Merge pull request #3 from facebook/fix_singleton_thread_safety

Added threadsafe singleton implementation to CCManager and BandwidthSampler

Author: rrong

connectionclass/src/main/java/com/facebook/network/connectionclass/ConnectionClassManager.java

@@ -64,19 +64,17 @@ public class ConnectionClassManager {
   static final long BANDWIDTH_LOWER_BOUND = 10;
 
   // Singleton.
-  @Nullable
-  private static ConnectionClassManager sInstance;
+  private static class ConnectionClassManagerHolder {
+      public static final ConnectionClassManager instance = new ConnectionClassManager();
+  }
 
   /**
    * Retrieval method for the DownloadBandwidthManager singleton.
    * @return The singleton instance of DownloadBandwidthManager.
    */
   @Nonnull
   public static ConnectionClassManager getInstance() {
-    if (sInstance == null) {
-      sInstance = new ConnectionClassManager();
-    }
-    return sInstance;
+      return ConnectionClassManagerHolder.instance;
   }
 
   // Force constructor to be private.

connectionclass/src/main/java/com/facebook/network/connectionclass/DeviceBandwidthSampler.java

@@ -41,20 +41,19 @@ public class DeviceBandwidthSampler {
 
   private long mLastTimeReading;
 
-  // Make BandwidthTimerManager a singleton.
-  @Nullable
-  private static DeviceBandwidthSampler sInstance;
+  // Singleton.
+  private static class DeviceBandwidthSamplerHolder {
+      public static final DeviceBandwidthSampler instance =
+              new DeviceBandwidthSampler(ConnectionClassManager.getInstance());
+  }
 
   /**
-   * Retrieval method for the BandwidthTimerManager singleton.
-   * @return The singleton instance of BandwidthTimerManager.
+   * Retrieval method for the DeviceBandwidthSampler singleton.
+   * @return The singleton instance of DeviceBandwidthSampler.
    */
   @Nonnull
   public static DeviceBandwidthSampler getInstance() {
-    if (sInstance == null) {
-      sInstance = new DeviceBandwidthSampler(ConnectionClassManager.getInstance());
-    }
-    return sInstance;
+    return DeviceBandwidthSamplerHolder.instance;
   }
 
   private DeviceBandwidthSampler(