how-do-servlets-work-initialization-shared-variables-and-multithreading


http://stackoverflow.com/questions/3106452/how-do-servlets-work-instantiation-shared-variables-and-multithreading/3106909#3106909

question:

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?

answer:

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

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.
}
}