The NetBeans E-commerce Tutorial - Managing Sessions

Content on this page applies to NetBeans IDE, versions 6.8 and 6.9

Every e-commerce application that offers some form of shopping cart functionality needs to be able to remember user-specific data as users click through the website. Unfortunately for you the developer, the HTTP protocol, over which communication on the Internet takes place, is a stateless protocol. Each request received by your server is an independent piece of information that has no relation to previously received requests. Therefore, if a customer clicks a button to add an item to his or her shopping cart, your application must take measures to ensure not only that the state of the user's cart is updated, but that the action doesn't affect the cart of another user who happens to be browsing the site at the same time.

In order to properly handle the above-described scenario, you need to implement functionality so that a session can be created and maintained for the duration of a user's visit to the site. Servlet technology, which is the foundation of all Java-based web applications, provides for this with its HttpSession interface. You also need to define several classes, namely ShoppingCart and ShoppingCartItem, that allow the application to temporarily store user data while the session is being maintained.

This tutorial unit takes a different approach from others in the NetBeans E-commerce Tutorial. Instead of having you create project files and providing steps with code snippets for you to copy and paste into your own project, you open the completed project snapshot for this unit, and examine the code using the IDE's debugger and other tools. In the process, you'll learn how to apply an HttpSession object to your code so that each visit to the website results in a dedicated session. You also learn about scoped variables, and their usage in both Java classes and JSP pages. This unit also discusses HttpSession's default mechanism for maintaining sessions (i.e., cookies) and shows what steps need to be taken in the event that cookies are deactivated in a user's browser. Finally, session time-outs are covered, and the unit demonstrates how to handle them by creating a simple filter that intercepts requests to check whether a session exists.

You can view a live demo of the application that you build in this tutorial: NetBeans E-commerce Tutorial Demo Application.



Software or Resource Version Required
NetBeans IDE Java bundle, 6.8 or 6.9
Java Development Kit (JDK) version 6
GlassFish server v3 or Open Source Edition 3.0.1
MySQL database server version 5.1
AffableBean project snapshot 5

Notes:

  • The NetBeans IDE requires the Java Development Kit (JDK) to run properly. If you do not have any of the resources listed above, the JDK should be the first item that you download and install.
  • The NetBeans IDE Java Bundle includes Java Web and EE technologies, which are required for the application you build in this tutorial.
  • The NetBeans IDE Java Bundle also includes the GlassFish server, which you require for this tutorial. You could download the GlassFish server independently, but the version provided with the NetBeans download has the added benefit of being automatically registered with the IDE.
  • You can follow this tutorial unit without having completed previous units. To do so, see the setup instructions, which describe how to prepare the database and establish connectivity between the IDE, GlassFish, and MySQL.

Handling Session Data

Applications can manage user sessions with the HttpSession object. You can bind user-specific data to the HttpSession object, then access this data at a later stage. Both bind and access actions can be done from Java classes, as well as from session-scoped variables in EL expressions.

Working with an HttpSession Object

The AffableBean application uses the HttpSession object to identify users over multiple requests. An HttpSession object is obtained using getSession() on a given request:

HttpSession session = request.getSession();

If a session object doesn't yet exist for the request, the method creates and returns a new session object.

You can use the session object as a vehicle for passing data between requests. You use the setAttribute method to bind objects to the session. Likewise, you use getAttribute to retrieve objects from the session. In the AffableBean application for example, the user's shopping cart is created and bound to the user session in the following manner:

ShoppingCart cart = new ShoppingCart();
session.setAttribute("cart", cart);

In order to retrieve the cart from the session, the getAttribute method is applied:

cart = (ShoppingCart) session.getAttribute("cart");

In JSP pages, you can access objects bound to the session using EL expressions. Continuing with the above example, if a ShoppingCart object named 'cart' is bound to the session, you can access the object using the following EL expression:

${cart}

Accessing the ShoppingCart object on its own is of little value however. What you really want is a way to access values stored in the object. If you explore the new ShoppingCart class in the project snapshot, you'll note that it contains the following properties:

  • double total
  • int numberOfItems
  • List<String, ShoppingCartItem> items

Provided that properties have matching getter methods, you can access values for singular properties using simple dot notation in an EL expression. If you examine the cart.jsp page, you'll see that this is exactly how the value for numberOfItems is accessed:

<p>Your shopping cart contains ${cart.numberOfItems} items.</p>

In order to extract data from properties that contain multiple values, such as the above items list, the cart.jsp page uses a <c:forEach> loop:

<c:forEach var="cartItem" items="${cart.items}" varStatus="iter">

  <c:set var="product" value="${cartItem.product}"/>

    <tr class="${((iter.index % 2) == 0) ? 'lightBlue' : 'white'}">
        <td>
            <img src="${initParam.productImagePath}${product.name}.png"
                 alt="${product.name}">
        </td>

        <td>${product.name}</td>

        <td>
            &euro; ${cartItem.total}
            <br>
            <span class="smallText">( &euro; ${product.price} / unit )</span>
        </td>
        ...
    </tr>

</c:forEach>

ShoppingCartItem's product property identifies the product type for a cart item. The above loop takes advantage of this by first setting a product variable to the expression ${cartItem.product}. It then uses the variable to obtain information about that product (e.g., name, price).

