Struts 1 Actions are singletons therefore they must be thread-safe because only one instance of a class handles all the requests for that Action. The singleton strategy restricts to Struts 1 Actions and requires extra care to make the action resources thread safe or synchronized while developing an application.
Struts 2 doesn't have thread-safety issues as Action objects are instantiated for each request. A servlet container generates many throw-away objects per request, and one more object does not impose a performance penalty or impact garbage collection.
For more details please visit:http://www.roseindia.net/struts/struts1-vs-struts2.shtml