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

Question

Are there multiple instances of servlet class? As I hear "each instance of servlet" Can anybody elaborate on this?

Answer 1

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.     }  }