Working with Scoped Variables in Web Applications

When working with JSP/Servlet technology, there are four scope objects available to you within the realm of the application. JSP technology implements implicit objects that allows you to access classes defined by the Servlet API.

Scope Definition Servlet Class JSP Implicit Object
Application Global memory for a web application javax.servlet.ServletContext applicationScope
Session Data specific to a user session javax.servlet.http.HttpSession sessionScope
Request Data specific to an individual server request javax.servlet.HttpServletRequest requestScope
Page Data that is only valid in the context of a single page (JSPs only) [n/a] pageScope

If you open your project's category.jsp file in the editor, you'll see that EL expressions include various scoped variables, including ${categories}, ${selectedCategory} and ${categoryProducts}. The ${categories} variable is application-scoped, which is set in the ControllerServlet's init method:

// store category list in servlet context
getServletContext().setAttribute("categories", categoryFacade.findAll());

The other two, ${selectedCategory} and ${categoryProducts}, are placed in the application's session scope from the ControllerServlet. For example:

// place selected category in session scope
session.setAttribute("selectedCategory", selectedCategory);

Note: If you are continuing from the previous tutorial units, you'll likely note that ${selectedCategory} and ${categoryProducts} were originally placed in the request scope. In previous units this was fine, but consider now what happens if a user clicks the 'add to cart' button in a category page. The server responds to an addToCart request by returning the currently viewed category page. It therefore needs to know the selectedCategory and the categoryProducts pertaining to the selected category. Rather than establishing this information for each request, you place it in the session scope from a category request so that it is maintained across multiple requests, and can be accessed when you need it. Also, examine the functionality provided by the cart page. (A functional description is provided below.) The 'continue shopping' button returns the user to the previously viewed category. Again, the selectedCategory and the categoryProducts variables are required.

When referencing scoped variables in an EL expression, you do not need to specify the variable's scope (provided that you do not have two variables of the same name in different scopes). The JSP engine checks all four scopes and returns the first variable match it finds. In category.jsp for example, you can use the following expression:

${categoryProducts}

This expression is shorthand for the following expression:

${sessionScope.categoryProducts}
For more information, see the following resources:

Examining Session Data with the Java Debugger

Begin exploring how the application behaves during runtime. Use the IDE's debugger to step through code and examine how the HttpSession is created, and how other objects can be placed in the session scope to be retrieved at a later point.

  1. Open the project snapshot for this tutorial unit in the IDE. Click the Open Project ( Open Project button ) button and use the wizard to navigate to the location on your computer where you downloaded the project. If you are proceeding from the previous tutorial unit, note that this project snapshot includes a new cart package, containing ShoppingCart and ShoppingCartItem classes. Also, the following files have been modified:
    • WEB-INF/web.xml
    • css/affablebean.css
    • WEB-INF/jspf/header.jspf
    • WEB-INF/jspf/footer.jspf
    • WEB-INF/view/cart.jsp
    • WEB-INF/view/category.jsp
    • WEB-INF/view/checkout.jsp
    • controller/ControllerServlet
  2. Run the project ( Run Project button ) to ensure that it is properly configured with your database and application server.

    If you receive an error when running the project, revisit the setup instructions, which describe how to prepare the database and establish connectivity between the IDE, GlassFish, and MySQL.

  3. Test the application's functionality in your browser. If you are continuing directly from the previous tutorial unit, you'll note the following enhancements.

    category page

    • Clicking 'add to cart' for the first time enables the shopping cart and 'proceed to checkout' widgets to display in the header.
    • Clicking 'add to cart' results in an update to the number of cart items in the header's shopping cart widget.
    • Clicking 'view cart' results in the cart page displaying.
    • Clicking 'proceed to checkout' results in the checkout page displaying.
    Browser image of category page

    cart page

    • Clicking 'clear cart' results in shopping cart being emptied of items.
    • Clicking 'continue shopping' results in a return to the previously viewed category.
    • Clicking 'proceed to checkout' results in the checkout page displaying.
    • Entering a number (1 - 99) in an item's quantity field then clicking 'update' results in a recalculation of the total price for the item, and of the subtotal.
    • Entering zero in an item's quantity field then clicking 'update' results in the item being removed from the displayed table.
    Browser image of cart page

    checkout page

    • Clicking 'view cart' results in the cart page displaying.
    • Clicking 'submit purchase' results in the confirmation page displaying (without user-specific data).
    Browser image of checkout page
  4. Use the Go to File dialog to open the ControllerServlet in the editor. Press Alt-Shift-O (Ctrl-Shift-O on Mac), then type 'Controller' in the dialog and click OK.
    Go to File dialog
  5. Set a breakpoint in the doPost method on the line that creates an HttpSession object (line 150). To set a breakpoint, click in the left margin of the editor.
    Breakpoint set in editor

    To toggle line numbers for the editor, right-click in the left margin and choose Show Line Numbers.

  6. Run the debugger. Click the Debug Project ( Debug Project button ) button in the IDE's main toolbar. The GlassFish server starts (or restarts, if it is already running) and opens a socket on its debug port number. The application welcome page opens in your browser.

