How do servlets work? Instantiation, sessions, shared variables and multithreading
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?
ServletContext
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 file 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 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.
When a Servlet has a <servlet><load-on-startup> or @WebServlet(loadOnStartup) value greater than 0 , 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 -> 1st, 2 -> 2nd, etc). If the same value is specified for more than one servlet, then each of those servlets is loaded in the order they appear in the web.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.
HttpServletRequest and HttpServletResponse
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 (user with a web browser) sends an HTTP request, the servlet container creates new HttpServletRequest and HttpServletResponse objects and passes them through any defined Filter chain and, eventually, the Servlet instance.
In the case of filters, the doFilter() method is invoked. When its 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 headers 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 for reuse.
HttpSession
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 store 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 a decent web browser and web server have to 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 (ie 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 not been used 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 begin used.
In a nutshell
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. Thread Safety
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 of 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.
}
}
See also:
Sessions
In short: the web server issues a unique identifier to each visitor on his first visit. The visitor must bring back that ID for him to be recognised next time around. This identifier also allows the server to properly segregate objects owned by one session against that of another.
Servlet Instantiation
If load-on-startup is false :
If load-on-startup is true :
Once he's on the service mode and on the groove, the same servlet will work on the requests from all other clients.
Why isn't it a good idea to have one instance per client? Think about this: Will you hire one pizza guy for every order that came? Do that and you'd be out of business in no time.
It comes with a small risk though. Remember: this single guy holds all the order information in his pocket: so if you're not cautious about thread safety on servlets, he may end up giving the wrong order to a certain client.
Session in Java servlets is the same as session in other languages such as PHP. It is unique to the user. The server can keep track of it in different ways such as cookies, url rewriting etc. This Java doc article explains it in the context of Java servlets and indicates that exactly how session is maintained is an implementation detail left to the designers of the server. The specification only stipulates that it must be maintained as unique to a user across multiple connections to the server. Check out this article from Oracle for more information about both of your questions.
Edit There is an excellent tutorial here on how to work with session inside of servlets. And here is a chapter from Sun about Java Servlets, what they are and how to use them. Between those two articles, you should be able to answer all of your questions.
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