Configuring Navigation Rules

As explained in Navigation Model (page 292), navigation is a set of rules for choosing the next page to be displayed after a button or hyperlink component is clicked. Navigation rules are defined in the application configuration resource file.

Each navigation rule specifies how to navigate from one page to a set of other pages. The JavaServer Faces implementation chooses the proper navigation rule according to which page is currently displayed.

After the proper navigation rule is selected, the choice of which page to access next from the current page depends on the action method that was invoked when the component was clicked and the logical outcome that is referenced by the component's tag or was returned from the action method.

The outcome can be anything the developer chooses, but Table 13-3 lists some outcomes commonly used in web applications.

Table 13-3 Common Outcome Strings
Outcome	What It Means
`success`	Everything worked. Go on to the next page.
`failure`	Something is wrong. Go on to an error page.
`logon`	The user needs to log on first. Go on to the logon page.
`no results`	The search did not find anything. Go to the search page again.

Usually, the action method performs some processing on the form data of the current page. For example, the method might check whether the user name and password entered in the form match the user name and password on file. If they match, the method returns the outcome success. Otherwise, it returns the outcome failure. As this example demonstrates, both the method used to process the action and the outcome returned are necessary to determine the proper page to access.

<navigation-rule>
  <from-view-id>/logon.jsp</from-view-id>
  <navigation-case>
    <from-action>#{LogonForm.logon}</from-action>
    <from-outcome>success</from-outcome>
    <to-view-id>/storefront.jsp</to-view-id>
  </navigation-case>
  <navigation-case>
    <from-action>#{LogonForm.logon}</from-action>
    <from-outcome>failure</from-outcome>
    <to-view-id>/logon.jsp</to-view-id>
    </navigation-case>
</navigation-rule>

This navigation rule defines the possible ways to navigate from logon.jsp. Each navigation-case element defines one possible navigation path from logon.jsp. The first navigation-case says that if LogonForm.logon returns an outcome of success, then storefront.jsp will be accessed. The second navigation-case says that logon.jsp will be rerendered if LogonForm.logon returns failure.

An application's navigation configuration consists of a set of navigation rules. Each rule is defined by the navigation-rule element in the faces-config.xml file.

The navigation rules of the Duke's Bookstore application are very simple. Here are two complex navigation rules that could be used with the Duke's Bookstore application:

<navigation-rule>
  <from-view-id>/catalog.jsp</from-view-id>
  <navigation-case>
    <from-outcome>success</from-outcome>
    <to-view-id>/bookcashier.jsp</to-view-id>
  </navigation-case>
  <navigation-case>
    <from-outcome>out of stock</from-outcome>
    <from-action>
      #{catalog.buy}
    </from-action>
    <to-view-id>/outofstock.jsp</to-view-id>
  </navigation-case>
  <navigation-case>
    <from-outcome>error</from-outcome>
    <to-view-id>/error.jsp</to-view-id>
  </navigation-case>
</navigation-rule>

The first navigation rule in this example says that the application will navigate from catalog.jsp to

bookcashier.jsp if the item ordered is in stock

outofstock.jsp if the item is out of stock

The second navigation rule says that the application will navigate from any page to error.jsp if the application encountered an error.

Each navigation-rule element corresponds to one component tree identifier defined by the optional from-view-id element. This means that each rule defines all the possible ways to navigate from one particular page in the application. If there is no from-view-id element, the navigation rules defined in the navigation-rule element apply to all the pages in the application. The from-view-id element also allows wildcard matching patterns. For example, this from-view-id element says that the navigation rule applies to all the pages in the books directory:

As shown in the example navigation rule, a navigation-rule element can contain zero or more navigation-case elements. The navigation-case element defines a set of matching criteria. When these criteria are satisfied, the application will navigate to the page defined by the to-view-id element contained in the same navigation-case element.

The navigation criteria are defined by optional from-outcome and from-action elements. The from-outcome element defines a logical outcome, such as success. The from-action element uses a method expression to refer to an action method that returns a String, which is the logical outcome. The method performs some logic to determine the outcome and returns the outcome.

The navigation-case elements are checked against the outcome and the method expression in this order:

Cases specifying both a from-outcome value and a from-action value. Both of these elements can be used if the action method returns different outcomes depending on the result of the processing it performs.

Cases specifying only a from-outcome value. The from-outcome element must match either the outcome defined by the action attribute of the UICommand component or the outcome returned by the method referred to by the UICommand component.

Cases specifying only a from-action value. This value must match the action expression specified by the component tag.

When any of these cases is matched, the component tree defined by the to-view-id element will be selected for rendering.