Session and Cookies

转载自：http://stackoverflow.com/questions/16099752/session-or-cookie-confusion

[PART 1] : SESSION OBJECT

HTTP-request are processed separately, so in order to keep information between each request (for instance, information about the user), a session object has to be created on server-side.

Some websites doesn't need a session at all. A website where users can't modify any content won't have to manage a session (for instance, an online CV). You won't need any cookie or session on such a website.

Create a session :

In a servlet, use the method request.getSession(true) from the HttpServletRequest object to create a new HttpSession object. Note that if you use request.getSession(false), null will be returned if the session has not already been created. Look at this answer for more details.

Set / Get attributes :

The purpose of a session is to keep information on server-side between each request. For instance, keeping the user's name :

session.setAttribute("name","MAGLEFF");
// Cast
String name = (String) session.getAttribute("name");

Destroy a session :

A session will be automatically destroyed if kept inactive too much time. Look at this answer for more details. But you can force manually the session to be destroyed, in the case of a logout action for example :

HttpSession session = request.getSession(true); 
session.invalidate();

 [PART 2] : So... join the dark side, we have COOKIES ?

There comes the cookies.

JSESSIONID :

A JSESSIONID cookie is created on the user's computer each time a session is created withrequest.getSession(). Why ? Because each session created on server side has an ID. You can't acces another user's session, unless you don't have the right ID. This ID is kept in JSESSIONID cookie, and allow the user to find his information. Look at this answer for more details !

When does a JSESSIONID is deleted ?

JSESSIONID doesn't have an expiration date : it's a session cookie. As all session cookies, it will be deleted when the broswer is closed. If you use the basic JSESSIONID mechanism, then the session will become unreachable after you close and re-open the browser, because JSESSIONID cookie is deleted.

Note that the session is unreachable by the client, but is still running on server-side. Setting aMaxInactiveInterval allows the server to automatically invalidate the session when it has been inactive for too long.

Evil destruction of JSESSIONID

Just for fun, one day I found this code on a project. It was used to invalidate the session by deleting the JSESSIONID cookie with javascript :

<SCRIPT language="JavaScript" type="text/javascript">

    function delete_cookie( check_name ) {
        // first we'll split this cookie up into name/value pairs
        // note: document.cookie only returns name=value, not the other components
        var a_all_cookies = document.cookie.split( ';' );
        var a_temp_cookie = '';
        var cookie_name = '';
        var cookie_value = '';
        var b_cookie_found = false; // set boolean t/f default f
        // var check_name = 'JSESSIONID';
        var path = null;

        for ( i = 0; i < a_all_cookies.length; i++ )
        {
            // now we'll split apart each name=value pair
            a_temp_cookie = a_all_cookies[i].split( '=' );
            // and trim left/right whitespace while we're at it
            cookie_name = a_temp_cookie[0].replace(/^\s+|\s+$/g, '');
            // alert (cookie_name);

            // if the extracted name matches passed check_name
            if ( cookie_name.indexOf(check_name) > -1 )
            {
                b_cookie_found = true;
                // we need to handle case where cookie has no value but exists (no = sign, that is):
                if ( a_temp_cookie.length > 1 )
                {
                    cookie_value = unescape( a_temp_cookie[1].replace(/^\s+|\s+$/g, '') );
                    document.cookie = cookie_name + "=" + cookie_value +
                    ";path=/" +
                    ";expires=Thu, 01-Jan-1970 00:00:01 GMT";
                    // alert("cookie deleted " + cookie_name);
                }
            }
            a_temp_cookie = null;
            cookie_name = '';
        }
        return true;
    }
    // DESTROY
    delete_cookie("JSESSIONID");

</SCRIPT>

 Give another look to this answer. With javascript, JSESSIONID can be red, modified, etc. and the session lost, or hijacked.

[PART 3] : KEEPING A SESSION AFTER CLOSING YOUR BROWSER

After closed and re-opened the browser and their accounts are still signed in. But some websites, cannot do like that. I'm confused that it's considered session or cookie??

It's cookie.

We saw that when the JSESSIONID session cookie has been deleted by the web browser, the session object on server-side is lost. There is no way to access it again without the right ID.

If I want my website to be signed in like that, do I have to set session.setMaxInactiveInterval() or cookie.setMaxAge()?

We also saw that session.setMaxInactiveInterval() was to prevent from running a lost session indefinitely. JSESSIONID cookie cookie.setMaxAge() won't get us anywhere either.

Use a persistent cookie with the session Id :

I came to this solution after reading the following topics :

• Java EE 6: How to implement "Stay Logged In" when user login in to the web application by BalusC
• http://simple.souther.us/not-so-simple.html by Ben Souther; [email protected]

The main idea is to register the user's session in a Map, put into the servlet context. Each time a session is created, it is added to the Map with the JSESSIONID value for key; A persistent cookie is also created to memorize the JSESSIONID value, in order to find the session after the JSESSIONID cookie has been destroyed.

When you close the web browser, JSESSIONID is destroyed. But all the HttpSession objects adress have been kept into a Map on server-side, and you can access the right session with the value saved into the persistent cookie.

First, add two listeners in your web.xml deployment descriptor.

<listener>
    <listener-class>
        fr.hbonjour.strutsapp.listeners.CustomServletContextListener
    </listener-class>
</listener>

<listener>
    <listener-class>
        fr.hbonjour.strutsapp.listeners.CustomHttpSessionListener
    </listener-class>
</listener>

 The CustomServletContextListener creates a map at context initialization. This map will register all the sessions created by the user on this application.

