Java – How to share a variable or object between two or more Servlets

javaservletsweb-applications

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

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. (it's just for Testing)

Best Solution

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");
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