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

*** 44,98 ****
          boolean enqueue(Reference<? extends S> r) {
              return false;
          }
      }
  
!     static ReferenceQueue<Object> NULL = new Null<>();
!     static ReferenceQueue<Object> ENQUEUED = new Null<>();
  
!     static private class Lock { };
!     private Lock lock = new Lock();
!     private volatile Reference<? extends T> head = null;
!     private long queueLength = 0;
  
      boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
!         synchronized (lock) {
!             // Check that since getting the lock this reference hasn't already been
              // enqueued (and even then removed)
!             ReferenceQueue<?> queue = r.queue;
              if ((queue == NULL) || (queue == ENQUEUED)) {
                  return false;
              }
              assert queue == this;
!             r.queue = ENQUEUED;
!             r.next = (head == null) ? r : head;
!             head = r;
!             queueLength++;
!             if (r instanceof FinalReference) {
!                 sun.misc.VM.addFinalRefCount(1);
!             }
              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);
              }
!             return r;
          }
!         return null;
      }
  
      /**
       * Polls this queue to see if a reference object is available.  If one is
       * available without further delay then it is removed from the queue and
--- 44,106 ----
          boolean enqueue(Reference<? extends S> r) {
              return false;
          }
      }
  
!     static final ReferenceQueue<Object> NULL = new Null<>();
!     static final ReferenceQueue<Object> ENQUEUED = new Null<>();
  
!     static private class Lock { }
!     private final Lock lock = new Lock();
!     private volatile int waiters;
!     @SuppressWarnings("unused")
!     private volatile Reference<? extends T> head; // we assign using Unsafe CAS
  
      boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
!         ReferenceQueue<?> queue;
!         do {
!             // Check that this reference hasn't already been
              // enqueued (and even then removed)
!             queue = r.queue;
              if ((queue == NULL) || (queue == ENQUEUED)) {
                  return false;
              }
+         } while (!r.casQueue(queue, ENQUEUED));
          assert queue == this;
!         Reference<? extends T> h;
!         do {
!             h = head;
!             r.next = (h == null) ? r : h;
!         } while (!casHead(h, r));
! 
!         if (waiters > 0) {
!             synchronized (lock) {
!                 if (waiters > 0) {
                      lock.notifyAll();
                  }
              }
+         }
+         return true;
+     }
  
!     private Reference<? extends T> reallyPoll() {
!         Reference<? extends T> r;
!         while ((r = head) != null) {
!             @SuppressWarnings("unchecked") // due to cast to raw type
!                 Reference<? extends T> nh = (r.next == r)
!                 ? null : (Reference) r.next;
!             if (casHead(r, nh)) {
!                 r.lazySetQueue(NULL);
!                 UNSAFE.storeFence();
                  r.next = r;
                  if (r instanceof FinalReference) {
                      sun.misc.VM.addFinalRefCount(-1);
                  }
!                 break;
              }
!         }
!         return r;
      }
  
      /**
       * Polls this queue to see if a reference object is available.  If one is
       * available without further delay then it is removed from the queue and
*** 100,115 ****
       *
       * @return  A reference object, if one was immediately available,
       *          otherwise <code>null</code>
       */
      public Reference<? extends T> poll() {
-         if (head == null)
-             return null;
-         synchronized (lock) {
              return reallyPoll();
          }
-     }
  
      /**
       * Removes the next reference object in this queue, blocking until either
       * one becomes available or the given timeout period expires.
       *
--- 108,119 ----
*** 133,156 ****
          throws IllegalArgumentException, InterruptedException
      {
          if (timeout < 0) {
              throw new IllegalArgumentException("Negative timeout value");
          }
-         synchronized (lock) {
              Reference<? extends T> r = reallyPoll();
              if (r != null) return r;
!             long start = (timeout == 0) ? 0 : System.nanoTime();
              for (;;) {
!                 lock.wait(timeout);
!                 r = reallyPoll();
                  if (r != null) return r;
!                 if (timeout != 0) {
!                     long end = System.nanoTime();
!                     timeout -= (end - start) / 1000_000;
!                     if (timeout <= 0) return null;
!                     start = end;
                  }
              }
          }
      }
  
      /**
--- 137,171 ----
          throws IllegalArgumentException, InterruptedException
      {
          if (timeout < 0) {
              throw new IllegalArgumentException("Negative timeout value");
          }
          Reference<? extends T> r = reallyPoll();
          if (r != null) return r;
!         return reallyRemove(timeout);
!     }
! 
!     private Reference<? extends T> reallyRemove(long timeout) throws InterruptedException {
!         long deadline = (timeout == 0)
!                 ? 0 : System.nanoTime() + timeout * 1000_000L;
!         synchronized (lock) {
!             int w = waiters;
!             waiters = w + 1;
!             try {
                  for (;;) {
!                     Reference<? extends T> r = reallyPoll();
                      if (r != null) return r;
!                     if (timeout == 0) {
!                         lock.wait(0);
!                     } else {
!                         long timeoutNanos = deadline - System.nanoTime();
!                         if (timeoutNanos <= 0) return null;
!                         lock.wait(timeoutNanos / 1000_000L, (int)(timeoutNanos % 1000_000L));
!                     }
                  }
+             } finally {
+                 waiters = w;
              }
          }
      }
  
      /**
*** 162,167 ****
--- 177,200 ----
       */
      public Reference<? extends T> remove() throws InterruptedException {
          return remove(0);
      }
  
+     // Unsafe machinery
+ 
+     private boolean casHead(Reference<?> cmp, Reference<? extends T> val) {
+         return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
+     }
+ 
+     private static final sun.misc.Unsafe UNSAFE;
+     private static final long headOffset;
+ 
+     static {
+         try {
+             UNSAFE = sun.misc.Unsafe.getUnsafe();
+             Class<ReferenceQueue> rqc = ReferenceQueue.class;
+             headOffset = UNSAFE.objectFieldOffset(rqc.getDeclaredField("head"));
+         } catch (Exception e) {
+             throw new Error(e);
+         }
+     }
  }