protected final ConfigurableFileCollection log4jConfigs;
 	protected final RegularFileProperty accessWidener;
+	protected final RegularFileProperty fabricModJsonPath;
 	protected final ManifestLocations versionsManifests;
 	protected final Property<String> customMetadata;
 	protected final SetProperty<String> knownIndyBsms;
@@ -118,6 +117,7 @@ protected LoomGradleExtensionApiImpl(Project project, LoomFiles directories) {
 				.empty();
 		this.log4jConfigs = project.files(directories.getDefaultLog4jConfigFile());
 		this.accessWidener = project.getObjects().fileProperty();
+		this.fabricModJsonPath = project.getObjects().fileProperty();
 		this.versionsManifests = new ManifestLocations();
 		this.versionsManifests.add("mojang", MirrorUtil.getVersionManifests(project), -2);
 		this.versionsManifests.add("fabric_experimental", MirrorUtil.getExperimentalVersions(project), -1);
@@ -205,6 +205,11 @@ public RegularFileProperty getAccessWidenerPath() {
 		return accessWidener;
 	}
 
+	@Override
+	public RegularFileProperty getFabricModJsonPath() {
+		return fabricModJsonPath;
+	}
+
 	@Override
 	public NamedDomainObjectContainer<DecompilerOptions> getDecompilerOptions() {
 		return decompilers;
@@ -293,17 +298,13 @@ public SetProperty<String> getKnownIndyBsms() {
 
 	@Override
 	public String getModVersion() {
-		try {
-			final FabricModJson fabricModJson = FabricModJsonFactory.createFromSourceSetsNullable(getProject(), SourceSetHelper.getMainSourceSet(getProject()));
+		var fabricModJsons = FabricModJsonHelpers.getModsInProject(getProject());
 
-			if (fabricModJson == null) {
-				throw new RuntimeException("Could not find a fabric.mod.json file in the main sourceset");
-			}
-
-			return fabricModJson.getModVersion();
-		} catch (IOException e) {
-			throw new UncheckedIOException("Failed to read mod version from main sourceset.", e);
+		if (fabricModJsons.isEmpty()) {
+			throw new RuntimeException("Could not find a fabric.mod.json file in the main sourceset");
 		}
+
+		return fabricModJsons.getFirst().getModVersion();
 	}
 
 	@Override

src/main/java/net/fabricmc/loom/util/fmj/FabricModJsonFactory.java

@@ -107,10 +107,29 @@ public static Optional<FabricModJson> createFromZipOptional(Path zipPath) {
 		return Optional.ofNullable(createFromZipNullable(zipPath));
 	}
 
+	@Nullable
+	public static FabricModJson createFromOverrideNullable(@Nullable File file) throws IOException {
+		var modJson = readFmjJsonObject(file);
+		if (modJson == null) {
+			return null;
+		}
+
+		return create(modJson, new FabricModJsonSource.DirectorySource(file.toPath().getParent()));
+	}
+
 	@Nullable
 	public static FabricModJson createFromSourceSetsNullable(Project project, SourceSet... sourceSets) throws IOException {
-		final File file = SourceSetHelper.findFirstFileInResource(FABRIC_MOD_JSON, project, sourceSets);
+		var file = SourceSetHelper.findFirstFileInResource(FABRIC_MOD_JSON, project, sourceSets);
+		var modJson = readFmjJsonObject(file);
+		if (modJson == null) {
+			return null;
+		}
 
+		return create(modJson, new FabricModJsonSource.SourceSetSource(project, sourceSets));
+	}
+
+	@Nullable
+	private static JsonObject readFmjJsonObject(@Nullable File file) {
 		if (file == null) {
 			return null;
 		}
@@ -121,10 +140,9 @@ public static FabricModJson createFromSourceSetsNullable(Project project, Source
 			if (modJson == null) {
 				// fromJson returns null if the file is empty
 				LOGGER.warn("Failed to parse empty fabric.mod.json: {}", file.getAbsolutePath());
-				return null;
 			}
 
-			return create(modJson, new FabricModJsonSource.SourceSetSource(project, sourceSets));
+			return modJson;
 		} catch (JsonSyntaxException e) {
 			LOGGER.warn("Failed to parse fabric.mod.json: {}", file.getAbsolutePath());
 			return null;

src/main/java/net/fabricmc/loom/util/fmj/FabricModJsonHelpers.java

@@ -24,39 +24,54 @@
 
 package net.fabricmc.loom.util.fmj;
 
+import java.io.File;
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
+import net.fabricmc.loom.api.LoomGradleExtensionAPI;
 import org.gradle.api.Project;
 import org.gradle.api.tasks.SourceSet;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
+import org.jetbrains.annotations.Nullable;
 
 public class FabricModJsonHelpers {
-	// Returns a list of Mods found in the provided project's main or client sourcesets
+	/**
+	 * Returns the list of mods provided by either {@link LoomGradleExtensionAPI#getFabricModJsonPath()}
+	 * or {@code fabric.mod.json} in main or client resources.
+	 */
 	public static List<FabricModJson> getModsInProject(Project project) {
-		final LoomGradleExtension extension = LoomGradleExtension.get(project);
+		LoomGradleExtension extension = LoomGradleExtension.get(project);
+		var overrideFile = extension.getFabricModJsonPath().getAsFile();
 		var sourceSets = new ArrayList<SourceSet>();
-		sourceSets.add(SourceSetHelper.getMainSourceSet(project));
 
