How to Use JavaScript in Struts2?

Question

I have an Interceptor that wants to put something in the session after the action has executed.

But if the session doesn't already exist I get an exception:

java.lang.IllegalStateException: Cannot create a session after the response has been committed

How can I determine if the session already exists from within an Interceptor?

 public String doIntercept(ActionInvocation invocation) throws Exception
 {
 String result = invocation.invoke();
 Map session = invocation.getInvocationContext().getSession();
 session.put(key, value); // throws exception if session doesn't exist
 return result;
 }