code: public void setNamespaceContext(Map<String, String> prefix2uri) {
          this.prefix2uri = prefix2uri == null
              ? Collections.<String, String> emptyMap()
              : Collections.unmodifiableMap(prefix2uri);
      }
docs: Establish a namespace context that will be used in for the
  XPath.
  
  <p>Without a namespace context (or with an empty context) the
  XPath expressions will only use local names for elements and
  attributes.</p>
  
  @param prefix2uri maps from prefix to namespace URI.