Work

junit/build.gradle

@@ -1,8 +1,9 @@
 plugins {
-    id 'java'
+    id 'java-library'
 }
 
 dependencies {
     implementation('org.junit.platform:junit-platform-launcher:1.9.3')
-    implementation('cpw.mods:bootstraplauncher:1.1.8-test')
+    // 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

@@ -1,3 +1,8 @@
+/*
+ * 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;
 

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

@@ -1,28 +1,23 @@
+/*
+ * 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 org.junit.platform.launcher.LauncherSession;
 import org.junit.platform.launcher.LauncherSessionListener;
 
-import java.nio.file.Files;
-import java.nio.file.Path;
-
 public class JUnitService implements LauncherSessionListener {
     private ClassLoader oldLoader;
     public JUnitService() {
+
     }
 
     @Override
     public void launcherSessionOpened(LauncherSession session) {
         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);
-        } catch (Exception exception) {
-            System.err.println("Failed to start Minecraft: " + exception);
-            throw new RuntimeException(exception);
-        }
+        Thread.currentThread().setContextClassLoader(LaunchWrapper.getTransformingLoader());
     }
 
     @Override

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;
+    }
+}

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

@@ -1,11 +1,16 @@
+/*
+ * 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.CLIENT;
+        return Dist.DEDICATED_SERVER;
     }
 
     @Override

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

@@ -1,11 +1,16 @@
+/*
+ * 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.CLIENT;
+        return Dist.DEDICATED_SERVER;
     }
 
     @Override