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

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  /**
   * Phantom reference objects, which are enqueued after the collector
   * determines that their referents may otherwise be reclaimed.  Phantom
   * references are most often used to schedule post-mortem cleanup actions.
+  * <p>
+  * The referent must not be an instance of a primitive class; such a value
+  * can never have another reference to it and cannot be held in a reference type.
   *
   * <p> Suppose the garbage collector determines at a certain point in time
   * that an object is <a href="package-summary.html#reachability">
   * phantom reachable</a>.  At that time it will atomically clear
   * all phantom references to that object and all phantom references to

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       * queue.  Such a reference will never be enqueued.
       *
       * @param referent the object the new phantom reference will refer to
       * @param q the queue with which the reference is to be registered,
       *          or {@code null} if registration is not required
+      * @throws IllegalArgumentException if the referent is an instance of an
+      *         {@link Class#isPrimitiveClass() primitive class}
       */
      public PhantomReference(T referent, ReferenceQueue<? super T> q) {
          super(referent, q);
      }