Drools JBoss Rules 5.X Developer’s Guide

Null-safe dereferencing operator in Drools patterns
The !. operator allows to derefencing in a null-safe way. More in details the matching algorithm requires the value to the left of the !. operator to be not null in order to give a positive result for pattern matching itself. In other words the pattern:
Person( $streetName : address!.street )
will be internally translated in:
Person( address != null, $streetName : address.street )