Threading model of Struts1:
Struts 1 Actions are singletons and must be thread-safe since there will only be one instance of a class to handle all requests for that Action. The singleton strategy places restrictions on what can be done with Struts 1 Actions and requires extra care to develop. Action resources must be thread-safe or synchronized.
and that of Struts2:
Struts 2 Action objects are instantiated for each request, so there are no thread-safety issues. (In practice, servlet containers generate many throw-away objects per request, and one more object does not impose a performance penalty or impact garbage collection.)
Like it's been pointed out,
Struts 2 actions are NOT singletons, they're instantiated for every user request. So you don't have to worry about thread-safety, your instance(and of course class) variables are inherently thread-safe.