WMSLayer display perfectly in arccatalog and arcmap but in flex

[DamonSong]

Hi,
    I publish a map service with WMS capability, it displays perfectly when added to arcmap and previews in arccatalog, but shows nothing in flex project using WMSLayer, the arcgis api for flex is 2.2. Any suggestions?
    thanks in advance for any help!
following is my code.

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
      xmlns:s="library://ns.adobe.com/flex/spark"
      xmlns:esri="http://www.esri.com/2008/ags">
 <fx:Script>
  <![CDATA[
   import com.esri.ags.Units;
  ]]>
 </fx:Script>
 <esri:Map units="{Units.DECIMAL_DEGREES}">
  <esri:WMSLayer 
   url="http://sdm:8399/arcgis/services/nef/MapServer/WMSServer"/>
 </esri:Map>
</s:Application>

[DasaPaddock]

Are you accessing your Flex app at http://sdm:8399/...? If not, you'll need a crossdomain.xml file at http://sdm:8399/crossdomain.xml

See References book at:
http://help.arcgis.com/en/webapi/flex/help/index.html

[DamonSong]

Are you accessing your Flex app at http://sdm:8399/...? If not, you'll need a crossdomain.xml file at http://sdm:8399/crossdomain.xml

See References book at:
http://help.arcgis.com/en/webapi/flex/help/index.html

thanks for your reply!
    i try your suggestion, but it still does not work. My crossdomain.xml is located at "Server10.0\java\manager\web_output\crossdomain.xml", and i can get it at http://sdm:8399/crossdomain.xml
    the screenshots are attached.

[DasaPaddock]

Can you use a tool like Fiddler or HttpFox to see what requests are being made to your service and what the responses are?

[HugoDurana]

@xgyb12458

From the code:
http://sdm:8399/arcgis/services/nef/MapServer/WMSServer

I am sorry to point out the possible obvious, but i think you forgot the rest part of the URL e.g:
http://sdm:8399/arcgis/rest/services/nef/MapServer/WMSServer

[DasaPaddock]

xgyb12458,

You need to specify at least one visible layer.

hugo.durana,

The WMS layer does not use the REST endpoint.

See sample at:
http://help.arcgis.com/en/webapi/flex/samples/index.html?sample=WMS