Question about thread-safe of JTidy

I know that JTidy is not thread safe.  My question is to the what parts are
not thread-safe.
For example, If keep many dom's (Nodes from parseDOM), is there any issue
with processing
the dom's in a mult-threaded system?  This assumes any call to JTidy's
engine , parseDOM and pprint, are
keep synchronized.

I know I could figure out this, but any help to reduce the time would be
helpful.

Thanks.