public int requiredIntegerAttribute(String name) {
        if (stringAttribute(name) == null)
            throw new IllegalArgumentException("Required attribute '" + name + "' is missing");
        return integerAttribute(name, null);
    }
Returns the given attribute or throws IllegalArgumentException if not present
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/ModelElement.java