How to validate against schema in JAXB 2.0 without marshalling?

Question

I need to validate my JAXB objects before marshalling to an XML file. Prior to JAXB 2.0, one could use a javax.xml.bind.Validator. But that has been deprecated so I'm trying to figure out the proper way of doing this. I'm familiar with validating at marshall time but in my case I just want to know if its valid. I suppose I could marshall to a temp file or memory and throw it away but wondering if there is a more elegant solution.

Answer 1

Firstly, javax.xml.bind.Validator has been deprecated in favour of javax.xml.validation.Schema. The idea is that you parse your schema, and inject that into the marshaller/unmarshaller.

As for your question regarding validation without marshalling, the problem here is that JAXB actually delegates the validation to Xerces (or whichever SAX processor you're using), and Xerces validates your document as a stream of SAX events. So in order to validate, you need to perform some kind of marshalling.

The lowest-impact implementation of this would be to use a "/dev/null" implementation of a SAX processor. Marshalling to a null OutputStream would still involve XML generation, which is wasteful. So I would suggest:

 SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
 Schema schema = schemaFactory.newSchema(locationOfMySchema); 

 Marshaller marshaller = jaxbContext.createMarshaller();
 marshaller.setSchema(schema);
 marshaller.marshal(objectToMarshal, new DefaultHandler());

DefaultHandler will discard all the events, and the marshal() operation will throw a JAXBException if validation against the schema fails.