    You can view and modify the debug port number from the Servers window (Tools > Servers). Select the Java tab for the server you are using. Specify the port number in the 'Address to use' field under Debug Settings.

  7. When the application's welcome page displays in the browser, click any category image to navigate to the category page. Recall that clicking the 'add to cart' button sends an addToCart request to the server:
    <form action="addToCart" method="post">
    As you may recall from Preparing the Page Views and Controller Servlet, the ControllerServlet's doPost method handles requests for the /addToCart URL pattern. You can therefore expect that when a user clicks an 'add to cart' button, the doPost method is called.
  8. Click 'add to cart' for any product in the category page. Switch back to the IDE and note that the debugger suspends on the breakpoint.
    Debugger suspended on breakpoint in editor
  9. Place your cursor on the call to getSession() and press Ctrl-Space to invoke the Javadoc documentation.
    Javadoc documentation displayed in editor
    According to the documentation, getSession() returns the HttpSession currently associated with the request, and if no session exists, the method creates a new session object.

  10. Hover your mouse over the session variable. Note that the debugger suspends on the line it is about to execute. The value returned by getSession() has not yet been saved into the session variable, and you see a popup stating that "session is not a known variable in the current context."
    Debugger popup displayed in editor
  11. Click the Step Over ( Step Over button ) button in the debugger toolbar located above the editor. The line is executed, and the debugger steps to the next line in the file.
  12. Hover your mouse over the session variable again. Now you see the value currently set to the session variable.
    Debugger popup displayed in editor

    In NetBeans 6.9, you can click the grey pointer ( Step Over button ) in the popup to expand a list of variable values contained in the highlighted element.

  13. Click the Step Over ( Step Over button ) button (F8; fn-F8 on Mac) to arrive at the if statement (line 154). Because you just clicked the 'add to cart' button in the browser, you know that the expression userPath.equals("/addToCart") should evaluate to true.
  14. Highlight the userPath.equals("/addToCart") expression (by control-clicking with your mouse). This time you see a popup indicating the value of the expression you highlighted.
    Evaluated expression displayed in popup
  15. Press F8 (fn-F8 on Mac) to step to the next line (line 158). The application has been designed so that the ShoppingCart object for the user session is only created when the user adds an item to the cart for the first time. Since this is the first time the addToCart request has been received in this debug session, you can expect the cart object to equal null.
    Evaluated expression displayed in popup
  16. Press F8 (fn-F8 on Mac) to step to the next line (line 160). Then, on line 160, where the ShoppingCart object is created, click the Step Into ( Step Into button ) button. The debugger steps into the method being called. In this case, you are taken directly to the ShoppingCart's constructor.
    Evaluated expression displayed in popup
  17. Press Ctrl-Tab to switch back to the ControllerServlet. Note that the IDE provides a Call Stack ( Call Stack badge ) badge on line 160, indicating that the debugger is currently suspended somewhere on a method higher up in the call stack.

    Press Alt-Shift-3 (Ctrl-Shift-3 on Mac) to open the IDE's Call Stack window.

  18. Press F8 (fn-F8 on Mac) to continue stepping through code. When the debugger completes the ShoppingCart constructor, you are taken back to the ControllerServlet.

    Line 161 of the ControllerServlet binds the newly-created cart object to the session.
    session.setAttribute("cart", cart);
    To witness this, open the debugger's Variables window. Choose Window > Debugging > Variables, or press Alt-Shift-1 (Ctrl-Shift-1 on Mac).
    Variables window
    If you expand the session > session > attributes node, you are able to view the objects that are bound to the session. In the above image, there are two items currently bound to the session (highlighted). These are selectedCategory and categoryProducts, instantiated in the ControllerServlet at lines 83 and 89, respectively. Both of these items were bound earlier, when you clicked a category image, and the ControllerServlet processed the category page request.
  19. Press F8 (fn-F8 on Mac) to execute line 161. The cart object is bound to the session, and the Variables window updates to reflect changes. In the Variables window, note that the session now contains three attributes, the third being the newly initialized ShoppingCart object (highlighted below).
    Variables window

    So far, we have not "proven" that the session, as listed in the Variables window, represents an HttpSession. As previously mentioned, HttpSession is actually an interface, so when we talk about an HttpSession object, or session object, we are in fact referring to any object that implements the HttpSession interface. In the Variables window, if you hover your cursor over 'session', a popup displays indicating that the variable represents an HttpSession object. The StandardSessionFacade type, as displayed, is the internal class that GlassFish uses to implement the HttpSession interface. If you are familiar with Tomcat and are puzzled by the 'org.apache.catalina' paths that appear in the Value column, this is because the GlassFish web/servlet container is in fact a derivative of the Apache Tomcat container.

