public boolean isConnected(Url url) {
        ensureStarted();
        Connection conn = this.rpcRemoting.connectionManager.get(url.getUniqueKey());
        if (null != conn) {
            return conn.isFine();
        }
        return false;
    }
check whether a {@link Url} connected

@param url url
@return {@code true} means url is connected and {@code false} means disconnected.
src/main/java/com/alipay/remoting/rpc/RpcServer.java