8269096: Add java.util.Objects.newIdentity method

src/java.base/share/classes/java/lang/Object.java

@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import java.util.Objects;
+
 import jdk.internal.vm.annotation.IntrinsicCandidate;
 
 /**
@@ -39,6 +41,7 @@ public class Object {
 
     /**
      * Constructs a new object.
+     * @see Objects#newIdentity()
      */
     @IntrinsicCandidate
     public Object() {}

src/java.base/share/classes/java/util/Objects.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2009, 2021, 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
@@ -486,4 +486,17 @@ long checkFromToIndex(long fromIndex, long toIndex, long length) {
     long checkFromIndexSize(long fromIndex, long size, long length) {
         return Preconditions.checkFromIndexSize(fromIndex, size, length, null);
     }
+
+    /**
+     * {@return a new instance of an unspecified class}
+     * The object has a unique identity; no other references to it exist.
+     * It can be used for synchronization, or where a placeholder Object is needed.
+     * The class does not override any of the methods of {@code java.lang.Object}.
+     * Use this method to avoid relying on the {@linkplain Object#Object() Object constructor}.
+     *
+     * @since 17
+     */
+    public static Object newIdentity() {
+        return new Object() {};
+    }
 }

test/jdk/java/util/Objects/BasicObjectsTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009, 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2009, 2021, 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
@@ -45,6 +45,7 @@ public static void main(String... args) {
         errors += testIsNull();
         errors += testNonNull();
         errors += testNonNullOf();
+        errors += testNewIdentity();
         if (errors > 0 )
             throw new RuntimeException();
     }
@@ -275,4 +276,31 @@ private static int testNonNullOf() {
         }
         return errors;
     }
+
+    // Shared counter for testNewIdentity
+    private static int counter;
+
+    private static int testNewIdentity() {
+        int errors = 0;
+
+        Object o1 = Objects.newIdentity();
+        Object o2 = Objects.newIdentity();
+
+        if (o1 == null || o2 == null)
+            errors += 1;
+
+        if (o1 == o2)
+            errors += 1;
+
+        try {
+            synchronized(o1) {
+                counter++;
+            }
+        } catch (RuntimeException rex) {
+            rex.printStackTrace();
+            errors += 1;
+        }
+
+        return errors;
+    }
 }