    A new ShoppingCart is added to the session, and the request continues to be processed. In order to complete implementation of the 'add to cart' functionality, the following actions are taken:
    • the ID of the selected product is retrieved from the request (line 165)
    • a Product object is created using the ID (line 169)
    • a new ShoppingCartItem is created using the product (line 170)
    • the ShoppingCartItem is added to ShoppingCart's items list (line 170)
  20. Press F8 (fn-F8 on Mac) to continue stepping through code while being mindful of the above-listed four actions. Pause when the debugger suspends on line 170.
  21. Create a watch on the session. This will allow you to view values contained in the session when you step into the addItem method in the next step. Right-click the session in the Variables window and choose Create Fixed Watch.
    Right-click menu displayed in Variables window

    Alternatively, you can place your cursor on the session variable in the editor, then right-click and choose New Watch. The New Watch dialog enables you to specify variables or expressions to watch continuously when debugging an application. (In the case of expressions, highlight the expression first, then right-click and choose New Watch.)
    New Watch dialog

    A new watch is created on the session variable and all variables it contains. The watch is visible from the Watches window (Window > Debugging > Watches) or, if you toggle the Watches ( Watches button ) button in the left margin of the Variables window, it displays in the top row of the Variables window.

    The debugger enables you to keep an eye on variables as it steps through code. This can be helpful, for example if you'd like to follow changes to specific variable values (and don't want to need to sift through the full list presented in the Variables window with each step), or if you temporarily step into a class that doesn't contain the variables you are interested in.
  22. Click the Step Into ( Step Into button ) button to step into ShoppingCart's addItem method.
  23. Step through the addItem method until you reach line 53. As the Javadoc states, addItem "adds a ShoppingCartItem to the ShoppingCart's items list. If item of the specified product already exists in shopping cart list, the quantity of that item is incremented."
  24. Examine the session variable which you created a watch on (step 21 above). The items.add(scItem) statement in line 51 added the new ShoppingCartItem to the items list in the ShoppingCart. This is evident by drilling into the third attribute (i.e., the cart variable) contained in the session.
    Variables window
    At this stage, you can see how an HttpSession is created for the request, how a ShoppingCart object is created and attached to the session, and how a ShoppingCartItem is created based on the user's product choice, then added to the ShoppingCart's list of items. The only remaining action is to forward the request to the category.jsp view.
  25. Open the header JSP fragment (header.jspf) in the editor and place a breakpoint on line 86. This line contains the EL statement within the shopping cart widget that displays the number of cart items.
    Breakpoint set in JSP page
  26. Click the Continue ( Continue button ) button in the debugger toolbar. The debugger continues until execution completes, or until it reaches another breakpoint. In this case, the debugger suspends on line 86 in the header JSP fragment.

    Note: In order to suspend the debugger in a JSP page, you need to set a breakpoint. For example, when the ControllerServlet forwards the request to the appropriate view, the debugger will not automatically suspend within the JSP page.

  27. Open the Variables window (Alt-Shift-1; Ctrl-Shift-1 on Mac) if it is not already open. Unlike with Java classes, the debugger does not provide tooltips when you hover your mouse over variables or expressions in a JSP page. However, the Variables window does enable you to determine variable values as you step through code. So, where can you find the value for ${cart.numberOfItems}?
  28. In the Variables window, expand the Implicit Objects > pageContext > session > session > attributes node. This provides access to the session object, just as you saw earlier when working in the ControllerServlet. In fact, you may note that the session which you created a watch on in step 21 above points to the very same object. Here you can verify that the value of ${cart.numberOfItems} equals '1'.
    Variables window

    Maximize the Variables window, or any window in the IDE, by right-clicking the window header, then choosing Maximize Window (Shift-Esc).

    The debugger gives you access to the pageContext implicit object. pageContext represents the context of the JSP page, and offers direct access to various objects including the HttpServletRequest, HttpSession, and ServletContext objects. For more information, see the Java EE 5 Tutorial: Implicit Objects.
  29. Click the Finish Session ( Finish Session button ) button. The runtime finishes executing, and the debug session terminates. The browser displays a fully-rendered category page, and you can see that the shopping cart widget in the page header contains one item.

Hopefully you now feel comfortable using the IDE's debugger not only to examine your project when it behaves unexpectedly, but also as a tool to become more familiar with code. Other useful buttons in the debugger toolbar include:

  • ( Step Out button ) Step Out: Steps you out of the current method call. Executes and removes the topmost method call in your call stack.
  • ( Run to Cursor button ) Run to Cursor: Executes up to the line on which your cursor is placed.
  • ( Apply Code Changes button ) Apply Code Changes: After editing a file, you can press this button so that the file is recompiled and changes are taken into account in the debug session.
  • ( Step Over Expression button ) Step Over Expression: Enables you to view the input parameters and resulting output values of each method call within an expression. You can inspect the output values for the previous method and the input parameters for the next method in the Local Variables window. When there are no further method calls, Step Over Expression behaves like the Step Over ( Step Over button ) command.

Examining Session Tracking Options

There are three conventional ways of tracking sessions between client and server. By far the most common is with cookies. URL rewriting can be applied in the event that cookies are not supported or disabled. Hidden form fields can also be used as a means of "maintaining state" over multiple requests, but these are limited to usage within forms.

The AffableBean project includes an example of the hidden field method in both the category and cart pages. The 'add to cart' and 'update' buttons that display for product items contain a hidden field which relays the product ID to the server when the button is clicked. If you open the cart.jsp page in the editor, you'll see that the <form> tags contain a hidden field.

<form action="updateCart" method="post">
    <input type="hidden"
           name="productId"
           value="${product.id}">
    ...
</form>

In this manner, the product ID is sent as a request parameter which the server uses to identify the item within the user's cart whose quantity needs to be modified.

