Add back java7/8 fairness optimization without breaking Java 6.

src/main/java/com/zaxxer/hikari/util/ConcurrentBag.java

@@ -119,8 +119,8 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
             }
         }
 
-        // Otherwise, scan the shared list ... for maximum of timeout
-        final long retryTimeout = (retries > 0 && timeoutMillis > 0) ? Math.max((timeoutMillis / (retries + 1)), 50) : timeoutMillis;
+        // Otherwise, scan the shared list ... for maximum of timeoutMillis
+        final long retryTimeoutMs = (retries > 0 && timeoutMillis > 0) ? Math.max((timeoutMillis / (retries + 1)), 50) : timeoutMillis;
         long totalTimeoutNs = TimeUnit.MILLISECONDS.toNanos(timeoutMillis);
         boolean tryAddItem = true;
         while (totalTimeoutNs > 0)
@@ -136,16 +136,17 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
 
             if (listener != null && tryAddItem)
             {
-                listener.addBagItem(retryTimeout);
+                listener.addBagItem(retryTimeoutMs);
             }
 
             totalTimeoutNs -= (System.nanoTime() - startAttempt);
             final long startTryAcquire = System.nanoTime();
             try
             {
-                long timeoutNs = TimeUnit.MILLISECONDS.toNanos(retryTimeout) - (System.nanoTime() - startAttempt);
+                long timeoutNs = TimeUnit.MILLISECONDS.toNanos(retryTimeoutMs) - (System.nanoTime() - startAttempt);
                 if (synchronizer.tryAcquireSharedNanos(startAttempt, timeoutNs))
                 {
+                    // Somebody requited and item while we were waiting, rescan the list but don't try to add a new item
                     tryAddItem = false;
                     continue;
                 }
@@ -319,11 +320,24 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
     private static class Synchronizer extends AbstractQueuedLongSynchronizer
     {
         private static final long serialVersionUID = 104753538004341218L;
+        private static boolean JAVA7;
+
+        static
+        {
+            try
+            {
+                JAVA7 = AbstractQueuedLongSynchronizer.class.getMethod("hasQueuedPredecessors", new Class<?>[0]) != null;
+            }
+            catch (Exception e)
+            {
+                // nothing
+            }
+        }
 
         @Override
         protected long tryAcquireShared(long startScanTime)
         {
-            return getState() > startScanTime ? 1 : -1;
+            return getState() >= startScanTime && !java67hasQueuedPredecessors() ? 1 : -1;
         }
 
         /** {@inheritDoc} */
@@ -334,5 +348,15 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
             
             return true;
         }
+
+        private boolean java67hasQueuedPredecessors()
+        {
+            if (JAVA7)
+            {
+                return hasQueuedPredecessors();
+            }
+
+            return false;
+        }
     }
 }