rule conditioned by attribute value

Question

Hi stackers,

I would like to parse an XML which sometimes have an attribute, sometimes not:

<tagName constructor="AdvancedCriteria" />

or

<tagName />

I added a rule to flag the attribute presence:

digester.addRule("*/tagName", new Rule()
{
    @Override
    public void begin(String namespace, String name, Attributes attributes) throws Exception
    {
        advCrit = attributes.getIndex("constructor") != -1;
    }
}

But it is not the correct way to do such a task. Do you have any clue?