The Servlet API provides a high-level mechanism for managing sessions. Essentially, it creates and passes a cookie between the client and server with each request-response cycle. If the client browser doesn't accept cookies, the servlet engine automatically reverts to URL rewriting. The following two exercises demonstrate this functionality.

Examining Client-Server Communication with the HTTP Monitor

By default, the servlet engine uses cookies to maintain and identify sessions between requests. A random, alphanumeric number is generated for each session object, which serves as a unique identifier. This identifier is passed as a 'JSESSIONID' cookie to the client. When the client makes a request, the servlet engine reads the value of the JSESSIONID cookie to determine the session which the request belongs to.

To demonstrate this, we'll use the debugger in tandem with the IDE's HTTP Monitor.

  1. Begin by activating the HTTP Monitor for the server you are using. Choose Tools > Servers. In the left column of the Servers window, select the server you are using (GlassFish). Then, in the main column, select the Enable HTTP Monitor option.
    Servers window
  2. If your server is already running, you need to restart it. However, since we plan to use the debugger, and running the debugger restarts the server to communicate on a different port, just click the Debug Project ( Debug Project button ) button in the IDE's main toolbar. The server restarts, a debug session begins and the application's welcome page opens in your browser. The HTTP Monitor displays in the bottom region of the IDE.
    HTTP Monitor
  3. Click the AffableBean record in the left column (as shown in the above image). When you select records in the left column, the right (i.e., main) column refreshes to display corresponding data. In the above image, the Request tab displays the requested URI (/AffableBean/), the HTTP method (GET), and points out that there was no query string sent with the request.
  4. Select the Session tab. Note that there is a statement, "The session was created as a result of this request." This is due to the fact that the server has sent a Set-Cookie header for the JSESSIONID cookie in its response. Also note that the new session ID is listed under 'Session properties'. As will later be shown, the session ID is the value of the JSESSIONID cookie.
    HTTP Monitor - Session tab
    You may wonder how a session object was created from a request for the site welcome page. After all, the ControllerServlet does not handle the initial request for /AffableBean/, and nowhere does this request encounter getSession(). Or does it? Recall that JSP pages are compiled into servlets upon deployment. Once you've deployed your project to the server, you can actually use the IDE to view the JSP's compiled servlet on your server.
  5. In the Projects window, right-click the index.jsp file and choose View Servlet. An index_jsp.java file opens in the editor. This is the servlet that was automatically compiled from the index.jsp page.
  6. Perform a search in the file for getSession. Press Ctrl-F (⌘-F on Mac), type 'getSession' in the search bar, then press Enter.

    Ctrl-F (⌘-F on Mac) is a keyboard shortcut for Edit > Find.

    Editor displaying getSession method

    The getSession method is in fact called. The reason this occurs is because JSP pages include the pageContext.session implicit object by default. If you wanted to deactivate this behavior, you could add the following directive to the top of a JSP file:

    <%@page session="false" %>

    If you add the directive the getSession method will be removed in the compiled servlet.

    To find out the location of the compiled servlet on your server, you can hover your mouse over the servlet's name tab above the editor. A popup displays the path to the file on your computer.

  7. In the browser, select a category then add an item to your cart. Switch back to the IDE. Note that the debugger suspends on the breakpoint in the ControllerServlet you set earlier (line 150). All breakpoints are remembered between sessions. To remove the breakpoint, you could click the breakpoint ( Breakpoint badge ) badge in the editor's left margin. However, since there are multiple breakpoints already set in the project, open the debugger's Breakpoints window (Window > Debugging > Breakpoints).
    Breakpoints window
    From the Breakpoints window, you can view and call actions on all breakpoints set in projects opened in the IDE.
  8. Right-click the breakpoint set in header.jspf and choose Delete. Then right-click the breakpoint set in the ControllerServlet and choose Disable. (You'll re-enable it later in this exercise.)
  9. Click the Continue ( Continue button ) button. The request finishes executing, and the category page displays in the browser with one item added to the cart.
  10. In the HTTP Monitor, search for the addToCart request in the left column, then select it to display details in the main column.

    Click the Ascending Sort ( Ascending Sort button ) button so that the most recent records are listed at the top.


    Under the Request tab, note the requested URI (/AffableBean/addToCart), the HTTP method (POST), and the request parameters (productId and submit).
    HTTP Monitor - Request tab
  11. Select the Cookies tab. Here you see that a cookie named JSESSIONID exists, and was sent from the client to the server. Note that the value for the cookie is the same as the Session ID displayed under the Session tab.
    HTTP Monitor - Cookies tab
    Likewise, if you click the Header tab, you see the cookie listed, since 'Cookie' is a request header that was sent by the client.
    HTTP Monitor - Cookies tab

    See Wikipedia's List of HTTP headers for more information on request and response headers.

