Add a subproject with a JUnit session listener

build.gradle

@@ -75,19 +75,22 @@ allprojects {
 subprojects { subProject ->
     subProject.version = rootProject.version
 
-    jar.doFirst {
+    jar {
         manifest.attributes(
                 'Git-Commit'  : gradleutils.gitInfo.abbreviatedId,
                 'Build-Number': "${subProject.version}",
                 'Automatic-Module-Name' : "fml_${subProject.name.replace("-", "_")}",
-                'FMLModType'  : 'LIBRARY',
                 'Specification-Title'   : "FML${subProject.name}",
                 'Specification-Vendor'  : 'NeoForged',
                 'Specification-Version' : "${subProject.version.toString().split('\\.')[0]}",
                 'Implementation-Title'  : "FML $subProject.name",
                 'Implementation-Version': "${subProject.version.toString().split('\\.')[0]}.${subProject.version.toString().split('\\.')[1]}",
                 'Implementation-Vendor' : 'NeoForged'
         )
+
+        if (subProject.name != 'junit') {
+            manifest.attributes.put('FMLModType', 'LIBRARY')
+        }
     }
 
     tasks.withType(JavaCompile) {

junit/build.gradle

@@ -0,0 +1,20 @@
+plugins {
+    id 'java-library'
+}
+
+archivesBaseName = 'junit-fml'
+
+dependencies {
+    implementation(platform("org.junit:junit-bom:$jupiter_version"))
+    implementation('org.junit.platform:junit-platform-launcher')
+    // BSL should not be exposed and the actual version should be provided by the neo dep
+    compileOnly("cpw.mods:bootstraplauncher:${project.bootstraplauncher_version}")
+}
+
+publishing {
+    publications {
+        maven(MavenPublication) {
+            artifactId = archivesBaseName
+        }
+    }
+}

junit/src/main/java/module-info.java

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) Forge Development LLC and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+import net.neoforged.fml.junit.JUnitService;
+import org.junit.platform.launcher.LauncherSessionListener;
+
+module net.neoforged.fml.junit {
+    requires org.junit.platform.launcher;
+    requires cpw.mods.bootstraplauncher;
+
+    provides LauncherSessionListener with JUnitService;
+}

junit/src/main/java/net/neoforged/fml/junit/JUnitService.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) Forge Development LLC and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.fml.junit;
+
+import org.junit.platform.launcher.LauncherSession;
+import org.junit.platform.launcher.LauncherSessionListener;
+
+/**
+ * A session listener for JUnit environments that will bootstrap a Minecraft (FML) environment.
+ */
+public class JUnitService implements LauncherSessionListener {
+    private ClassLoader oldLoader;
+
+    public JUnitService() {}
+
+    @Override
+    public void launcherSessionOpened(LauncherSession session) {
+        // When the tests are started we want to make sure that they run on the transforming class loader which is set up by
+        // bootstrapping BSL which will then load the launch target
+        oldLoader = Thread.currentThread().getContextClassLoader();
+        Thread.currentThread().setContextClassLoader(LaunchWrapper.getTransformingLoader());
+    }
+
+    @Override
+    public void launcherSessionClosed(LauncherSession session) {
+        // Reset the loader in case JUnit wants to execute some pre-shutdown commands
+        // and our custom class loader might throw it off
+        Thread.currentThread().setContextClassLoader(oldLoader);
+    }
+}

junit/src/main/java/net/neoforged/fml/junit/LaunchWrapper.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) Forge Development LLC and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.fml.junit;
+
+import cpw.mods.bootstraplauncher.BootstrapLauncher;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+public class LaunchWrapper {
+    private static ClassLoader transformingCL;
+
+    /**
+     * Lazily get the transforming module class loader used by the game at runtime by loading the game when needed.
+     * <p>
+     * Since in a JUnit environment we can't easily get the launch arguments, we will load them from the file specified via the {@code fml.junit.argsfile} system property.
+     */
+    public static synchronized ClassLoader getTransformingLoader() {
+        if (transformingCL != null) return transformingCL;
+        final var oldLoader = Thread.currentThread().getContextClassLoader();
+
+        try {
+            final String[] args = Files.readAllLines(Path.of(System.getProperty("fml.junit.argsfile", "mainargs.txt"))).toArray(String[]::new);
+            BootstrapLauncher.unitTestingMain(args);
+
+            transformingCL = Thread.currentThread().getContextClassLoader();
+        } catch (Exception exception) {
+            System.err.println("Failed to start Minecraft: " + exception);
+            throw new RuntimeException(exception);
+        } finally {
+            Thread.currentThread().setContextClassLoader(oldLoader);
+        }
+
+        return transformingCL;
+    }
+}

