Difference between each instance of servlet and each thread of servlet in servlets? [duplicate]

When the Servlet container starts, it:

1. reads web.xml;
2. finds the declared Servlets in the classpath; and
3. loads and instantiates each Servlet only once.

Roughly, like this:

String urlPattern = parseWebXmlAndRetrieveServletUrlPattern();
String servletClass = parseWebXmlAndRetrieveServletClass();
HttpServlet servlet = (HttpServlet) Class.forName(servletClass).newInstance();
servlet.init();
servlets.put(urlPattern, servlet); // Similar to a map interface.

Those Servlets are stored in memory and reused every time the request URL matches the Servlet's associated url-pattern. The servlet container then executes code similar to:

for (Entry<String, HttpServlet> entry : servlets.entrySet()) {
    String urlPattern = entry.getKey();
    HttpServlet servlet = entry.getValue();
    if (request.getRequestURL().matches(urlPattern)) {
        servlet.service(request, response);
        break;
    }
}

The GenericServlet#service() on its turn decides which of the doGet(), doPost(), etc.. to invoke based on HttpServletRequest#getMethod().

You see, the servletcontainer reuses the same servlet instance for every request. In other words: the servlets are shared among every request. That's why it's extremely important to write servlet code the threadsafe manner --which is actually simple: just do not assign request or session scoped data as servlet instance variables, but just as method local variables. E.g.

public class MyServlet extends HttpServlet {

    private Object thisIsNOTThreadSafe;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Object thisIsThreadSafe;

        thisIsNOTThreadSafe = request.getParameter("foo"); // BAD!! Shared among all requests!
        thisIsThreadSafe = request.getParameter("foo"); // OK, this is thread safe.
    } 
}

No, there is only one instance of the servlet which is reused for multiple requests from multiple clients. This leads to two important rules:

• don't use instance variables in a servlet, except for application-wide values, most often obtained from context parameters.
• don't make methods synchronized in a servlet

(same goes for servlet filters and jsps)

According to the Java Servlet Specification Version 3.0 (pp. 6-7), there will be one instance per declaration per JVM, unless the servlet implements SingleThreadModel in which case there may be multiple instances per JVM.