  12. Select the Session tab. There is a statement which indicates, "The session existed before this request." Also note that the cart attribute is listed under 'Session attributes after the request'. This makes sense, since we know that the cart object is bound to the session when the addToCart request is processed for the first time.
    HTTP Monitor - Session tab

    In the next few steps, locate the session ID and JSESSIONID cookie in the Variables window.
  13. Re-enable the breakpoint you set earlier in the ControllerServlet. Press Alt-Shift-5 (Ctrl-Shift-5 on Mac) to open the Breakpoints window, then click in the checkbox next to the breakpoint entry to re-enable it.
  14. In the browser, click the 'add to cart' button for one of the listed products.
  15. Switch to the IDE and note that the debugger is suspended on the breakpoint set in the ControllerServlet. Click the Step Over ( Step Over button ) button so that the session variable is assigned to the session object.
  16. Open the Variables window (Alt-Shift-1; Ctrl-Shift-1 on Mac) and expand session > session. You'll find the session ID listed as the value for the id variable.
  17. To locate the JSESSIONID cookie, recall that you can normally access cookies from a servlet by calling the getCookies method on the HttpServletRequest. Therefore, drill into the request object: request > Inherited > request > request > Inherited > cookies. Here you see the cookies ArrayList. If you expand the list, you'll find the JSESSIONID cookie, the value of which is the session ID.
  18. Click the Finish Session ( Finish Session button ) button to terminate the debug session.

Maintaining Sessions with URL Rewriting

As mentioned, the servlet engine detects whether cookies are supported for the client browser, and if not, it switches to URL rewriting as a means of maintaining sessions. This all happens transparently for the client. For you, the developer, the process isn't entirely transparent.

You need to ensure that the application is capable of rewriting URLs whenever cookies are disabled. You do this by calling the response’s encodeURL method on all URLs returned by servlets in your application. Doing so enables the session ID to be appended to the URL in the event that the use of cookies is not an option; otherwise, it returns the URL unchanged.

For example, the browser sends a request for AffableBean's third category (bakery): category?3. The server responds with session ID included in the URL:

/AffableBean/category;jsessionid=364b636d75d90a6e4d0085119990?3

As stated above, all URLs returned by your application's servlets must be encoded. Keep in mind that JSP pages are compiled into servlets. How can you encode URLs in JSP pages? JSTL's <c:url> tag serves this purpose. The following exercise demonstrates the problem and illustrates a solution.

  1. Temporarily disable cookies in your browser. If you are using Firefox, you can choose Tools > Options (Firefox > Preferences on Mac). In the window that displays, select the Privacy tab, then under History, select 'Use custom settings for history' in the provided drop-down. Deselect the 'Accept cookies from sites' option.
    Firefox Preferences window
  2. Run the AffableBean project. When the welcome page displays, click into a category, then try adding an item to your cart. You'll see that the application's functionality is severely compromised in its present state.
    Broken categories page
    As before, the server generates a session and binds objects to it. This is how the category page is able to display the selected category and products. However, the server has failed in its attempt to set a JSESSIONID cookie. Therefore, when the client makes a second request (when user clicks 'add to cart'), the server has no way of identifying the session which the request belongs to. It therefore cannot locate any of the attributes previously set in the session, such as selectedCategory and categoryProducts. This why the rendered response lacks the information specified by these attributes.
  3. Open the project's category.jsp page in the editor. Locate the line that implements the 'add to cart' button (line 58). The <form> element's action attribute determines the request sent to the server.
    <form action="addToCart" method="post">
  4. Modify the request so that it is passed through the <c:url> tag.
    <form action="<c:url value='addToCart'/>" method="post">
  5. Press Ctrl-S (⌘-S on Mac) to save changes to the file. Recall that the IDE provides the Deploy on Save feature, which is enabled by default. This means that any saved changes are automatically deployed to your server.
  6. In the browser, select a different category so that the application renders the newly modified category page.
  7. Examine the source code for the page. In Firefox, you can press Ctrl-U (⌘-U on Mac). The 'add to cart' button for each product displays with the session ID appended to the URL.
    <form action="addToCart;jsessionid=4188657e21d72f364e0782136dde" method="post">
  8. Click the 'add to cart' button for any item. You see that the server is now able to determine the session which the request belongs to, and renders the response appropriately.
  9. Before proceeding, make sure to re-enable cookies for your browser.

Again, every link that a user is able to click on within the application, whose response requires some form of session-related data, needs to be properly encoded. Sometimes implementation is not as straight-forward as the example shown above. For example, the 'clear cart' widget used in cart.jsp currently sets a clear parameter to true when the link is clicked.

<%-- clear cart widget --%>
<c:if test="${!empty cart && cart.numberOfItems != 0}">
    <a href="viewCart?clear=true" class="bubble hMargin">clear cart</a>
</c:if>

The <c:url> tag can be applied to the URL in the following manner:

<%-- clear cart widget --%>
<c:if test="${!empty cart && cart.numberOfItems != 0}">

    <c:url var="url" value="viewCart">
        <c:param name="clear" value="true"/>
    </c:url>

    <a href="${url}" class="bubble hMargin">clear cart</a>
</c:if>

The clear=true parameter is set by adding a <c:param tag between the <c:url> tags. A variable named 'url' is set using <c:url>'s var attribute, and var is then accessed in the HTML anchor tag using the ${url} expression.

You can download and examine snapshot 6 to see how all links in the project have been encoded.

