- [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();
        }
      ...
    }
```

String webappPath = servletContext.getContextPath();
            if (StringUtils.isNotEmpty(webappPath)) {
                webappPath = webappPath.substring(1);
                if (!contextPath.startsWith(webappPath)) {
                    throw new RpcException("Since you are using server='servlet', " +
                            "make sure that the 'contextpath' property starts with the path of external webapp");
                }
                contextPath = contextPath.substring(webappPath.length());
                if (contextPath.startsWith("/")) {
                    contextPath = contextPath.substring(1);
                }
            }
        }
        final Class resourceDef = GetRestful.getRootResourceClass(implClass) != null ? implClass : type;
        server.deploy(resourceDef, impl, contextPath);
        final RestServer s = server;
        return new Runnable() {
            @Override
            public void run() {
                
                
                s.undeploy(resourceDef);
            }
        };
    }
    @Override
    protected <T> T doRefer(Class<T> serviceType, URL url) throws RpcException {
        if (connectionMonitor == null) {
            connectionMonitor = new ConnectionMonitor();
        }