8351038: ConcurrentModificationException in EventType constructor

modules/javafx.base/src/main/java/javafx/event/EventType.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2010, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -64,8 +64,7 @@ public final class EventType<T extends Event> implements Serializable{
      * indirect sub types of it. It is also the only event type which
      * has its super event type set to {@code null}.
      */
-    public static final EventType<Event> ROOT =
-            new EventType<>("EVENT", null);
+    public static final EventType<Event> ROOT = new EventType<>("EVENT", true);
 
     @SuppressWarnings("doclint:missing")
     private WeakHashMap<EventType<? extends T>, Void> subTypes;
@@ -132,23 +131,11 @@ public EventType(final EventType<? super T> superType,
     }
 
     /**
-     * Internal constructor that skips various checks
+     * Internal constructor for the ROOT instance that skips various checks
      */
-    EventType(final String name,
-                      final EventType<? super T> superType) {
-        this.superType = superType;
+    private EventType(String name, boolean ignored) {
+        this.superType = null;
         this.name = name;
-        if (superType != null) {
-            if (superType.subTypes != null) {
-                for (Iterator i = superType.subTypes.keySet().iterator(); i.hasNext();) {
-                    EventType t  = (EventType) i.next();
-                    if (name == null && t.name == null || (name != null && name.equals(t.name))) {
-                        i.remove();
-                    }
-                }
-            }
-            superType.register(this);
-        }
     }
 
     /**
@@ -179,7 +166,7 @@ public String toString() {
         return (name != null) ? name : super.toString();
     }
 
-    private void register(javafx.event.EventType<? extends T> subType) {
+    private synchronized void register(javafx.event.EventType<? extends T> subType) {
         if (subTypes == null) {
             subTypes = new WeakHashMap<>();
         }

modules/javafx.base/src/test/java/test/javafx/event/EventTypeConcurrencyTest.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package test.javafx.event;
+
+import static org.junit.jupiter.api.Assertions.fail;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Callable;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import javafx.event.Event;
+import javafx.event.EventType;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Developers should be able to create EventTypes in background thread(s).
+ */
+public class EventTypeConcurrencyTest {
+    @Test
+    public void concurrentInitialization() {
+        int N = 1_000;
+        try (var executor = Executors.newCachedThreadPool()) {
+            try {
+                ArrayList<Callable<Object>> runs = new ArrayList<>(N);
+                for (int i = 0; i < N; i++) {
+                    String name = "TEST" + i;
+                    runs.add(() -> {
+                        return new EventType<>(Event.ANY, name);
+                    });
+                }
+
+                List<Future<Object>> futures = executor.invokeAll(runs);
+
+                for (Future<Object> future : futures) {
+                    future.get();
+                }
+            } catch (Throwable e) {
+                fail(e);
+            }
+        }
+    }
+}