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

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

@@ -66,6 +66,22 @@ public interface LoomGradleExtensionAPI {
 
 	RegularFileProperty getAccessWidenerPath();
 
+	/**
+	 * Specifies the {@code fabric.mod.json} file location used in injected interface processing.
+	 *
+	 * <p>
+	 *     By default, either {@code src/main/resources/fabric.mod.json}
+	 *     or {@code src/client/resources/fabric.mod.json} in the project directory is used.
+	 * </p>
+	 *
+	 * <p>
+	 *     Providing a path to a different location allows using a shared or preprocessed
+	 *     file. However, at the end it must be put into the root of the processed
+	 *     resources directory (usually {@code build/resources/main}).
+	 * </p>
+	 */
+	RegularFileProperty getFabricModJsonPath();
+
 	NamedDomainObjectContainer<DecompilerOptions> getDecompilerOptions();
 
 	void decompilers(Action<NamedDomainObjectContainer<DecompilerOptions>> action);

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

@@ -24,9 +24,13 @@
 
 package net.fabricmc.loom.api.fabricapi;
 
+import java.io.File;
+
 import org.gradle.api.file.RegularFileProperty;
 import org.gradle.api.provider.Property;
 
+import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
+
 /**
  * Represents the settings for data generation.
  */
@@ -67,4 +71,11 @@ 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) {
+		getModId().set(FabricModJsonFactory.createFromFile(fabricModJsonFile).getId());
+	}
 }

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

@@ -24,10 +24,14 @@
 
 package net.fabricmc.loom.api.fabricapi;
 
+import java.io.File;
+
 import org.gradle.api.provider.Property;
 import org.gradle.api.tasks.Optional;
 import org.jetbrains.annotations.ApiStatus;
 
+import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
+
 /**
  * Represents the settings for game and/or client tests.
  */
@@ -89,4 +93,11 @@ 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) {
+		getModId().set(FabricModJsonFactory.createFromFile(fabricModJsonFile).getId());
+	}
 }

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

@@ -24,8 +24,6 @@
 
 package net.fabricmc.loom.configuration.fabricapi;
 
-import java.io.IOException;
-
 import javax.inject.Inject;
 
 import org.gradle.api.Project;
@@ -64,17 +62,13 @@ protected SourceSet configureSourceSet(Property<String> modId, boolean isClient)
 		});
 
 		modId.convention(getProject().provider(() -> {
-			try {
-				final FabricModJson fabricModJson = FabricModJsonFactory.createFromSourceSetsNullable(getProject(), 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()");
-				}
+			final FabricModJson fabricModJson = FabricModJsonFactory.createFromSourceSetsNullable(getProject(), sourceSet);
 
-				return fabricModJson.getId();
-			} catch (IOException e) {
-				throw new org.gradle.api.UncheckedIOException("Failed to read mod id from the datagen source set.", e);
+			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();
 		}));
 
 		extension.getMods().create(modId.get(), mod -> {

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

@@ -25,7 +25,6 @@
 package net.fabricmc.loom.extension;
 
 import java.io.File;
-import java.io.IOException;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
@@ -34,7 +33,6 @@
 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 +71,7 @@
 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.fmj.FabricModJsonHelpers;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
 
 /**
@@ -86,6 +83,7 @@ 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 +116,7 @@ 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 +204,11 @@ public RegularFileProperty getAccessWidenerPath() {
 		return accessWidener;
 	}
 
+	@Override
+	public RegularFileProperty getFabricModJsonPath() {
+		return fabricModJsonPath;
+	}
+
 	@Override
 	public NamedDomainObjectContainer<DecompilerOptions> getDecompilerOptions() {
 		return decompilers;
@@ -293,17 +297,13 @@ 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

@@ -107,27 +107,38 @@ public static Optional<FabricModJson> createFromZipOptional(Path zipPath) {
 		return Optional.ofNullable(createFromZipNullable(zipPath));
 	}
 
+	public static FabricModJson createFromFile(File file) {
+		var modJson = readFmjJsonObject(file);
+		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);
+	public static FabricModJson createFromSourceSetsNullable(Project project, SourceSet... sourceSets) {
+		var file = SourceSetHelper.findFirstFileInResource(FABRIC_MOD_JSON, project, sourceSets);
 
 		if (file == null) {
 			return null;
 		}
 
+		try {
+			var modJson = readFmjJsonObject(file);
+			return create(modJson, new FabricModJsonSource.SourceSetSource(project, sourceSets));
+		} catch (JsonSyntaxException e) {
+			LOGGER.warn("Failed to parse fabric.mod.json: {}", file.getAbsolutePath());
+			return null;
+		}
+	}
+
+	private static JsonObject readFmjJsonObject(File file) {
 		try (Reader reader = Files.newBufferedReader(file.toPath(), StandardCharsets.UTF_8)) {
 			final JsonObject modJson = LoomGradlePlugin.GSON.fromJson(reader, JsonObject.class);
 
 			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));
-		} catch (JsonSyntaxException e) {
-			LOGGER.warn("Failed to parse fabric.mod.json: {}", file.getAbsolutePath());
-			return null;
+			return modJson;
 		} catch (IOException e) {
 			throw new UncheckedIOException("Failed to read " + file.getAbsolutePath(), e);
 		}

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

@@ -24,37 +24,41 @@
 
 package net.fabricmc.loom.util.fmj;
 
-import java.io.IOException;
-import java.io.UncheckedIOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
 import org.gradle.api.Project;
 import org.gradle.api.tasks.SourceSet;
 
-import net.fabricmc.loom.LoomGradleExtension;
+import net.fabricmc.loom.api.LoomGradleExtensionAPI;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
+import net.fabricmc.loom.LoomGradleExtension;
 
 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);
+		var overrideFile = extension.getFabricModJsonPath().getAsFile();
+
+		if (overrideFile.isPresent()) {
+			return List.of(FabricModJsonFactory.createFromFile(overrideFile.get()));
+		}
+
 		var sourceSets = new ArrayList<SourceSet>();
 		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));
+		final FabricModJson fabricModJson = FabricModJsonFactory.createFromSourceSetsNullable(project, sourceSets.toArray(SourceSet[]::new));
 
-			if (fabricModJson != null) {
-				return List.of(fabricModJson);
-			}
-		} catch (IOException e) {
-			throw new UncheckedIOException(e);
+		if (fabricModJson != null) {
+			return List.of(fabricModJson);
 		}
 
 		return Collections.emptyList();

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

@@ -0,0 +1,59 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration
+
+import org.gradle.testkit.runner.BuildResult
+import spock.lang.Specification
+import spock.lang.Unroll
+
+import net.fabricmc.loom.test.util.GradleProjectTestTrait
+
+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 +1,37 @@
+// 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"]
+    }
+  }
+}