Advanced Topics - eXist: A NoSQL Database and Application Development Platform - page 464

without modifications). When the events are forwarded, they are sent to the vali‐
dator , which ensures the resultant document is well formed and valid according
to any configured schemas. If you choose to throw a SAXException (or any Runti
meException ) from one of your SAX event handler methods, then you are effec‐
tively indicating to eXist that you consider the document to be invalid, which
stops the validation phase and aborts the store process.
Storage
If the validation phase completes, eXist then enters the storage phase. The entire
document is parsed for a second time, and the generated SAX events are again
sent to your trigger, which is responsible for forwarding them (with or without
modifications). When the events are forwarded, they are sent to the database
storage engine, which is responsible for writing the document into the database.
If you choose to throw a SAXException (or any RuntimeException ) from one of
your SAX event handler methods, you are signaling to eXist that there was a
problem with the store process, and eXist will abort the store and roll back the
transaction.
You can determine which phase your SAX event handler methods are being called in
by calling the function isValidating from the super FilteringTrigger class. An
interesting effect of having different validation and storage phases is that you can
modify the document stream in a different manner in each phase. This offers some
interesting possibilities, such as allowing the validation phase to pass, and then
rewriting the document into another form before it is stored.
Consider the startElement method from a fictional implementation of Filtering
Trigger shown in Example 16-12 . This example shows you how you can drop ele‐
ments, rename elements, and create new elements as the document is being stored.
Example 16-12. Event handling in a filtertrigger
@Override
public void startElement ( final String namespaceURI , final String localName ,
final String qname , final Attributes attributes ) throws SAXException {
if ( localName . equals ( "author" )) {
//drop an element
} else if ( localName . equals ( "color" )) {
//rename an element
super . startElement ( namespaceURI , "colour" ,
qname . replace ( "color" , "colour" ), attributes );
} else if ( localName . equals ( "isbn" )) {
//encapsulate an element
super . startElement ( namespaceURI , "reference" , "reference" , null );
super . startElement ( namespaceURI , localName , qname , attributes );