后端代码：
var reactHtml = ReactDom.renderToString(<App />);
res.render('index', {title:'test',html:reactHtml});
前端代码：
ReactDOM.render(<App />, document.getElementById("example"));
无论这个<App />多么简单都会被重新替换掉，不知道怎么办？
浏览器提示：Warning: render(…): Replacing React-rendered children with a new root component. If you intended to update the children of this node, you should instead have the existing children update their state and render the new components instead of calling ReactDOM.render.