- [x] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.6.5


### Confusion

the code below  create a ```connectionMonitor``` instance, but ```Start``` method seems not called
```
@Override
    protected <T> T doRefer(Class<T> serviceType, URL url) throws RpcException {
        if (connectionMonitor == null) {
            connectionMonitor = new ConnectionMonitor();
        }
      ...
    }
```

private final List<ResteasyClient> clients = Collections.synchronizedList(new LinkedList<ResteasyClient>());
    private volatile ConnectionMonitor connectionMonitor;
    public RestProtocol() {
        super(WebApplicationException.class, ProcessingException.class);
    }
    public void setHttpBinder(HttpBinder httpBinder) {
        serverFactory.setHttpBinder(httpBinder);
    }
    @Override
    public int getDefaultPort() {
        return DEFAULT_PORT;
    }
    @Override
    protected <T> Runnable doExport(T impl, Class<T> type, URL url) throws RpcException {
        String addr = getAddr(url);
        Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
        RestServer server = servers.get(addr);
        if (server == null) {
            server = serverFactory.createServer(url.getParameter(Constants.SERVER_KEY, DEFAULT_SERVER));
            server.start(url);
            servers.put(addr, server);
        }
        String contextPath = getContextPath(url);
        if ("servlet".equalsIgnoreCase(url.getParameter(Constants.SERVER_KEY, DEFAULT_SERVER))) {
            ServletContext servletContext = ServletManager.getInstance().getServletContext(ServletManager.EXTERNAL_SERVER_PORT);
            if (servletContext == null) {
                throw new RpcException("No servlet context found. Since you are using server='servlet', " +
                        "make sure that you've configured " + BootstrapListener.class.getName() + " in web.xml");
            }