+		sourceSets.add(SourceSetHelper.getMainSourceSet(project));
 		if (extension.areEnvironmentSourceSetsSplit()) {
 			sourceSets.add(SourceSetHelper.getSourceSetByName("client", project));
 		}
 
-		try {
-			final FabricModJson fabricModJson = FabricModJsonFactory.createFromSourceSetsNullable(project, sourceSets.toArray(SourceSet[]::new));
+		return getModsInProject(project, overrideFile.getOrNull(), sourceSets.toArray(SourceSet[]::new));
+	}
 
+	/**
+	 * Returns the list of mods provided by either {@code overrideFile} property
+	 * or {@code fabric.mod.json} in the {@code sourceSets} array.
+	 */
+	public static List<FabricModJson> getModsInProject(Project project, @Nullable File overrideFile, SourceSet... sourceSets) {
+		try {
+			var fabricModJson = overrideFile != null
+				? FabricModJsonFactory.createFromOverrideNullable(overrideFile)
+				: FabricModJsonFactory.createFromSourceSetsNullable(project, sourceSets);
 			if (fabricModJson != null) {
 				return List.of(fabricModJson);
 			}
 		} catch (IOException e) {
 			throw new UncheckedIOException(e);
 		}
-
 		return Collections.emptyList();
 	}
 }

src/test/groovy/net/fabricmc/loom/test/integration/FabricModJsonPathTest.groovy

@@ -0,0 +1,34 @@
+package net.fabricmc.loom.test.integration
+
+import net.fabricmc.loom.test.util.GradleProjectTestTrait
+import org.gradle.testkit.runner.BuildResult
+import spock.lang.Specification
+import spock.lang.Unroll
+
+import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
+
+class FabricModJsonPathTest extends Specification implements GradleProjectTestTrait {
+    @Unroll
+    def "Resolve custom FMJ"() {
+        setup:
+        GradleProject gradle = gradleProject(project: "fmjPathConfig")
+
+        when:
+        BuildResult result = gradle.run(task: "build", args: ["-PoverrideFMJ=true"])
+
+        then:
+        result.task(":build").outcome == SUCCESS
+    }
+
+    @Unroll
+    def "Fail to find FMJ"() {
+        setup:
+        GradleProject gradle = gradleProject(project: "fmjPathConfig")
+
+        when:
+        BuildResult result = gradle.run(task: "build", expectFailure: true)
+
+        then:
+        result.task(":build") == null
+    }
+}

src/test/resources/projects/fmjPathConfig/build.gradle

@@ -0,0 +1,38 @@
+// This is used by a range of tests that append to this file before running the gradle tasks.
+// Can be used for tests that require minimal custom setup
+plugins {
+    id 'fabric-loom'
+    id 'maven-publish'
+}
+
+version = "1.0.0"
+group = "com.example"
+
+// In multi-version setup this would be a separate project,
+// but a source set will suffice for a test.
+sourceSets {
+    custom {
+
+    }
+    main {
+        compileClasspath += sourceSets.custom.compileClasspath
+        runtimeClasspath += sourceSets.custom.runtimeClasspath
+    }
+}
+
+
+dependencies {
+    minecraft "com.mojang:minecraft:1.17.1"
+    mappings "net.fabricmc:yarn:1.17.1+build.59:v2"
+    modImplementation "net.fabricmc:fabric-loader:0.11.6"
+}
+
+base {
+    archivesName = "fabric-example-mod"
+}
+
+if (project.hasProperty("overrideFMJ")) {
+    loom {
+        fabricModJsonPath = file("src/custom/resources/fabric.mod.json")
+    }
+}

src/test/resources/projects/fmjPathConfig/src/custom/resources/fabric.mod.json

@@ -0,0 +1,11 @@
+{
+  "schemaVersion": 1,
+  "id": "testmod",
+  "version": "1",
+  "name": "Test Mod",
+  "custom": {
+    "loom:injected_interfaces": {
+      "net/minecraft/class_310": ["InjectedInterface"]
+    }
+  }
+}

src/test/resources/projects/fmjPathConfig/src/main/java/ExampleMod.java

@@ -0,0 +1,10 @@
+import net.minecraft.client.MinecraftClient;
+
+import net.fabricmc.api.ModInitializer;
+
+public class ExampleMod implements ModInitializer {
+    @Override
+    public void onInitialize() {
+        MinecraftClient.getInstance().newMethodThatDidNotExist();
+    }
+}

src/test/resources/projects/fmjPathConfig/src/main/java/InjectedInterface.java

@@ -0,0 +1,4 @@
+public interface InjectedInterface {
+    default void newMethodThatDidNotExist() {
+    }
+}