Suppose, I have a webserver which holds numerous Servlets . For information passing among those Servlets I am getting the `Servlets` context and setting session variables.
Now, if 2 or more users send request to this server then what happens to the session variables? Will they all be common for all the users or they will be different for each user. If they are different, then how was the server able to differentiate between different users?
One more similar question, if there are n users accessing a particular Servlets , then this Servlets gets instantiated only the first time the first user accessed it or does it get instantiated for all the users separately?
##ServletContext
When the servletcontainer like Apache Tomcat starts up, it will deploy and load all webapplications. When a webapplication get loaded, the servletcontainer will create the ServletContext once and keep in server's memory. The webapp's `web.xml` will be parsed and every servlet filter and listener found in web.xml, or annotated with respectively @WebServlet, `@WebFilter` and @WebListener, will be created once and kept in server's memory as well. For all filters, the `init()` method will also be invoked immediately. When the servletcontainer shuts down, it will unload all webapplications, invoke the `destroy()` of all initialized servlets and filters, and finally the `ServletContext` and all `Servlet`, `Filter` and `Listener` instances will be trashed.
When the `Servlet` in question has a servlet load-on-startup or @WebServlet(loadOnStartup) value greater than `0`, then its `init()` method will also immediately be invoked during startup. Those servlets are initialized in the same order as "load-on-startup" value represents, or if they are the same, then the order in the `web.xml` or `@WebServlet` classloading. Or, if the "load-on-startup" value is absent, then the `init()` method will only be invoked on very first HTTP request hitting the servlet in question.
##HttpServletRequest and HttpServletResponse
The servletcontainer is attached to a webserver which listens on HTTP requests on a certain port number, which is usually 8080 in development and 80 in production. When a client (user with a webbrowser) sends a HTTP request, the servletcontainer will create new HttpServletRequest and HttpServletResponse objects and pass it through the methods of the already-created `Filter` and Servlet instances whose `url-pattern` matches the request URL, all in the same thread.
The request object provides access to all information of the HTTP request, such as the request headers and the request body. The response object provides facility to control and send the HTTP response the way you want, such as setting headers and the body (usually with HTML content from a JSP file). When the HTTP response is committed and finished, then both the request and response objects will be trashed.
##HttpSession
When a client visits the webapp for the first time and/or the HttpSession is to be obtained for the first time by request.getSession(), then the servletcontainer will create it, generate a long and unique ID (which you can get by `session.getId()`) and store it in server's memory. The servletcontainer will also set a [`Cookie`][6] in the `Set-Cookie` header of the HTTP response with `JSESSIONID` as cookie name and the unique session ID as cookie value.
As per the HTTP cookie specification (a contract a decent webbrowser and webserver has to adhere), the client (the webbrowser) is required to send this cookie back in the subsequent requests in the `Cookie` header as long as the cookie is valid. Using browser builtin HTTP traffic monitor you can check them (press F12 in Chrome / Firefox23+ / IE9+ and check *Net/Network* tab). The servletcontainer will determine the `Cookie` header of every incoming HTTP request for the presence of the cookie with the name `JSESSIONID` and use its value (the session ID) to get the associated `HttpSession` from server's memory.
The `HttpSession` lives until it has not been used for more than the session-timeout time, a setting you can specify in web.xml, which defaults to 30 minutes. So when the client doesn't visit the webapp anymore for over 30 minutes, then the servletcontainer will trash the session. Every subsequent request, even though with the cookie specified, will not have access to the same session anymore. The servletcontainer will create a new one.
On the other hand, the session cookie on the client side has a default lifetime which is as long as the browser instance is running. So when the client closes the browser instance (all tabs/windows), then the session will be trashed at the client side. In a new browser instance the cookie associated with the session won't be sent anymore. A new `request.getSession()` would return a brand new `HttpSession` and set a cookie with a brand new session ID.
##In a nutshell
- The `ServletContext` lives as long as the webapp lives. It's been shared among all requests in all sessions.
- The `HttpSession` lives as long as the client is interacting with the webapp with the same browser instance and the session hasn't timed out at the server side yet. It's been shared among all requests in the same session.
- The `HttpServletRequest` and `HttpServletResponse` lives as long as the client has sent it until the complete response (the webpage) is arrived. It is not being shared elsewhere.
- Any `Servlet`, `Filter` and `Listener` lives as long as the webapp lives. They are being shared among all requests in all sessions.
- Any `attribute` which you set in `ServletContext`, `HttpServletRequest` and `HttpSession` will live as long as the object in question lives.
##Threadsafety
That said, your major concern is possibly *threadsafety*. You should now have learnt that Servlets and filters are shared among all requests. That's the nice thing of Java, it's multithreaded and different threads (read: HTTP requests) can make use of the same instance. It would otherwise have been too expensive to recreate it on every request.
But you should also realize that you should never assign any request or session scoped data as an instance variable of a servlet or filter. It will be shared among all other requests in other sessions. That's threadunsafe ! The below example illustrates that:
public class ExampleServlet 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.
