public <T, R> void registerHandler(String handlerName, WVJBHandler<T, R> handler) {
        if (handlerName == null || handlerName.length() == 0 || handler == null) {
            return;
        }
        messageHandlers.put(handlerName, handler);
    }
set a listener for javascript closing the current activity.
app/src/main/java/com/example/jingbin/webviewstudy/view/WVJBWebView.java