In XWiki 9.7RC1, we have introduced an Abstract Syntax Tree (AST) to generate the query that fetch the events we want to display as notifications.

This AST has been created for 2 reasons:

• in the future, the events might be stored in a different way, and the current HQL query might be not effective on a different storage (such as NoSQL). This AST abstracts the language so the query could still be effective, as soon as we have a translator for the target language.
• it's easier for developers to write a NotificationFilter by generating an AST than generating a good HQL query that must escape its values and co-exists with other filters...

In this release, we have rewritten the whole internal query, and we had to introduce new types of ExpressionNode to handle all use-cases. This new nodes are:

• BooleanValueNode to handle a boolean value
• DateValueNode to handle a date
• EntityReferenceNode to handle an EntityReference without manual serialization
• GreaterThanNode to handle >= comparison
• InNode to check if a property or a value is contained in a list
• LesserThanNode to handle <= comparison
• StartsWith to check if a value or a property starts with some other value

Since theses nodes have been created for our own implementation needs, please open a JIRA issue if you wish to have more.