public static Object getAdditionalInstanceField(Object obj, String key) {
        if (obj == null)
            throw new NullPointerException("object must not be null");
        if (key == null)
            throw new NullPointerException("key must not be null");

        HashMap<String, Object> objectFields;
        synchronized (additionalFields) {
            objectFields = additionalFields.get(obj);
            if (objectFields == null)
                return null;
        }

        synchronized (objectFields) {
            return objectFields.get(key);
        }
    }
Returns a value which was stored with {@link #setAdditionalInstanceField}.

@param obj The object instance for which the value has been stored.
@param key The key in the value map for this object instance.
@return The stored value for this instance/key combination, or {@code null} if there is none.
android/android_framework/base/src/main/java/util/XposedHelpers.java