Export parameter REALLY work in message mapping???????

I wrote a java version transformation and it worked good with export parameter.

However I can not work it out in graphical message mapping. I refered http://help.sap.com/saphelp_nwpi71/helpdata/en/43/c3e1fa6c31599ee10000000a1553f6/frameset.htm.

In PI7.1, I add an export parameter "EXPORT_PARA" in the "Signature" tab. Are there any special step to assign this parameter to an UDF? I did not find a way to do it.

And then I define an UDF,

public String SetExportPara(String sss, Container container) throws StreamTransformationException{

String str = "GOOD";

GlobalContainer gc = container.getGlobalContainer();

OutputParameters paras= gc.getOutputParameters();

if(paras.exists("EXPORT_PARA ") == true) {

paras.setString("EXPORT_PARA ", str);

}else {

str = "NOT FOUND EXPORT_PARA";

}

return str;

}

And I assign this UDF to one element of my output XML, I tried to run it in IE, it always output "NOT FOUND EXPORT_PARA.

why?????

I searched the forum, no answer found.