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

@@ -23,10 +23,12 @@
  * 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

@@ -81,11 +83,11 @@
         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;
+            r.next = Reference.DEQUEUED;
             queueLength--;
             if (r instanceof FinalReference) {
                 sun.misc.VM.addFinalRefCount(-1);
             }
             return r;

@@ -162,6 +164,25 @@
      */
     public Reference<? extends T> remove() throws InterruptedException {
         return remove(0);
     }
 
+    /**
+     * Iterate queue and invoke given action with each Reference.
+     * Suitable for diagnostic purposes.
+     */
+    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) {
+                // end of chain
+                r = null;
+            } else if (rn == Reference.DEQUEUED) {
+                // restart from head if overtaken by queue poller(s)
+                r = head;
+            } else {
+                r = rn;
+            }
+        }
+    }
 }