Why Not a Function #23: validate-xml

(import
  '(javax.xml.validation SchemaFactory)
  '(javax.xml XMLConstants)
  '(javax.xml.transform.stream StreamSource)
  '(org.xml.sax ErrorHandler)
  '(java.net URL))

(defn validate-xml
  [xml ^URL xsd]
  {:pre [xml]}
  (let [factory (SchemaFactory/newInstance
                  XMLConstants/W3C_XML_SCHEMA_NS_URI)
        schema (.newSchema factory xsd)
        validator (.newValidator schema)
        source (-> xml (io/input-stream) (StreamSource.))
        warnings-as-errors (reify ErrorHandler
                             (warning [_ ex] (throw ex))
                             (error [_ ex] (throw ex))
                             (fatalError [_ ex] (throw ex)))]
    (.setErrorHandler validator warnings-as-errors)
    (.validate validator source)))

Validating an XML document against an XML Schema (XSD) can be done in pure JVM and this is a Clojure code to do it. The first argument to validate-xml is anything that can be converted into input stream (the XML) and the second argument is an URL (the XSD). In case of any validation warnings or errors this routine will throw a SAXParseException. Otherwise it will just return nil.

Below we are additionally using the output-xml function defined in the previous blog.

(output-xml [:yeehaw])
=> #object["[B" 0x3dd69b7f "[B@3dd69b7f"]

(validate-xml *1 (URL. "https://www.w3.org/2001/03/xml.xsd"))
Execution error (SAXParseException) at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper/createSAXParseException (ErrorHandlerWrapper.java:203).
cvc-elt.1: Cannot find the declaration of element 'yeehaw'.

Written on April 14, 2020