Improvements in ReadableNativeMap locking mechanism (a follow up to D…

…43398416)

Summary:
[Changelog][Internal]

A follow-up, based on javache comments on D43398416 (9aac13d) (post-land) - just some minor potential improvements for locking efficiency.

Reviewed By: javache

Differential Revision: D43438414

fbshipit-source-id: 0cf807a045a1f132d5481d3f6115a97869e93d65

ReactAndroid/src/main/java/com/facebook/react/bridge/ReadableNativeMap.java

@@ -40,20 +40,21 @@ public static int getJNIPassCounter() {
   }
 
   private void ensureKeysAreImported() {
+    if (mKeys != null) {
+      return;
+    }
     synchronized (this) {
-      if (mKeys == null) {
-        mKeys = Assertions.assertNotNull(importKeys());
-        mJniCallCounter++;
-      }
+      mKeys = Assertions.assertNotNull(importKeys());
+      mJniCallCounter++;
     }
   }
 
   private HashMap<String, Object> getLocalMap() {
     if (mLocalMap != null) {
       return mLocalMap;
     }
-    ensureKeysAreImported();
     synchronized (this) {
+      ensureKeysAreImported();
       if (mLocalMap == null) {
         Object[] values = Assertions.assertNotNull(importValues());
         mJniCallCounter++;
@@ -75,9 +76,8 @@ private HashMap<String, Object> getLocalMap() {
     if (mLocalTypeMap != null) {
       return mLocalTypeMap;
     }
-    ensureKeysAreImported();
     synchronized (this) {
-      // check that no other thread has already updated
+      ensureKeysAreImported();
       if (mLocalTypeMap == null) {
         Object[] types = Assertions.assertNotNull(importTypes());
         mJniCallCounter++;
@@ -190,9 +190,10 @@ public int getInt(@NonNull String name) {
 
   @Override
   public @NonNull Iterator<Map.Entry<String, Object>> getEntryIterator() {
-    ensureKeysAreImported();
-    final String[] iteratorKeys = mKeys;
     synchronized (this) {
+      ensureKeysAreImported();
+
+      final String[] iteratorKeys = mKeys;
       final Object[] iteratorValues = Assertions.assertNotNull(importValues());
 
       return new Iterator<Map.Entry<String, Object>>() {