方式1，通过 webpackDevMiddleware 和 webpackHotMiddleware：
server.js:

const webpackDevMiddleware = require("webpack-dev-middleware");
const webpackHotMiddleware = require("webpack-hot-middleware");
const compiler = webpack(config);
const app = express();

// auto refresh
app.use(webpackDevMiddleware(compiler, {
    publicPath: '/',
}));
app.use(webpackHotMiddleware(compiler));

方式2，通过WebpackDevServer：
Webpack.config.js:

var server = new webpackDevServer(compiler, {
  hot: true
  ...
});
server.listen(8000);