Use WeakReferences to avoid Tomcat ThreadLocal leak warning.

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

@@ -15,6 +15,7 @@
  */
 package com.zaxxer.hikari.util;
 
+import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.LinkedList;
 import java.util.List;
@@ -63,7 +64,7 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
 	    boolean compareAndSetState(int expectedState, int newState);
 	}
 
-    private ThreadLocal<LinkedList<T>> threadList;
+    private ThreadLocal<LinkedList<WeakReference<T>>> threadList;
     private CopyOnWriteArraySet<T> sharedList;
     private Synchronizer synchronizer;
 
@@ -74,7 +75,7 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
     {
         this.sharedList = new CopyOnWriteArraySet<T>();
         this.synchronizer = new Synchronizer();
-        this.threadList = new ThreadLocal<LinkedList<T>>();
+        this.threadList = new ThreadLocal<LinkedList<WeakReference<T>>>();
     }
 
     /**
@@ -89,19 +90,20 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
     public T borrow(long timeout, TimeUnit timeUnit) throws InterruptedException
     {
         // Try the thread-local list first
-        LinkedList<T> list = threadList.get();
+        LinkedList<WeakReference<T>> list = threadList.get();
         if (list == null)
         {
-            list = new LinkedList<T>();
+            list = new LinkedList<WeakReference<T>>();
             threadList.set(list);
         }
 
         while (!list.isEmpty())
         {
-            final T reference = list.removeFirst();
-            if (reference.compareAndSetState(STATE_NOT_IN_USE, STATE_IN_USE))
+            final WeakReference<T> reference = list.removeFirst();
+            T element = reference.get();
+            if (element != null && element.compareAndSetState(STATE_NOT_IN_USE, STATE_IN_USE))
             {
-                return reference;
+                return element;
             }
         }
 
@@ -144,7 +146,7 @@ public class ConcurrentBag<T extends com.zaxxer.hikari.util.ConcurrentBag.IBagMa
         if (value.compareAndSetState(STATE_IN_USE, STATE_NOT_IN_USE))
         {
         	final long returnTime = System.nanoTime();
-            threadList.get().addLast(value);
+            threadList.get().addLast(new WeakReference<T>(value));
             synchronizer.releaseShared(returnTime);
         }
         else