I discovered a problem today with some code that was reusing a custom Rule in multiple threads, even though each thread was creating its own digester. It seems that Digester.addRule is calling rule.setDigester and if the rule is shared across multiple threads, the calls to begin/end can get tangled across threads.
It is obvious that Rules are not meant to be shared, but the javadoc <http://commons.apache.org/digester/apidocs/org/apache/commons/digester3/Rule.html> seems to be implying the opposite and is confusing at best. It talks about the rules being stateless, even though the framework itself is changing its state with rule.setDigester(digester). It further states that since all state is part of the digester, the rule is safe under all cases, which is very misleading.
" ... Rule objects should be stateless, ie they should not update any instance member during the parsing process. A rule instance that changes state will encounter problems if invoked in a "nested" manner; this can happen if the same instance is added to digester multiple times or if a wildcard pattern is used which can match both an element and a child of the same element. The digester object stack and named stacks should be used to store any state that a rule requires, making the rule class safe under all possible uses. ..."
I think the statement above should be reworded to be more correct and avoid confusion. Down the line, maybe the digester accessed by the rule should be a ThreadLocal.