8201793: (ref) Reference object should not support cloning

Reviewed-by: psandoz, kbarrett

src/java.base/share/classes/java/lang/ref/Reference.java

@@ -300,6 +300,20 @@ public abstract class Reference<T> {
         return this.queue.enqueue(this);
     }
 
+    /**
+     * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
+     * meaningfully cloned. Construct a new {@code Reference} instead.
+     *
+     * @returns never returns normally
+     * @throws  CloneNotSupportedException always
+     *
+     * @since 11
+     */
+    @Override
+    protected Object clone() throws CloneNotSupportedException {
+        throw new CloneNotSupportedException();
+    }
+
     /* -- Constructors -- */
 
     Reference(T referent) {

test/jdk/java/lang/ref/ReferenceClone.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright (c) 2018, 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.
+ *
+ * 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.
+ */
+
+/*
+ * @test
+ * @bug 8201793
+ * @summary Test Reference::clone to throw CloneNotSupportedException
+ */
+
+import java.lang.ref.*;
+
+public class ReferenceClone {
+    private static final ReferenceQueue<Object> QUEUE = new ReferenceQueue<>();
+    public static void main(String... args) {
+        ReferenceClone refClone = new ReferenceClone();
+        refClone.test();
+    }
+
+    public void test() {
+        // test Reference::clone that throws CNSE
+        Object o = new Object();
+        assertCloneNotSupported(new SoftRef(o));
+        assertCloneNotSupported(new WeakRef(o));
+        assertCloneNotSupported(new PhantomRef(o));
+
+        // Reference subclass may override the clone method
+        CloneableReference ref = new CloneableReference(o);
+        try {
+            ref.clone();
+        } catch (CloneNotSupportedException e) {}
+    }
+
+    private void assertCloneNotSupported(CloneableRef ref) {
+        try {
+            ref.clone();
+            throw new RuntimeException("Reference::clone should throw CloneNotSupportedException");
+        } catch (CloneNotSupportedException e) {}
+    }
+
+    // override clone to be public that throws CNSE
+    interface CloneableRef extends Cloneable {
+        public Object clone() throws CloneNotSupportedException;
+    }
+
+    class SoftRef extends SoftReference<Object> implements CloneableRef {
+        public SoftRef(Object referent) {
+            super(referent, QUEUE);
+        }
+        public Object clone() throws CloneNotSupportedException {
+            return super.clone();
+        }
+    }
+
+    class WeakRef extends WeakReference<Object> implements CloneableRef {
+        public WeakRef(Object referent) {
+            super(referent, QUEUE);
+        }
+        public Object clone() throws CloneNotSupportedException {
+            return super.clone();
+        }
+    }
+
+    class PhantomRef extends PhantomReference<Object> implements CloneableRef {
+        public PhantomRef(Object referent) {
+            super(referent, QUEUE);
+        }
+
+        public Object clone() throws CloneNotSupportedException {
+            return super.clone();
+        }
+    }
+
+    // override clone to return a new instance
+    class CloneableReference extends WeakReference<Object> implements Cloneable {
+        public CloneableReference(Object referent) {
+            super(referent, QUEUE);
+        }
+
+        public Object clone() throws CloneNotSupportedException {
+            return new CloneableReference(this.get());
+        }
+    }
+
+}