Suppose, I have a webserver which holds numerous servlets. For information passing among those servlets I am setting session and instance 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 servlet, then this servlet gets instantiated only the first time the first user accessed it or does it get instantiated for all the users separately?
In other words, what happens to the instance variables?
When the servlet container (like Apache Tomcat) starts up, it will deploy and load all its web applications. When a web application is loaded, the servlet container creates the ServletContext
once and keeps it in the server's memory. The web app's web.xml
and all of included web-fragment.xml
files is parsed, and each <servlet>
, <filter>
and <listener>
found (or each class annotated with @WebServlet
, @WebFilter
and @WebListener
respectively) is instantiated once and kept in the server's memory as well. For each instantiated filter, its init()
method is invoked with a new FilterConfig
.
When a Servlet
has a <servlet><load-on-startup>
or @WebServlet(loadOnStartup)
value greater than 0
, then its init()
method is also invoked during startup with a new ServletConfig
. Those servlets are initialized in the same order specified by that value (1
is 1st, 2
is 2nd, etc). If the same value is specified for more than one servlet, then each of those servlets is loaded in the same order as they appear in the web.xml
, web-fragment.xml
, or @WebServlet
classloading. In the event the "load-on-startup" value is absent, the init()
method will be invoked whenever the HTTP request hits that servlet for the very first time.
When the servlet container is finished with all of the above described initialization steps, then the ServletContextListener#contextInitialized()
will be invoked.
When the servlet container shuts down, it unloads all web applications, invokes the destroy()
method of all its initialized servlets and filters, and all ServletContext
, Servlet
, Filter
and Listener
instances are trashed. Finally the ServletContextListener#contextDestroyed()
will be invoked.
The servlet container is attached to a web server that listens for HTTP requests on a certain port number (port 8080 is usually used during development and port 80 in production). When a client (e.g. user with a web browser, or programmatically using URLConnection
) sends an HTTP request, the servlet container creates new HttpServletRequest
and HttpServletResponse
objects and passes them through any defined Filter
in the chain and, eventually, the Servlet
instance.
In the case of filters, the doFilter()
method is invoked. When the servlet container's code calls chain.doFilter(request, response)
, the request and response continue on to the next filter, or hit the servlet if there are no remaining filters.
In the case of servlets, the service()
method is invoked. By default, this method determines which one of the doXxx()
methods to invoke based off of request.getMethod()
. If the determined method is absent from the servlet, then an HTTP 405 error is returned in the response.
The request object provides access to all of the information about the HTTP request, such as its URL, headers, query string and body. The response object provides the ability to control and send the HTTP response the way you want by, for instance, allowing you to set the headers and the body (usually with generated HTML content from a JSP file). When the HTTP response is committed and finished, both the request and response objects are recycled and made available for reuse.
When a client visits the webapp for the first time and/or the HttpSession
is obtained for the first time via request.getSession()
, the servlet container creates a new HttpSession
object, generates a long and unique ID (which you can get by session.getId()
), and stores it in the server's memory. The servlet container also sets a Cookie
in the Set-Cookie
header of the HTTP response with JSESSIONID
as its name and the unique session ID as its value.
As per the HTTP cookie specification (a contract any decent web browser and web server must adhere to), the client (the web browser) is required to send this cookie back in subsequent requests in the Cookie
header for as long as the cookie is valid (i.e. the unique ID must refer to an unexpired session and the domain and path are correct). Using your browser's built-in HTTP traffic monitor, you can verify that the cookie is valid (press F12 in Chrome / Firefox 23+ / IE9+, and check the Net/Network tab). The servlet container will check 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
stays alive until it has been idle (i.e. not used in a request) for more than the timeout value specified in <session-timeout>
, a setting in web.xml
. The timeout value defaults to 30 minutes. So, when the client doesn't visit the web app for longer than the time specified, the servlet container trashes the session. Every subsequent request, even with the cookie specified, will not have access to the same session anymore; the servlet container will create a new session.
On the client side, the session cookie stays alive for as long as the browser instance is running. So, if the client closes the browser instance (all tabs/windows), then the session is trashed on the client's side. In a new browser instance, the cookie associated with the session wouldn't exist, so it would no longer be sent. This causes an entirely new HttpSession
to be created, with an entirely new session cookie being used.
ServletContext
lives for as long as the web app lives. It is shared among all requests in all sessions.HttpSession
lives for as long as the client is interacting with the web app with the same browser instance, and the session hasn't timed out at the server side. It is shared among all requests in the same session.HttpServletRequest
and HttpServletResponse
live from the time the servlet receives an HTTP request from the client, until the complete response (the web page) has arrived. It is not shared elsewhere.Servlet
, Filter
and Listener
instances live as long as the web app lives. They are shared among all requests in all sessions.attribute
that is defined in ServletContext
, HttpServletRequest
and HttpSession
will live as long as the object in question lives. The object itself represents the "scope" in bean management frameworks such as JSF, CDI, Spring, etc. Those frameworks store their scoped beans as an attribute
of its closest matching scope.That said, your major concern is possibly thread safety. You should now know that servlets and filters are shared among all requests. That's the nice thing about Java, it's multithreaded and different threads (read: HTTP requests) can make use of the same instance. It would otherwise be too expensive to recreate, init()
and destroy()
them for every single request.
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 not thread-safe! The below example illustrates this:
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.
}
}