URL rewriting should only be used in the event that cookies are not an available tracking method. URL rewriting is generally considered a suboptimal solution because it exposes the session ID in logs, bookmarks, referer headers, and cached HTML, in addition to the browser's address bar. It also requires more server-side resources, as the server needs to perform additional steps for each incoming request in order to extract the session ID from the URL and pair it with an existing session.


Handling Session Time-Outs

Setting Session Time Intervals

You should consider the maximum time interval which your server maintains sessions for. If your website receives heavy traffic, a large number of sessions could expend your server's memory capacity. You might therefore shorten the interval in hopes of removing unused sessions. On the other hand, you certainly wouldn't want to cut sessions too short, as this could become a usability issue that might have a negative impact on the business behind the website. Taking the AffableBean application as an example, a user proceeds to checkout after filling her shopping cart with items. She then realizes she needs to enter her credit card details and goes off to find her purse. After returning to her computer with credit card in hand, she fills in the checkout form and clicks submit. During this time however, her session has expired on the server. The user sees that her shopping cart is empty and is redirected to the homepage. Will she really take the time to step through the process again?

The following steps demonstrate how to set the session time-out interval in the AffableBean project to 10 minutes. Of course, the actual duration ultimately depends on your server resources, the business objectives of the application, and the popularity of your website.

  1. Open the application's deployment descriptor in the editor. Press Alt-Shift-O (Ctrl-Shift-O on Mac) to use the IDE's Go to File dialog. Type in 'web', then click OK.
    Go to File dialog
    The editor displays the web.xml file in the XML view. The template that NetBeans provides for the web.xml file includes a default setting for 30 minutes.
    <session-config>
        <session-timeout>
            30
        </session-timeout>
    </session-config>
  2. Click the General tab, and type in '10' in the Session Timeout field.
    General tab of web.xml file
  3. Save the file (Ctrl-S; ⌘-S on Mac).

    If you switch back to the XML view, you'll see that the <session-timeout> element has been updated.
    <session-config>
        <session-timeout>10</session-timeout>
    </session-config>

Note: Alternatively, you could remove the <session-timeout> element altogether, and edit the session-properties element in the GlassFish-specific deployment descriptor (sun-web.xml). This would set the global time-out for all applications in the server's web module. See the Oracle GlassFish Server 3.0.1 Application Development Guide: Creating and Managing Sessions for more details.

Programmatically Handling Session Time-Outs

If your application relies on sessions, you need to take measures to ensure that it can gracefully handle situations in which a request is received for a session that has timed out or cannot be identified. You can accomplish this in the AffableBean application by creating a simple filter that intercepts requests heading to the ControllerServlet. The filter checks if a session exists, and if not, it forwards the request to the site's welcome page.

  1. Start by examining the problem that arises when a session times out midway through a user's visit to the site. Temporarily reset the session time-out interval to one minute. Open the web deployment descriptor (web.xml) and enter '1' between the <session-timeout> tags.
    <session-config>
        <session-timeout>1</session-timeout>
    </session-config>
  2. Run the AffableBean project. In the browser, click into a category page, add several items to your cart, then click 'view cart'.
    Cart page displaying items in shopping cart
  3. Wait at least one full minute.
  4. Update the quantity for one of the items displayed in the cart page. (Any number between 1 and 99 is acceptable.) Click 'update'. The server returns an HTTP Status 500 message.
    GlassFish error report displayed in browser
  5. Examine the GlassFish server log in the IDE. Open the Output window (Ctrl-4; ⌘-4 on Mac) and select the GlassFish Server tab. Scroll to the bottom of the log to examine the error's stack trace.
    GlassFish error report displayed in browser
    The server log indicates that a NullPointerException occurred at line 184 in the ControllerServlet. The Output window forms a link to the line where the exception occurred.
  6. Click the link. You navigate directly to line 184 in the ControllerServlet. Hovering your mouse over the error badge in the editor's left margin provides a tooltip describing the exception.
    Error badge and tooltip displayed in editor
    Because the session had already expired before the request was received, the servlet engine was unable to associate the request with its corresponding session. It was therefore unable to locate the cart object (line 151). The exception finally occurred in line 184 when the engine attempted to call a method on a variable equating to null.

    Now that we've identified the problem, let's fix it by implementing a filter.
  7. Click the New File ( New File button ) button in the IDE's toolbar. (Alternatively, press Ctrl-N; ⌘-N on Mac.)
  8. Select the Web category, then select Filter and click Next.
  9. Name the filter SessionTimeoutFilter. Type filter into the Packages field so that the filter class is placed in a new package when created.
  10. Click Next. Accept default settings and click Finish. A template for the SessionTimeoutFilter is generated and opens in the editor.

    Note: Currently, in NetBeans 6.9, it isn't possible to use the wizard to set a mapping to a servlet that isn't registered in the web deployment descriptor. (ControllerServlet was registered using the @WebServlet annotation.) We'll therefore modify the generated code in the next step.

  11. Modify the @WebFilter annotation signature so that it appears as follows.
    @WebFilter(servletNames = {"Controller"})
    public class SessionTimeoutFilter implements Filter {
    This sets the filter to intercept any requests that are handled by the ControllerServlet. (Alternatively, you could have kept the urlPatterns attribute, and listed all patterns that the ControllerServlet handles.)

