launcher/launcher/src/main/java/sashegdev/zernmc/launcher/minecraft/launch/LaunchCommandBuilder.java

@@ -63,13 +63,19 @@ public class LaunchCommandBuilder {
             // always use ModLauncher as main class (version.json might have
             // BootstrapLauncher which breaks with Java module system)
             if ("forge".equals(loaderType) || "neoforge".equals(loaderType)) {
-                // Forge: vanilla version jar on -cp creates automatic module _1._20._1
+                // Ensure securejarhandler can create the "minecraft" module:
-                // which conflicts with Forge's "minecraft" module → split-package error.
+                // It looks for versions/<versionId>/<versionId>.jar.
-                // Fix: use patched Forge client jar instead of vanilla version jar.
+                // The Forge installer only creates the JSON there, not the jar.
-                // The patched jar (forge-1.20.1-47.4.20-client.jar) is created by the
+                // We copy the vanilla version jar (e.g. versions/1.20.1/1.20.1.jar)
-                // Forge installer in libraries/ and contains all MC classes including
+                // to that location so securejarhandler finds it and creates the
-                // PreparableReloadListener. It is NOT listed in version.json manifest,
+                // "minecraft" module (identified by Automatic-Module-Name: minecraft
-                // so we find and add it separately.
+                // in the jar's MANIFEST.MF).
+                ensureVersionJarForForge();
+
+                // Build classpath from manifest libraries only (no version jar on -cp).
+                // Putting the vanilla jar on -cp would create automatic module _1._20._1
+                // (from filename 1.20.1.jar) which conflicts with the "minecraft" module
+                // created by securejarhandler → split-package ResolutionException.
                 String classpath;
                 if (manifest != null) {
                     classpath = buildClasspathFromManifest(manifest, false);
@@ -80,21 +86,6 @@ public class LaunchCommandBuilder {
                     classpath = buildVanillaClasspath();
                 }
 
-                // Find and add the patched Forge client jar
-                Path patchedJar = findPatchedForgeJar();
-                if (patchedJar != null) {
-                    String sep = System.getProperty("os.name").toLowerCase().contains("win") ? ";" : ":";
-                    classpath = patchedJar.toAbsolutePath().toString() + sep + classpath;
-                    System.out.println(ZAnsi.green("  Added patched Forge jar: " + patchedJar.getFileName()));
-                } else {
-                    System.out.println(ZAnsi.yellow("  No patched Forge client jar found, falling back to vanilla version jar"));
-                    Path versionJar = findVersionJar();
-                    if (versionJar != null && isValidJar(versionJar)) {
-                        String sep = System.getProperty("os.name").toLowerCase().contains("win") ? ";" : ":";
-                        classpath = versionJar.toAbsolutePath().toString() + sep + classpath;
-                    }
-                }
-
                 command.add("-cp");
                 String cpFile = writeClasspathFile(classpath);
                 command.add(cpFile);
@@ -102,8 +93,7 @@ public class LaunchCommandBuilder {
                 // Add JVM arguments from version.json manifest as-is.
                 // Forge's -p is a HARDCODED list of 8 bootstrap JARs (bootstraplauncher,
                 // securejarhandler, asm-*, JarJarFileSystems) — NOT ${classpath}.
-                // Do NOT intercept it. Version jar stays on -cp only (not -p),
+                // Do NOT intercept it.
-                // preventing _1._20._1 vs minecraft split-package conflicts.
                 if (manifest != null) {
                     Map<String, String> vars = buildVariableMap(options);
                     vars.put("classpath", cpFile);
@@ -657,6 +647,47 @@ public class LaunchCommandBuilder {
         return null;
     }
 
+    /**
+     * Ensure the version jar exists at versions/<versionId>/<versionId>.jar
+     * so that securejarhandler can create the "minecraft" module from it.
+     *
+     * The Forge installer only creates the JSON at that path, not the jar.
+     * We copy the vanilla version jar (e.g. versions/1.20.1/1.20.1.jar) there.
+     *
+     * securejarhandler reads Automatic-Module-Name: minecraft from the jar's
+     * MANIFEST.MF and creates a named "minecraft" module. This is the module
+     * that Forge's code expects classes like PreparableReloadListener to be in.
+     *
+     * Critical: the jar is NOT put on -cp. Only the copy at versions/<versionId>/
+     * is used by securejarhandler. The original 1.20.1.jar must stay off the
+     * classpath to prevent automatic module _1._20._1 from being created,
+     * which would conflict with the "minecraft" module (split-package error).
+     */
+    private void ensureVersionJarForForge() throws IOException {
+        String versionId = getVersionId();
+        String mcVersion = instance.getMinecraftVersion();
+        Path versionsDir = instance.getPath().resolve("versions");
+
+        Path targetDir = versionsDir.resolve(versionId);
+        Path targetJar = targetDir.resolve(versionId + ".jar");
+
+        if (Files.exists(targetJar) && isValidJar(targetJar)) {
+            System.out.println(ZAnsi.green("  Version jar already exists: " + targetJar));
+            return;
+        }
+
+        // Source: the vanilla version jar
+        Path sourceJar = versionsDir.resolve(mcVersion).resolve(mcVersion + ".jar");
+        if (!Files.exists(sourceJar) || !isValidJar(sourceJar)) {
+            System.out.println(ZAnsi.yellow("  Vanilla version jar not found at " + sourceJar));
+            return;
+        }
+
+        Files.createDirectories(targetDir);
+        Files.copy(sourceJar, targetJar);
+        System.out.println(ZAnsi.green("  Copied version jar for securejarhandler: " + sourceJar.getFileName() + " → " + targetJar));
+    }
+
     /**
      * Find the patched Forge client jar in libraries/.
      * The Forge installer creates it via binarypatcher processor: