Is HttpSession is ThreadSafe:-

Multiple servlets executing request threads may have active access to the same session object at the same time. The container must ensure that manipulation of internal data structures representing the session attributes is performed in a threadsafe manner. The Developer has the responsibility for threadsafe access to the attribute objects themselves. This will protect the attribute collection inside the HttpSession object from concurrent access, eliminating the opportunity for an application to cause that collection to become corrupted.

This is safe:

// guaranteed by the spec to be safe
request.getSession().setAttribute("foo", 1);

This is not safe:

HttpSession session = request.getSession();
Integer n = (Integer) session.getAttribute("foo");
// not thread safe
// another thread might be have got stale value between get and set
session.setAttribute("foo", (n == null) ? 1 : n + 1);

This is not guaranteed to be safe:

// no guarantee that same instance will be returned,
// nor that session will lock on "this"
HttpSession session = request.getSession();
synchronized (session) {
  Integer n = (Integer) session.getAttribute("foo");
  session.setAttribute("foo", (n == null) ? 1 : n + 1);
}

I have seen this last approach advocated (including in J2EE books), but it is not guaranteed to work by the Servlet specification. You could use the session ID to create a mutex, but there must be a better approach.

Reference:-

http://stackoverflow.com/questions/616601/is-httpsession-thread-safe-are-set-get-attribute-thread-safe-operations