Add nestJars API for nesting locally built mod jars (#1427)

* Add nestJars API for FileCollection support

* Add missin new-line

* Fix imports

* Rework constructor to avoid ugliness

* Update java docs

* Update java docs

* Implement asked improvements

* Fix checkstyle

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

@@ -40,6 +40,8 @@ import org.gradle.api.provider.Provider;
 import org.gradle.api.provider.SetProperty;
 import org.gradle.api.publish.maven.MavenPublication;
 import org.gradle.api.tasks.SourceSet;
+import org.gradle.api.tasks.TaskProvider;
+import org.gradle.jvm.tasks.Jar;
 import org.jetbrains.annotations.ApiStatus;
 
 import net.fabricmc.loom.api.decompilers.DecompilerOptions;
@@ -272,4 +274,26 @@ public interface LoomGradleExtensionAPI {
 	 * @return A lazily evaluated {@link FileCollection} containing the named minecraft jars.
 	 */
 	FileCollection getNamedMinecraftJars();
+
+	/**
+	 * Nest mod jars from a {@link FileCollection} into the specified jar task.
+	 * This is useful for including locally built mod jars or jars that don't come from Maven.
+	 *
+	 * <p>Important: The jars must already be valid mod jars (containing a fabric.mod.json file).
+	 * Non-mod jars will be rejected.
+	 *
+	 * <p>Example usage:
+	 * {@snippet lang=groovy :
+	 * loom {
+	 *     nestJars(tasks.jar, files('local-mod.jar'))
+	 *     nestJars(tasks.remapJar, tasks.named('buildOtherMod'))
+	 * }
+	 * }
+	 *
+	 * @param jarTask the jar task to nest jars into (can be jar or remapJar)
+	 * @param jars the file collection containing mod jars to nest
+	 * @since 1.14
+	 */
+	@ApiStatus.Experimental
+	void nestJars(TaskProvider<? extends Jar> jarTask, FileCollection jars);
 }

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

@@ -45,6 +45,8 @@ import org.gradle.api.provider.Provider;
 import org.gradle.api.provider.SetProperty;
 import org.gradle.api.publish.maven.MavenPublication;
 import org.gradle.api.tasks.SourceSet;
+import org.gradle.api.tasks.TaskProvider;
+import org.gradle.jvm.tasks.Jar;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.api.InterfaceInjectionExtensionAPI;
@@ -543,5 +545,10 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		public MixinExtension getMixin() {
 			throw new RuntimeException("Yeah... something is really wrong");
 		}
+
+		@Override
+		public void nestJars(TaskProvider<? extends Jar> jarTask, FileCollection jars) {
+			throw new RuntimeException("Yeah... something is really wrong");
+		}
 	}
 }

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

@@ -40,6 +40,8 @@ import org.gradle.api.file.ConfigurableFileCollection;
 import org.gradle.api.file.FileCollection;
 import org.gradle.api.provider.ListProperty;
 import org.gradle.api.provider.Property;
+import org.gradle.api.tasks.TaskProvider;
+import org.gradle.jvm.tasks.Jar;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.LoomNoRemapGradlePlugin;
@@ -57,6 +59,8 @@ import net.fabricmc.loom.configuration.providers.minecraft.MinecraftProvider;
 import net.fabricmc.loom.configuration.providers.minecraft.library.LibraryProcessorManager;
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.IntermediaryMinecraftProvider;
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.NamedMinecraftProvider;
+import net.fabricmc.loom.task.NestJarsAction;
+import net.fabricmc.loom.task.RemapJarTask;
 import net.fabricmc.loom.util.Constants;
 import net.fabricmc.loom.util.download.Download;
 import net.fabricmc.loom.util.download.DownloadBuilder;
@@ -339,4 +343,17 @@ public abstract class LoomGradleExtensionImpl extends LoomGradleExtensionApiImpl
 	public boolean disableObfuscation() {
 		return disableObfuscation.get();
 	}
+
+	@Override
+	public void nestJars(TaskProvider<? extends Jar> jarTask, FileCollection jars) {
+		jarTask.configure(task -> {
+			if (task instanceof RemapJarTask remapJarTask) {
+				// For RemapJarTask, add to the nestedJars property
+				remapJarTask.getNestedJars().from(jars);
+			} else {
+				// For regular Jar tasks (non-remap mode), add a NestJarsAction with the FileCollection
+				task.doLast(new NestJarsAction(jars));
+			}
+		});
+	}
 }

src/main/java/net/fabricmc/loom/task/NestJarsAction.java

@@ -26,12 +26,12 @@ package net.fabricmc.loom.task;
 
 import java.io.File;
 import java.io.Serializable;
-import java.util.Arrays;
+import java.util.ArrayList;
+import java.util.List;
 
 import org.gradle.api.Action;
 import org.gradle.api.Task;
-import org.gradle.api.file.Directory;
-import org.gradle.api.provider.Provider;
+import org.gradle.api.file.FileCollection;
 import org.gradle.jvm.tasks.Jar;
 import org.jetbrains.annotations.NotNull;
 
@@ -39,38 +39,26 @@ import net.fabricmc.loom.build.nesting.JarNester;
 
 /**
  * Configuration-cache-compatible action for nesting jars.
- * Uses a provider to avoid capturing task references at configuration time.
+ * Uses a FileCollection to avoid capturing task references at configuration time.
  * Do NOT turn me into a record!
  */
-class NestJarsAction implements Action<Task>, Serializable {
-	private final Provider<Directory> nestedJarsDir;
+public class NestJarsAction implements Action<Task>, Serializable {
+	private final FileCollection jars;
 
