Add locking around CatalystInstance.getJavaScriptContext()

Reviewed By: kathryngray

Differential Revision: D5861693

fbshipit-source-id: 226ff15622d5e1a8ae3ad4c63f1434bd95c1fa21

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

@@ -0,0 +1,29 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+package com.facebook.react.bridge;
+
+import javax.annotation.concurrent.GuardedBy;
+
+/**
+ * Wrapper for JavaScriptContext native pointer. CatalystInstanceImpl creates this on demand, and
+ * will call clear() before destroying the VM. People who need the raw JavaScriptContext pointer
+ * can synchronize on this wrapper object to guarantee that it will not be destroyed.
+ */
+public class JavaScriptContextHolder {
+  @GuardedBy("this")
+  private long mContext;
+
+  public JavaScriptContextHolder(long context) {
+    mContext = context;
+  }
+
+  @GuardedBy("this")
+  public long get() {
+    return mContext;
+  }
+
+  public synchronized void clear() {
+    mContext = 0;
+  }
+
+}