junit/src/main/resources/META-INF/services/org.junit.platform.launcher.LauncherSessionListener

@@ -0,0 +1 @@
+net.neoforged.fml.junit.JUnitService

loader/build.gradle

@@ -51,6 +51,11 @@ dependencies {
     testImplementation("org.assertj:assertj-core:3.25.3")
     testImplementation("org.junit.jupiter:junit-jupiter-engine:$jupiter_version")
     testImplementation('com.google.jimfs:jimfs:1.3.0')
+
+    // Provides the JUnit project as a BOM entry
+    constraints {
+        api(project(':junit'))
+    }
 }
 
 spotless {

loader/src/main/java/net/neoforged/fml/loading/targets/JUnitDevLaunchTarget.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) Forge Development LLC and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.fml.loading.targets;
+
+import net.neoforged.api.distmarker.Dist;
+
+/**
+ * A launch target for bootstrapping a slim Minecraft environment in forgedev, to be used in JUnit tests.
+ */
+public class JUnitDevLaunchTarget extends CommonDevLaunchHandler {
+    @Override
+    public Dist getDist() {
+        return Dist.DEDICATED_SERVER;
+    }
+
+    @Override
+    protected void runService(String[] arguments, ModuleLayer gameLayer) throws Throwable {
+        Class.forName(gameLayer.findModule("neoforge").orElseThrow(), "net.neoforged.neoforge.junit.JUnitMain").getMethod("main", String[].class).invoke(null, (Object) arguments);
+    }
+
+    @Override
+    public String name() {
+        return "forgejunitdev";
+    }
+}

loader/src/main/java/net/neoforged/fml/loading/targets/JUnitUserDevLaunchTarget.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) Forge Development LLC and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.fml.loading.targets;
+
+import net.neoforged.api.distmarker.Dist;
+
+/**
+ * A launch target for bootstrapping a slim Minecraft environment in userdev, to be used in JUnit tests.
+ */
+public class JUnitUserDevLaunchTarget extends NeoForgeUserdevLaunchHandler {
+    @Override
+    public Dist getDist() {
+        return Dist.DEDICATED_SERVER;
+    }
+
+    @Override
+    protected void runService(String[] arguments, ModuleLayer gameLayer) throws Throwable {
+        Class.forName(gameLayer.findModule("neoforge").orElseThrow(), "net.neoforged.neoforge.junit.JUnitMain").getMethod("main", String[].class).invoke(null, (Object) arguments);
+    }
+
+    @Override
+    public String name() {
+        return "forgejunituserdev";
+    }
+}

loader/src/main/resources/META-INF/services/cpw.mods.modlauncher.api.ILaunchHandlerService

@@ -6,3 +6,5 @@ net.neoforged.fml.loading.targets.NeoForgeServerDevLaunchHandler
 net.neoforged.fml.loading.targets.NeoForgeServerUserdevLaunchHandler
 net.neoforged.fml.loading.targets.NeoForgeDataDevLaunchHandler
 net.neoforged.fml.loading.targets.NeoForgeDataUserdevLaunchHandler
+net.neoforged.fml.loading.targets.JUnitDevLaunchTarget
+net.neoforged.fml.loading.targets.JUnitUserDevLaunchTarget

settings.gradle

@@ -24,3 +24,4 @@ rootProject.name = 'FancyModLoader'
 
 include 'loader'
 include 'earlydisplay'
+include 'junit'