    Note that 'Controller' is the name of the ControllerServlet, as specified in the servlet's @WebServlet annotation signature. Also note that you've removed the filterName attribute, since the name of the filter class is used by default.

    The IDE's filter template provides a lot of interesting code which is worth inspecting in its own right. However, most of it is not needed for our purposes here. Any filter class must implement the Filter interface, which defines three methods:
    • init: performs any actions after the filter is initialized but before it is put into service
    • destroy: removes the filter from service. This method can also be used to perform any cleanup operations.
    • doFilter: used to perform operations for each request the filter intercepts

    Use the Javadoc Index Search to pull up documentation on the Filter interface. Press Shift-F1 (fn-Shift-F1 on Mac), then type 'Filter' into the search field and hit Enter. Select the 'Interface in javax.servlet' entry. The Javadoc documentation displays in the lower pane of the index search tool.

  12. Replace the body of the SessionTimeoutFilter with the following contents.
    @WebFilter(servletNames = {"Controller"})
    public class SessionTimeoutFilter implements Filter {
    
        public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
                throws IOException, ServletException {
    
            HttpServletRequest req = (HttpServletRequest) request;
    
            HttpSession session = req.getSession(false);
    
            // if session doesn't exist, forward user to welcome page
            if (session == null) {
                try {
                    req.getRequestDispatcher("/index.jsp").forward(request, response);
                } catch (Exception ex) {
                    ex.printStackTrace();
                }
                return;
            }
    
            chain.doFilter(request, response);
        }
    
        public void init(FilterConfig filterConfig) throws ServletException {}
    
        public void destroy() {}
    
    }
  13. Press Ctrl-Shift-I (⌘-Shift-I on Mac) to fix import statements. (Imports need to be added for HttpServletRequest and HttpSession.) Also, use the editor hints to add the @Override annotation to the init, destroy, and doFilter methods.

    In the coming steps, you run the debugger on the project and step through the doFilter method to see how it determines whether the request is bound to an existing session.
  14. Open the Breakpoints window (Alt-Shift-5; Ctrl-Shift-5 on Mac) and ensure that you do not have any existing breakpoints set. To delete a breakpoint, right-click the breakpoint and choose Delete. (If you completed the above exercise, Examining Client-Server Communication with the HTTP Monitor, you may have an outstanding breakpoint set in the ControllerServlet.)
  15. Run the debugger. Click the Debug Project ( Debug Project button ) button in the IDE's main toolbar.
  16. When the welcome page displays in the browser, select a category, then add several items to your shopping cart.
  17. Set a breakpoint on the line in SessionTimeoutFilter's doFilter method that tries to access the session (line 32).
    Breakpoint set in editor
  18. In the browser, click the 'view cart' button. Switch to the IDE and note that the debugger has suspended on the breakpoint.

    Recall that getSession() creates a new session object if the current one doesn't exist. Here, we use getSession(false), which refrains from creating a new object if none is found. In other words, the method returns null if the session doesn't exist.
  19. Click the Step Over ( Step Over button ) button, then hover your mouse over the session variable. Provided that a minute hasn't passed since the previous request was sent, you'll see that the variable has been assigned to a StandardSessionFacade. This represents the session object for the request.
    Popup displaying 'session' variable assigned to a session object
  20. Continue stepping through the method until the request is processed. Since session doesn't equal null, you skip the if statement and chain.doFilter then forwards the request to the ControllerServlet (line 44).
  21. In the browser, make sure a full minute has passed, then update a quantity for one of the product items in your cart. This is the same procedure we went through earlier in the exercise when the status 500 message was returned. Now that the filter intercepts requests heading to the ControllerServlet, let's see what happens when a session time-out occurs.
  22. After clicking 'update', switch to the IDE and note that the debugger is again suspended on the breakpoint set in the filter.
  23. Highlight the req.getSession(false) expression, then hover your mouse over it. Here you see the expression equates to null, as the session has already expired.
    Popup displaying 'session' variable equates to 'null'
  24. Continue stepping through the code. Now that the session variable equals null, the if statement on line 35 is processed, and the request is forwarded to /index.jsp. When the debugger finishes executing, you'll see that the browser displays the site's welcome page.
  25. Click the Finish Session ( Finish Session button ) button to terminate the debug session.
  26. Open the project's web.xml file and change the session time-out interval back to 10 minutes.
    <session-config>
        <session-timeout>10</session-timeout>
    </session-config>
  27. Save (Ctrl-S; ⌘-S on Mac) the file.

Snapshot 6 provides you with the completed project version for this tutorial unit. One final topic concerning session management should be mentioned. You can explicitly terminate a session by calling the invalidate method on the session object. If the session is no longer needed, it should be removed in order to conserve the memory available to your server. After you complete the next unit, Integrating Transactional Business Logic, you will see how the ControllerServlet, upon successfully processing a customer order, destroys the user's cart object and terminates the session using the invalidate method.

// if order processed successfully send user to confirmation page
if (orderId != 0) {

    // dissociate shopping cart from session
    cart = null;

    // end session
    session.invalidate();

    ...
}

This is demonstrated in project snapshot 8 (and later snapshots).


See Also

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