Groovy:GPath - reading a xml file in which a node attribute is a map

Question

I have following xml file:

<doc_xml>
<nodes>
  <node id='1' spec="{spec_a=0.9, spec_b=0.1}" />
  <node id='2' spec="{spec_a=0.1, spec_b=0.3}" />
  <node id='3' spec="{}" />
</nodes>
</doc_xml>

This code was created using Groovy MarkupBuilder.

Now I would like to parse this file in a groovy script:

def xml = new XmlParser().parseText(getDocXmlAsString());

xml.nodes.node.each {
   Map spec = it.@spec; // here I got an exception org.codehaus.groovy.runtime.typehandling.GroovyCastException

}

but I keep getting this exception:

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '{spec_a=0.9, spec_b=0.1}' with class 'java.lang.String' to class 'java.util.Map'

My question, how to parse an xml attribute which is a map?