/**
 * Instanciates a HashMap for holding references to session objects, and
 * binds it to context scope.
 * Also instanciates the mock database (UserDB) and binds it to 
 * context scope.
 * @author Ben Souther; [email protected]
 * @since Sun May  8 18:57:10 EDT 2005
 */
public class CustomServletContextListener implements ServletContextListener{

    public void contextInitialized(ServletContextEvent event){
        ServletContext context = event.getServletContext();

        //
        // instanciate a map to store references to all the active
        // sessions and bind it to context scope.
        //
        HashMap activeUsers = new HashMap();
        context.setAttribute("activeUsers", activeUsers);
    }

    /**
     * Needed for the ServletContextListener interface.
     */
    public void contextDestroyed(ServletContextEvent event){
        // To overcome the problem with losing the session references
        // during server restarts, put code here to serialize the
        // activeUsers HashMap.  Then put code in the contextInitialized
        // method that reads and reloads it if it exists...
    }
}

 The CustomHttpSessionListener will put the session into the activeUsers map when it is created.

/**
 * Listens for session events and adds or removes references to 
 * to the context scoped HashMap accordingly.
 * @author Ben Souther; [email protected]
 * @since Sun May  8 18:57:10 EDT 2005
 */
public class CustomHttpSessionListener implements HttpSessionListener{

    public void init(ServletConfig config){
    }

    /**
     * Adds sessions to the context scoped HashMap when they begin.
     */
    public void sessionCreated(HttpSessionEvent event){
        HttpSession    session = event.getSession();
        ServletContext context = session.getServletContext();
        HashMap<String, HttpSession> activeUsers =  (HashMap<String, HttpSession>) context.getAttribute("activeUsers");

        activeUsers.put(session.getId(), session);
        context.setAttribute("activeUsers", activeUsers);
    }

    /**
     * Removes sessions from the context scoped HashMap when they expire
     * or are invalidated.
     */
    public void sessionDestroyed(HttpSessionEvent event){
        HttpSession    session = event.getSession();
        ServletContext context = session.getServletContext();
        HashMap<String, HttpSession> activeUsers = (HashMap<String, HttpSession>)context.getAttribute("activeUsers");
        activeUsers.remove(session.getId());
    }

}

 Use a basic form to test a user authentification by name/password. This login.jsp form is meant for test only.

<!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><bean:message key="formulaire1Title" /></title>
    </head>
    <body>
        <form action="login.go" method="get">
            <input type="text" name="username" />
            <input type="password" name="password" />
            <input type="submit" />
        </form>
    </body>
</html>

 There we go. This java servlet is forwarding to a login page when the user is not in session, and to another page when he is. It is only meant for testing the persistent session!

public class Servlet2 extends AbstractServlet {

    @Override
    protected void doGet(HttpServletRequest pRequest,
            HttpServletResponse pResponse) throws IOException, ServletException {
        String username = (String) pRequest.getParameter("username");
        String password = (String) pRequest.getParameter("password");
        // Session Object
        HttpSession l_session = null;

        String l_sessionCookieId = getCookieValue(pRequest, "JSESSIONID");
        String l_persistentCookieId = getCookieValue(pRequest, "MY_SESSION_COOKIE");

        // If a session cookie has been created
        if (l_sessionCookieId != null)
        {
            // If there isn't already a persistent session cookie
            if (l_persistentCookieId == null)
            {
                addCookie(pResponse, "MY_SESSION_COOKIE", l_sessionCookieId, 1800);
            }
        }
        // If a persistent session cookie has been created
        if (l_persistentCookieId != null)
        {
            HashMap<String, HttpSession> l_activeUsers = (HashMap<String, HttpSession>) pRequest.getServletContext().getAttribute("activeUsers");
            // Get the existing session
            l_session = l_activeUsers.get(l_persistentCookieId);
        }
        // Otherwise a session has not been created
        if (l_session == null)
        {
                    // Create a new session
            l_session = pRequest.getSession();
        }

            //If the user info is in session, move forward to another page
        String forward = "/pages/displayUserInfo.jsp";

        //Get the user
        User user = (User) l_session.getAttribute("user");

        //If there's no user
        if (user == null)
        {
                    // Put the user in session
            if (username != null && password != null)
            {
                l_session.setAttribute("user", new User(username, password));
            }
                    // Ask again for proper login
            else
            {
                forward = "/pages/login.jsp";
            }
        }
        //Forward
        this.getServletContext().getRequestDispatcher(forward).forward( pRequest, pResponse );

    }

 The MY_SESSION_COOKIE cookie save the value of the JSESSIONID cookie. When the JSESSIONID cookie is destroyed, the MY_SESSION_COOKIE is still there with the session ID.

JSESSIONID is gone with the web browser session, but we chose to use a persistent and simple cookie, along with a map of all active sessions put into the application context. The persistent cookie allow us to find the right session in the map.

Don't forget these useful methods made by BalusC to add/get/remove cookies :

/**
 * 
 * @author BalusC
 */
public static String getCookieValue(HttpServletRequest request, String name) {
    Cookie[] cookies = request.getCookies();
    if (cookies != null) {
        for (Cookie cookie : cookies) {
            if (name.equals(cookie.getName())) {
                return cookie.getValue();
            }
        }
    }
    return null;
}

/**
 * 
 * @author BalusC
 */
public static void addCookie(HttpServletResponse response, String name, String value, int maxAge) {
    Cookie cookie = new Cookie(name, value);
    cookie.setPath("/");
    cookie.setMaxAge(maxAge);
    response.addCookie(cookie);
}

/**
 * 
 * @author BalusC
 */
public static void removeCookie(HttpServletResponse response, String name) {
    addCookie(response, name, null, 0);
}

}

 The last solution was tested with glassfish on localhost, with chrome for webbrowser, on windows. It only depends on a single cookie, and you don't need a database. But actually, I don't know what are the limits of such a mechanism. I only spent the night coming to this solution, without knowing if it will be a good or a bad one.