}
}
###See also:
Servlets and Multithreading
Difference between JSP and Servlet
Session management in Java
Now, if 2 or more users send request to this server then what happens to the session variables? Will they all be common for all the users or they will be different for each user. If they are different, then how was the server able to differentiate between different users?
One more similar question, if there are n users accessing a particular Servlets , then this Servlets gets instantiated only the first time the first user accessed it or does it get instantiated for all the users separately?
##ServletContext
When the servletcontainer like Apache Tomcat starts up, it will deploy and load all webapplications. When a webapplication get loaded, the servletcontainer will create the ServletContext once and keep in server's memory. The webapp's `web.xml` will be parsed and every servlet filter and listener found in web.xml, or annotated with respectively @WebServlet, `@WebFilter` and @WebListener, will be created once and kept in server's memory as well. For all filters, the `init()` method will also be invoked immediately. When the servletcontainer shuts down, it will unload all webapplications, invoke the `destroy()` of all initialized servlets and filters, and finally the `ServletContext` and all `Servlet`, `Filter` and `Listener` instances will be trashed.
When the `Servlet` in question has a servlet load-on-startup or @WebServlet(loadOnStartup) value greater than `0`, then its `init()` method will also immediately be invoked during startup. Those servlets are initialized in the same order as "load-on-startup" value represents, or if they are the same, then the order in the `web.xml` or `@WebServlet` classloading. Or, if the "load-on-startup" value is absent, then the `init()` method will only be invoked on very first HTTP request hitting the servlet in question.
##HttpServletRequest and HttpServletResponse
The servletcontainer is attached to a webserver which listens on HTTP requests on a certain port number, which is usually 8080 in development and 80 in production. When a client (user with a webbrowser) sends a HTTP request, the servletcontainer will create new HttpServletRequest and HttpServletResponse objects and pass it through the methods of the already-created `Filter` and Servlet instances whose `url-pattern` matches the request URL, all in the same thread.
The request object provides access to all information of the HTTP request, such as the request headers and the request body. The response object provides facility to control and send the HTTP response the way you want, such as setting headers and the body (usually with HTML content from a JSP file). When the HTTP response is committed and finished, then both the request and response objects will be trashed.
##HttpSession
When a client visits the webapp for the first time and/or the HttpSession is to be obtained for the first time by request.getSession(), then the servletcontainer will create it, generate a long and unique ID (which you can get by `session.getId()`) and store it in server's memory. The servletcontainer will also set a [`Cookie`][6] in the `Set-Cookie` header of the HTTP response with `JSESSIONID` as cookie name and the unique session ID as cookie value.
As per the HTTP cookie specification (a contract a decent webbrowser and webserver has to adhere), the client (the webbrowser) is required to send this cookie back in the subsequent requests in the `Cookie` header as long as the cookie is valid. Using browser builtin HTTP traffic monitor you can check them (press F12 in Chrome / Firefox23+ / IE9+ and check *Net/Network* tab). The servletcontainer will determine the `Cookie` header of every incoming HTTP request for the presence of the cookie with the name `JSESSIONID` and use its value (the session ID) to get the associated `HttpSession` from server's memory.
The `HttpSession` lives until it has not been used for more than the session-timeout time, a setting you can specify in web.xml, which defaults to 30 minutes. So when the client doesn't visit the webapp anymore for over 30 minutes, then the servletcontainer will trash the session. Every subsequent request, even though with the cookie specified, will not have access to the same session anymore. The servletcontainer will create a new one.
On the other hand, the session cookie on the client side has a default lifetime which is as long as the browser instance is running. So when the client closes the browser instance (all tabs/windows), then the session will be trashed at the client side. In a new browser instance the cookie associated with the session won't be sent anymore. A new `request.getSession()` would return a brand new `HttpSession` and set a cookie with a brand new session ID.
##In a nutshell
- The `ServletContext` lives as long as the webapp lives. It's been shared among all requests in all sessions.
- The `HttpSession` lives as long as the client is interacting with the webapp with the same browser instance and the session hasn't timed out at the server side yet. It's been shared among all requests in the same session.
- The `HttpServletRequest` and `HttpServletResponse` lives as long as the client has sent it until the complete response (the webpage) is arrived. It is not being shared elsewhere.
- Any `Servlet`, `Filter` and `Listener` lives as long as the webapp lives. They are being shared among all requests in all sessions.
- Any `attribute` which you set in `ServletContext`, `HttpServletRequest` and `HttpSession` will live as long as the object in question lives.
##Threadsafety
That said, your major concern is possibly *threadsafety*. You should now have learnt that Servlets and filters are shared among all requests. That's the nice thing of Java, it's multithreaded and different threads (read: HTTP requests) can make use of the same instance. It would otherwise have been too expensive to recreate it on every request.
But you should also realize that you should never assign any request or session scoped data as an instance variable of a servlet or filter. It will be shared among all other requests in other sessions. That's threadunsafe ! The below example illustrates that:
public class ExampleServlet 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.
}
}
###See also:
Servlets and Multithreading
Difference between JSP and Servlet
Session management in Java
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