使用xerces作为jaxp的缺省实现，如何设置factory

javax.xml.datatype.DatatypeFactory --- org.apache.xerces.jaxp.datatype.DatatypeFactoryImpl
javax.xml.validation.SchemaFactory --- org.apache.xerces.jaxp.validation.XMLSchemaFactory
javax.xml.parsers.SAXParserFactory --- org.apache.xerces.jaxp.SAXParserFactoryImpl
javax.xml.parsers.DocumentBuilderFactory --- org.apache.xerces.jaxp.DocumentBuilderFactoryImpl