Page History

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Ordinary page-persistent fields

won't work for this, since persistent fields are available only to a specific page, not shared across multiple pages.

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A field holding an SSO is marked with the @SessionState annotation.

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public class MyPage
{
  @SessionState
  private ShoppingCart shoppingCart;
  
  . . .
}

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With @SessionState, you are creating a session-wide data storage area that is tied to the type (class) of the variable you annotate. It is not specifically tied to the variable itself, or even to the class in which that variable was annotated. As with all session data, there is the serious possibility of collisions, not just within your application but with other modules/libraries:

  @SessionState
  private String userName;     // Unsafe -- String is not a custom type

  ... then, later in this class or any other:

  @sessionState
  private String userCity;     // This overwrites value in userName, because it's also a String!

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Instead, create a second field with a matching name but with "Exists" appended:

  private boolean shoppingCartExists;

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Alternately, you may allow for the state being null:

  @SessionState(create=false)
  private ShoppingCart shoppingCart;

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Persistence Strategies

Main Article: Persistent Page Data PersistentPage

Each SSO is managed according to a persistence strategy. The default persistence strategy, "session", stores the SSOs inside the session. The session is created as needed.

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A Session State Object is configured using contributions to the ApplicationStateManager service. From your application's module:

  public void contributeApplicationStateManager(MappedConfiguration<Class, ApplicationStateContribution> configuration)
  {
    ApplicationStateCreator<MyState> creator = new ApplicationStateCreator<ShoppingCart>()
    {
      public ShoppingCart create()
      {
        return new ShoppingCart(new Date());
      }
    };
  
    configuration.add(ShoppingCart.class, new ApplicationStateContribution("session", creator));
  }

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As an alternative to SSOs, Tapestry provides a Session Attribute mechanism, which lets you store data in the session by name (rather than type). It is particularly useful when integrating Tapestry with legacy applications that directly manipulate the HttpSession.

public class PageMyPage {
    @Inject
    private Request request;
    
    public User getUser() {
        return (User) request.getSession(true).getAttribute("loggedInUserName");
    }
}

Starting with Tapestry 5.2, this can be accomplished just by annotating a page or component property with @SessionAttribute. This annotation is used to map a property of a page or component to value stored in session. Unlike Session State Objects, the name (not the type) of the annotated property is used as the name of the session attribute to look for.

public class PageMyPage {
    @SessionAttribute
    private User loggedInUserName;
}

You can also provide a name using the annotation's value parameter:

public class PageMyPage {
    @SessionAttribute("loggedInUserName")
    private User userName;
}

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It's best to define the session attribute name as constant, and use that in the annotation's value parameter, rather then defaulting to the instance variable name. This will help prevent subtle runtime errors due to misspellings. For example:

public static final String USER_NAME_SESSION_ATTRIBUTE = "com.example.shoppingapp.username";

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public class PageMyPage {
    @SessionAttribute(USER_NAME_SESSION_ATTRIBUTE)
    private User userName;
}

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Starting with version 5.4, by default Tapestry will apply locking semantics around access to the HttpSession. Reading attribute names occurs with a shared read lock, and getting or setting an attribute upgrades the lock to an exclusive write lock. This can tend to serialize threads when a number of simultaneous (Ajax) requests from the client arrive. However, many implementations of HttpSession are not thread safe, and often mutable objects
are stored in the session and shared between threads.

The tapestry.session-locking-enabled configuration symbol can control this behavior. Setting this to true (the default) will yield a more robust application; setting it to false may speed up processing for more Ajax intensive applications (but care should then be given to ensuring that objects shared inside the session are themeselves immutable or thread-safe).

  public static void contributeApplicationDefaults(MappedConfiguration<String,String> configuration)
  {
    configuration.add(SymbolConstants.SESSION_LOCKING_ENABLED, true);
    ...
  }