How can I share a variable or object between two or more Servlets?

Question

I would like to know if there is some way to share a variable or an object between two or more Servlets, I mean some "standard" way. I suppose that this is not a good practice but is a easier way to build a prototype.

I don't know if it depends on the technologies used, but I'll use Tomcat 5.5

Thanks.

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I want to share a Vector of objects of a simple class (just public attributes, strings, ints, etc). My intention is to have a static data like in a DB, obviously it will be lost when the Tomcat is stopped. (is Just for Testing)

Answer 1

Couldn't you just put the object in the HttpSession and then refer to it by its attribute name in each of the servlets?

e.g:

getSession().setAttribute("thing", object);

...then in another servlet:

Object obj = getSession.getAttribute("thing");

Answer 2

Depends on the scope of the intended use of the data.

If the data is only used on a per-user basis, like user login info, page hit count, etc. use the session object (httpServletRequest.getSession().get/setAttribute(String [,Object]))

If it is the same data across multiple users (total web page hits, worker threads, etc) use the ServletContext attributes. servlet.getServletCongfig().getServletContext().get/setAttribute(String [,Object])). This will only work within the same war file/web applicaiton. Note that this data is not persisted across restarts either.

Answer 3

I think what you're looking for here is request, session or application data.

In a servlet you can add an object as an attribute to the request object, session object or servlet context object:

protected void doGet(HttpServletRequest request, HttpServletResponse response) {
    String shared = "shared";
    request.setAttribute("sharedId", shared); // add to request
    request.getSession().setAttribute("sharedId", shared); // add to session
    this.getServletConfig().getServletContext().setAttribute("sharedId", shared); // add to application context
    request.getRequestDispatcher("/URLofOtherServlet").forward(request, response);
}

If you put it in the request object it will be available to the servlet that is forwarded to until the request is finished:

request.getAttribute("sharedId");

If you put it in the session it will be available to all the servlets going forward but the value will be tied to the user:

request.getSession().getAttribute("sharedId");

Until the session expires based on inactivity from the user.

Is reset by you:

request.getSession().invalidate();

Or one servlet removes it from scope:

request.getSession().removeAttribute("sharedId");

If you put it in the servlet context it will be available while the application is running:

this.getServletConfig().getServletContext().getAttribute("sharedId");

Until you remove it:

this.getServletConfig().getServletContext().removeAttribute("sharedId");

Answer 4

Put it in one of the 3 different scopes.

request - lasts life of request

session - lasts life of user's session

application - lasts until applciation is shut down

You can access all of these scopes via the HttpServletRequest variable that is passed in to the methods that extend from the HttpServlet class