Why instance variable in Servlet is not thread-safe [duplicate]

Question

When I read Head First Servlet and JSP, they say that instance variable is non-thread safe.

I don't understand this statement so much. For example: I have a servlet which name is ActionServlet.java. Each time, each user's request is sent to server, container will create a new thread and create new ActionServlet instance.

ActionServlet may be has a structure:

public class ActionServlet extends HttpServlet {
   // example of instance variable
   Instance variable;
   public void processRequest(HttpServletRequest request, HttpServletResponse response) {   
       // process something relating to instance variable
   }
}

So, because all these threads create a new class instance for ActionServlet, so I don't see any problem here. because instances of these thread is separate of each other.

Please figure out where problem when use instance variable in multithread environment.

Thanks :)

Answer 1

The mistake you are making is here:

So, because all these threads create a new class instance for ActionServlet, so I don't see any problem here. because instances of these thread is separate of each other.

The container does not create a new instance of the Servlet class for each request. It reuses an existing one. This is why they are not thread safe.

The Stripes Action Framework DOES create a new instance for each request, so that's an okay assumption under that framework. However, for example, Struts 1 follows the Servlet model and does not create a new action per request.

That does not mean that the container is limited to a single instance, it in theory can create more than one, but it's not a specified behavior so can not be relied upon. Most of the popular ones do not.

Answer 2

because all these threads create a new class instance (action.java), so I don't see any problem

You are supposing that every thread create a class instance that will be used only by that thread, that's why you don't have any problem.

But try to imagine, with your specific sample, that the same instance is accessed from two threads. What's happend if both use in the same time your request and response members? Maybe you will read data from unidentifiable request, and you will write an inconsistent response that mix two parts.

So in you case too, instance variable are not thread safe, because if two thread access the same instance they can disturb each other.