@Override
    public void loadUrl(String url) {
        if (mainThreadHandler != null) {
            Message msg = mainThreadHandler.obtainMessage(LOAD_URL, url);
            mainThreadHandler.sendMessage(msg);
        }
    }
This method can be called in any thread, and if it is not called in the main thread,
it will be automatically distributed to the main thread.

@param url
app/src/main/java/com/example/jingbin/webviewstudy/view/WVJBWebView.java