Add a subproject with a JUnit session listener

junit/build.gradle

@@ -0,0 +1,9 @@
+plugins {
+    id 'java-library'
+}
+
+dependencies {
+    implementation('org.junit.platform:junit-platform-launcher:1.9.3')
+    // BSL should not be exposed and the actual version should be provided by the neo dep
+    compileOnly("cpw.mods:bootstraplauncher:${project.bootstraplauncher_version}")
+}

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

@@ -0,0 +1,13 @@
+/*
+ * 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.fancymodlauncher.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 +1,27 @@
+/*
+ * 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;
+
+public class JUnitService implements LauncherSessionListener {
+    private ClassLoader oldLoader;
+    public JUnitService() {
+
+    }
+
+    @Override
+    public void launcherSessionOpened(LauncherSession session) {
+        oldLoader = Thread.currentThread().getContextClassLoader();
+        Thread.currentThread().setContextClassLoader(LaunchWrapper.getTransformingLoader());
+    }
+
+    @Override
+    public void launcherSessionClosed(LauncherSession session) {
+        Thread.currentThread().setContextClassLoader(oldLoader);
+    }
+}

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

@@ -0,0 +1,34 @@
+/*
+ * 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;
+
+    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.main(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/src/main/java/net/neoforged/fml/loading/targets/JUnitDevLaunchTarget.java

@@ -0,0 +1,25 @@
+/*
+ * 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;
+
+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 "junitfmldev";
+    }
+}

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

@@ -0,0 +1,25 @@
+/*
+ * 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;
+
+public class JUnitUserDevLaunchTarget extends ForgeUserdevLaunchHandler {
+    @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 "junitfmluserdev";
+    }
+}

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

@@ -6,3 +6,5 @@ net.neoforged.fml.loading.targets.ForgeServerDevLaunchHandler
 net.neoforged.fml.loading.targets.ForgeServerUserdevLaunchHandler
 net.neoforged.fml.loading.targets.ForgeDataDevLaunchHandler
 net.neoforged.fml.loading.targets.ForgeDataUserdevLaunchHandler
+net.neoforged.fml.loading.targets.JUnitDevLaunchTarget
+net.neoforged.fml.loading.targets.JUnitUserDevLaunchTarget

settings.gradle

@@ -31,6 +31,7 @@ include 'loader'
 include 'core'
 include 'events'
 include 'earlydisplay'
+include 'junit'
 
 includeLanguage 'minecraft'
 includeLanguage 'java'