How Do Servlets Work? Instantiation, Sessions, Shared Variables and Multithreading

How do servlets work? Instantiation, sessions, shared variables and multithreading

`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 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) will be instantiated once and be kept in the server's memory as well, registred via the ServletContext. For each instantiated filter, its init() method is invoked with a new FilterConfig argument which in turn contains the involved ServletContext.

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 argument which in turn contains the involved ServletContext. 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 with a ServletContextEvent argument which in turn contains the involved ServletContext. This will allow the developer the opportunity to programmatically register yet another Servlet, Filter or Listener.

When the servlet container shuts down, it unloads all web applications, invokes the destroy() method of all its initialized servlets and filters, and all Servlet, Filter and Listener instances registered via the ServletContext are trashed. Finally the ServletContextListener#contextDestroyed() will be invoked and the ServletContext itself will be trashed.

`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 (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.

`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 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.

In a nutshell

The ServletContext lives for as long as the web app lives. It is shared among all requests in all sessions.
The 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.
The 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.
All Servlet, Filter and Listener instances live as long as the web app lives. They are shared among all requests in all sessions.
Any 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 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.
    } 
}

How do servlets instances work?

Only one instance of the servlet is created. This is why it is so important not to include fields in your servlet that are used within the get and post methods. A class instance variable (field) within a servlet is not thread safe and could be modified by multiple requests resulting in unexpected behavior.

Consider the following example:

ServletTest.java

@WebServlet("/ServletTest")
public class ServletTest extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private Integer increment = 0;

    public ServletTest() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println(increment++);
    }

}

test.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>  
<form action="ServletTest" method="post">
    <button type="submit">Submit</button>
</form>
</body>
</html>

If you run this example and hit the submit button it will print 0 to the console. Subsequent submits of the form will print 1, then 2, etc... This proves the servlet is the same instance.

How does multithreading work for a java Servlet?

An object instance('s methods) can be called simultaneously by more than one thread. This isn't specific for servlets though and is true in general.

What happens when this occurs? Each thread will still have their own stack, which means, each thread will have a different copy of local variables to work on. As a result there will be no interference between threads and you won't have to worry about concurrent calls. Only when a shared resource, e.g. instance/class variable is accessed, there can be a problem. Same thing if instance/class variable is directly accessed concurrently.

EJBs in contrast do exactly what you seem to suggest. The EJB container makes sure that only one thread enters an EJB instance at a time, and hence an EJB programmer doesn't have to worry about concurrency so long as he/she doesn't break the EJB programming contract. There is no reason why the servlet spec didn't do this. Most likely no body came up with it during the meetings? It does have the advantage though that you can use a more efficient concurrency management than EJB's "one thread per instance".

Why instance variable in Servlet is not thread-safe

The mistake you are making is here:

So, because all these threads create a new class instance for
ActionServlet, so I don't see any problem here. because instances of
these thread is separate of each other.

The container does not create a new instance of the Servlet class for each request. It reuses an existing one. This is why they are not thread safe.

The Stripes Action Framework DOES create a new instance for each request, so that's an okay assumption under that framework. However, for example, Struts 1 follows the Servlet model and does not create a new action per request.

That does not mean that the container is limited to a single instance, it in theory can create more than one, but it's not a specified behavior so can not be relied upon. Most of the popular ones do not.

Towards understanding servlets and multi-threading

This is not specific to Servlets. This is specific to Java. The Java language and VM supports invoking a single method by multiple threads. Each thread will just have its own share of method-local variables. If you want to restrict this, you've to add a synchronized lock to the method or to let the servlet implement the (deprecated!) SingleThreadModel interface. But that's not necessary if you write servlet methods on a threadsafe manner (i.e. do not assign request/session scoped data as instance variable).

Are instance variables of non-servlet classes are thread safe?

Are instance variables of non-servlet classes are thread safe?

In your case, it is not an instance variable but a locale variable in the execution scope of the doGet() method.

 protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
     ...
     x xx = new x();
     ...
    }

Locale variables declared and used in a method are isolated between the calls to it of multiple threads. So you have no potential concurrency here.

If you xx variable was a instance variable, you would have a race condition since multiple threads could access them in a concurrent way if the servlet was called multiple times in a close timing.

 public class myServlet extends HttpServlet {
     x xx = new x();
     ...
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
     ...
     xx.doSomething(); // concurrency race 
     xx.var1 = varA;   // concurrency race 
     ...
    }

Difference between each instance of servlet and each thread of servlet in servlets?

When the Servlet container starts, it:

reads web.xml;
finds the declared Servlets in the classpath; and
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.
    } 
}

How Do Servlets Work? Instantiation, Sessions, Shared Variables and Multithreading