Re-use SAXParserFactory in AbstractUnmarshallerImpl for better performance

[winne42]

The current design of AbstractUnmarshallerImpl has a serious negative performance impact when unmarshalling many XML files, which could easily be avoided.

Most JAXB documentation found anywhere including the JAXB Users Guide recommend creating a single JAXBContext and then new Unmarshallers for every unmarshalling tasks (the Users Guide recommends using object pooling for best performance, but many JAXB users will never have read that remark; also, this approach is more complex).

My issue with using JAXB in this "standard way" is that AbstractUnmarshallerImpl.getXMLReader() creates a new SAXParserFactory for every new Unmarshaller:

• This is potentially quite an expensive operation, because the JAXP Lookup Mechanism uses the notoriously slow ServiceLoader mechanism. (this can be avoided by using system environment variables or jaxp.properties, but I highly doubt that this is a well-known fact)
• It also seems unintuitive that a new factory with the same parameters is built over-and-over-again (e.g., we don't build a new car factory for every car we build, do we?)

My suggestion would be to make the well-documented default use-case of unmarshalling faster by creating a singleton SAXParserFactory in AbstractUnmarshallerImpl and re-using it in getXMLReader().