Optimize gencode <clinit> handling of FileDescriptorProto and extensions

java/core/src/main/java/com/google/protobuf/BuiltinExtensionRegistry.java

@@ -0,0 +1,33 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2025 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+package com.google.protobuf;
+
+import static com.google.protobuf.Internal.checkNotNull;
+
+final class BuiltinExtensionRegistry {
+
+  private static volatile ExtensionRegistry instance = null;
+
+  static ExtensionRegistry getInstance() {
+    ExtensionRegistry instance = BuiltinExtensionRegistry.instance;
+    if (instance == null) {
+      synchronized (BuiltinExtensionRegistry.class) {
+        instance = BuiltinExtensionRegistry.instance;
+        if (instance == null) {
+          instance = ExtensionRegistry.newInstance();
+          instance.add(checkNotNull(JavaFeaturesProto.java_));
+          instance = instance.getUnmodifiable();
+          BuiltinExtensionRegistry.instance = instance;
+        }
+      }
+    }
+    return instance;
+  }
+
+  private BuiltinExtensionRegistry() {}
+}