dubbox中rest服务使用外部容器部署的小问题

使用dubbox发布rest服务，使用外部容器部署应用的时候，web.xml中有如下配置请求服务路径

    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>com.alibaba.dubbo.remoting.http.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/services/*</url-pattern>
    </servlet-mapping>

在service-provider.xml配置如下

<!-- use the external tomcat or other server with the servlet approach; the port and contextpath must be exactly the same as those in external server -->
<dubbo:protocol name="rest" port="8888" contextpath="services" server="servlet"/>

这样的配置是在应用发布在webapps的ROOT根路径下的配置，访问地址如下：
http://localhost:8888/services/...
当应用发布在非ROOT中是，比如webapps/provider/路径下，此时rest服务总是访问不到，而在dubbox的说明中有如下文字：

注意：如果你是选用外部应用服务器做rest server，即配置:

<dubbo:protocol name="rest" port="8888" contextpath="services" server="servlet"/>
则必须保证这里设置的port、contextpath，与外部应用服务器的端口、DispatcherServlet的上下文路径（即webapp path加上servlet url pattern）保持一致。例如，对于部署为tomcat ROOT路径的应用，这里的contextpath必须与web.xml中DispacherServlet的<url-pattern/>完全一致：

<servlet-mapping>
     <servlet-name>dispatcher</servlet-name>
     <url-pattern>/services/*</url-pattern>
</servlet-mapping>

但是修改了service-provider.xml中contextpath为/provider/services也不行

报错信息如下：

 com.alibaba.dubbo.rpc.RpcException: Since you are using server='servlet', make sure that the 'contextpath' property starts with the path of external webapp
	at com.alibaba.dubbo.rpc.protocol.rest.RestProtocol.doExport(RestProtocol.java:104)

查看RestProtocol源码

            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);
                }
            }

代码逻辑一目了然，if (!contextPath.startsWith(webappPath)) {

需要将service-provider.xml中contextpath修改为provider/services即可，不带/前缀