JAVA-1164: Replaced use of ThreadLocal<Random> subclass (used to override initialValue()) with a method that explicitly gets the thread local value and sets it if it's null.

This avoids some nasty classloader issues that show up particularly in web applications, where this anonymous subclass is holding a reference to the entire web app classloader,
which is not released until the thread exits.

Author: jyemin

src/main/com/mongodb/BaseCluster.java

@@ -36,12 +36,7 @@ abstract class BaseCluster implements Cluster {
 
     private final AtomicReference<CountDownLatch> phase = new AtomicReference<CountDownLatch>(new CountDownLatch(1));
     private final ClusterableServerFactory serverFactory;
-    private final ThreadLocal<Random> random = new ThreadLocal<Random>() {
-        @Override
-        protected Random initialValue() {
-            return new Random();
-        }
-    };
+    private final ThreadLocal<Random> random = new ThreadLocal<Random>();
     private final String clusterId;
     private final ClusterSettings settings;
     private final ClusterListener clusterListener;
@@ -220,8 +215,15 @@ private void throwIfIncompatible(final ClusterDescription curDescription) {
         }
     }
 
-    protected Random getRandom() {
-        return random.get();
+    // it's important that Random instances are created in this way instead of via subclassing ThreadLocal and overriding the
+    // initialValue() method.
+    private Random getRandom() {
+        Random result = random.get();
+        if (result == null) {
+            result = new Random();
+            random.set(result);
+        }
+        return result;
     }
 
     protected ClusterableServer createServer(final ServerAddress serverAddress, final ChangeListener<ServerDescription>