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

*** 1,7 ****
  /*
!  * Copyright (c) 1997, 2013, 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
--- 1,7 ----
  /*
!  * Copyright (c) 1997, 2015, 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
*** 23,32 ****
--- 23,34 ----
   * questions.
   */
  
  package java.lang.ref;
  
+ import java.util.function.Consumer;
+ 
  /**
   * Reference queues, to which registered reference objects are appended by the
   * garbage collector after the appropriate reachability changes are detected.
   *
   * @author   Mark Reinhold
*** 73,89 ****
              lock.notifyAll();
              return true;
          }
      }
  
!     @SuppressWarnings("unchecked")
      private Reference<? extends T> reallyPoll() {       /* Must hold lock */
          Reference<? extends T> r = head;
          if (r != null) {
!             head = (r.next == r) ?
                  null :
!                 r.next; // Unchecked due to the next field having a raw type in Reference
              r.queue = NULL;
              r.next = r;
              queueLength--;
              if (r instanceof FinalReference) {
                  sun.misc.VM.addFinalRefCount(-1);
--- 75,92 ----
              lock.notifyAll();
              return true;
          }
      }
  
!     @SuppressWarnings({"unchecked","rawtypes"})
      private Reference<? extends T> reallyPoll() {       /* Must hold lock */
          Reference<? extends T> r = head;
          if (r != null) {
!             Reference rn = r.next;
!             head = (rn == r) ?
                  null :
!                 rn; // Unchecked due to the next field having a raw type in Reference
              r.queue = NULL;
              r.next = r;
              queueLength--;
              if (r instanceof FinalReference) {
                  sun.misc.VM.addFinalRefCount(-1);
*** 162,167 ****
--- 165,198 ----
       */
      public Reference<? extends T> remove() throws InterruptedException {
          return remove(0);
      }
  
+     /**
+      * Iterate queue and invoke given action with each Reference.
+      * Suitable for diagnostic purposes.
+      * WARNING: any use of this method should make sure to not
+      * retain the referents of iterated references (in case of
+      * FinalReference(s)) so that their life is not prolonged more
+      * than necessary.
+      */
+     @SuppressWarnings({"unchecked","rawtypes"})
+     void forEach(Consumer<? super Reference<? extends T>> action) {
+         for (Reference<? extends T> r = head; r != null;) {
+             action.accept(r);
+             Reference rn = r.next;
+             if (rn == r) {
+                 if (r.queue == ENQUEUED) {
+                     // still enqueued -> we reached end of chain
+                     r = null;
+                 } else {
+                     // already dequeued: r.queue == NULL; ->
+                     // restart from head when overtaken by queue poller(s)
+                     r = head;
+                 }
+             } else {
+                 // next in chain
+                 r = rn;
+             }
+         }
+     }
  }