SLING-12519 - Use AtomicReference in ScriptableBase.java for lazy initialization of NativeJavaObject

* Use AtomicReference for thread-safe lazy initialization of NativeJavaObject.
* Introduce a special NULL_OBJECT placeholder as an intermediate state to avoid
  repeated construction of NativeJavaObject in each compareAndSet(...) operation.

Author: scottyuancoc

src/main/java/org/apache/sling/scripting/javascript/wrapper/ScriptableBase.java

@@ -21,6 +21,7 @@
 import java.lang.reflect.Method;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.mozilla.javascript.NativeJavaObject;
 import org.mozilla.javascript.Scriptable;
@@ -31,12 +32,13 @@
  */
 public abstract class ScriptableBase extends ScriptableObject {
 
-    private NativeJavaObject njo;
+    private static final NativeJavaObject NULL_OBJECT = new NativeJavaObject();
+    private final AtomicReference<NativeJavaObject> atomicNjoReference = new AtomicReference<NativeJavaObject>(null);
     private final Set<String> jsMethods = getJsMethodNames();
 
     public static final String JSFUNC_PREFIX = "jsFunction_";
 
-    protected synchronized Object getNative(String name, Scriptable start) {
+    protected Object getNative(String name, Scriptable start) {
         final Object wrapped = getWrappedObject();
 
         if (wrapped == null) {
@@ -47,11 +49,11 @@ protected synchronized Object getNative(String name, Scriptable start) {
             return Scriptable.NOT_FOUND;
         }
 
-        if (njo == null) {
-            njo = new NativeJavaObject(start, wrapped, getStaticType());
+        // Use a NULL_OBJECT refernece to ensure only one initiation the real NatvieJavaObject.
+        if (atomicNjoReference.compareAndSet(null, NULL_OBJECT)) {
+            atomicNjoReference.compareAndSet(NULL_OBJECT, new NativeJavaObject(start, wrapped, getStaticType()));
         }
-
-        return njo.get(name, start);
+        return atomicNjoReference.get().get(name, start);
     }
 
     /** @return the Java object that we're wrapping, used to create a NativeJavaObject