-	NestJarsAction(Provider<Directory> nestedJarsDir) {
-		this.nestedJarsDir = nestedJarsDir;
+	public NestJarsAction(FileCollection jars) {
+		this.jars = jars;
 	}
 
 	@Override
 	public void execute(@NotNull Task t) {
 		final Jar jarTask = (Jar) t;
 		final File jarFile = jarTask.getArchiveFile().get().getAsFile();
+		final List<File> allJars = new ArrayList<>(jars.getFiles());
 
-		if (!nestedJarsDir.isPresent()) {
-			return;
-		}
-
-		final File outputDir = nestedJarsDir.get().getAsFile();
-
-		if (outputDir.exists() && outputDir.isDirectory()) {
-			final File[] jars = outputDir.listFiles((dir, name) -> name.endsWith(".jar"));
-
-			if (jars != null && jars.length > 0) {
-				JarNester.nestJars(
-						Arrays.asList(jars),
-						jarFile,
-						jarTask.getLogger()
-				);
-				jarTask.getLogger().lifecycle("Nested {} jar(s) into {}", jars.length, jarFile.getName());
-			}
+		// Nest all collected jars
+		if (!allJars.isEmpty()) {
+			JarNester.nestJars(allJars, jarFile, jarTask.getLogger());
+			jarTask.getLogger().lifecycle("Nested {} jar(s) into {}", allJars.size(), jarFile.getName());
 		}
 	}
-}
+}

src/main/java/net/fabricmc/loom/task/NonRemappedJarTaskConfiguration.java

@@ -52,10 +52,11 @@ public class NonRemappedJarTaskConfiguration {
 		project.getTasks().named(JavaPlugin.JAR_TASK_NAME, Jar.class).configure(task -> {
 			task.dependsOn(processIncludeJarsTask);
 			// Use JarNester to properly add jars and update fabric.mod.json
-			task.doLast(new NestJarsAction(processIncludeJarsTask.flatMap(NestableJarGenerationTask::getOutputDirectory)));
+			task.doLast(new NestJarsAction(project.fileTree(processIncludeJarsTask.flatMap(NestableJarGenerationTask::getOutputDirectory))
+					.matching(pattern -> pattern.include("*.jar"))));
 		});
 
 		// Add jar task to unmapped collection
 		extension.getUnmappedModCollection().from(project.getTasks().getByName(JavaPlugin.JAR_TASK_NAME));
 	}
-}
+}

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

@@ -0,0 +1,54 @@
+/*
+ * 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 spock.lang.Specification
+import spock.lang.Unroll
+
+import net.fabricmc.loom.test.util.GradleProjectTestTrait
+
+import static net.fabricmc.loom.test.LoomTestConstants.STANDARD_TEST_VERSIONS
+import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
+
+class NestJarsApiTest extends Specification implements GradleProjectTestTrait {
+	@Unroll
+	def "nest jars using loom.nestJars() API (gradle #version)"() {
+		setup:
+		def gradle = gradleProject(project: "nestJarsApi", version: version)
+
+		when:
+		def result = gradle.run(tasks: ["remapJar"])
+
+		then:
+		result.task(":remapJar").outcome == SUCCESS
+		result.task(":createNestedMod").outcome == SUCCESS
+
+		// Assert the locally built mod jar is nested using the new API
+		gradle.hasOutputZipEntry("nestJarsApi.jar", "META-INF/jars/nested-mod.jar")
+
+		where:
+		version << STANDARD_TEST_VERSIONS
+	}
+}

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

@@ -0,0 +1,30 @@
+plugins {
+	id 'fabric-loom'
+}
+
+repositories {
+	mavenCentral()
+}
+
+dependencies {
+	minecraft 'com.mojang:minecraft:1.20.1'
+	mappings loom.officialMojangMappings()
+}
+
+// Create a simple mod jar file to nest
+task createNestedMod(type: Jar) {
+	archiveBaseName = 'nested-mod'
+	destinationDirectory = layout.buildDirectory.dir('nested-mods')
+
+	from('nested-mod-content') {
+		include 'fabric.mod.json'
+	}
+}
+
+// Use the new nestJars API to nest the locally built jar
+loom {
+	nestJars(tasks.named('remapJar'), files(createNestedMod.outputs.files))
+}
+
+// Make sure remapJar depends on the nested mod being created
+tasks.remapJar.dependsOn(createNestedMod)

src/test/resources/projects/nestJarsApi/nested-mod-content/fabric.mod.json

@@ -0,0 +1,8 @@
+{
+  "schemaVersion": 1,
+  "id": "nestedmod",
+  "version": "1.0.0",
+  "name": "Nested Mod",
+  "description": "This mod will be nested using the nestJars API",
+  "environment": "*"
+}

src/test/resources/projects/nestJarsApi/settings.gradle

@@ -0,0 +1 @@
+rootProject.name = 'nestJarsApi'

src/test/resources/projects/nestJarsApi/src/main/resources/fabric.mod.json

@@ -0,0 +1,8 @@
+{
+  "schemaVersion": 1,
+  "id": "mainmod",
+  "version": "1.0.0",
+  "name": "Main Mod",
+  "description": "Main mod that nests another mod using nestJars API",
+  "environment": "*"
+}