Is HttpSession thread safe, are set/get Attribute thread safe operations?

Servlet 2.5 spec:

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.

No. And since you don't want the same client (with session) to be doing concurrent requests, you should serialize these requests like the AbstractController does in Spring MVC

In some ways, this depends on your client design.

Do you have the opportunity, in your web design, for a single client to have multiple outstanding simultaneous requests using the same HTTP session? This seems difficult to do unless you tie a single HTTP session to multiple sockets. (aka, AJAX) Short of doing this, a given client's HTTP access will be single-threaded as far as the server is concerned, which means a single session is effectively Thread safe.

Synchronization of your session objects will make the application safer against future changes that make your web application capable of having multiple simultaneous requests, so it's not a bad idea. In modern Java implementations, synchronization does not have the large cost that was previously associated with it, especially when the synchronization is usually uncontended. If your application uses AJAX, which implies that you expect multiple in-flight simultaneous requests to your web server, then synchronization is a must.

They are not, but most of the times, your clients will only access them with a single thread.

Different clients will have different threads and each one will have its own Session.

As Eddie points out, one situation where you may face two thread accessing the same session is two ajax calls are attempting to modify the same session attribute. Otherwise you won't have problems.