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x belongs or not. If changing x requires changing multiple layers or a seemingly irrelevant layer (as in this case), then there is something wrong with the application design. In this case, a change to the business logic resulted in a change to the flow definition, indicating the flow definition is not the right place for this logic.
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So what is the solution Simple: Move the business rule out of the web flow. Does this prevent the web flow from referencing the business rule Of course not, it simply means that the web flow does not implement the business rule. Refactoring might lead to something like Listing 12-2. Listing 12-2. Spring Web Flow Fragment Referencing Business Rules <action-state id="checkRights "> <action bean="checkRightsAction"/> <transition on="error" to="rightsViolation.view"/> <transition on="success" to="nextAction"/> </action-state> The flow now drives the execution of the business rule, but does not define it. When implementing Spring Web Flow, be sure to avoid inadvertently coding business logic inside of your flow.
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CHAPTER 12 ADVANCED SPRING WEB FLOW
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Flows come in varying granularities, from large top-level flows composed of many smaller subflows to self-contained flows with no dependencies. Spring Web Flow gains much of its power from its ability to compose multiple flows together, creating modular and reusable application components. Up to this point, you have seen flow definitions that are self-contained with no composition. In this section we will show you how to create modular flow definitions with subflows and inner flows.
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Subflows
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A subflow is simply a flow called by another flow. Any flow can be a subflow, and any subflow is a flow. This is possible because a flow is a coarse-grained component with a well-defined contract that acts as a self-contained black box. What happens inside a flow is hidden from any other flows, including the calling flow.
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Note When a flow is spawned by another flow, the spawning flow is often referred to as the parent flow
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or calling flow, while the flow being spawned is often referred to as the subflow or child flow.
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Architecturally, subflows are a powerful mechanism that enable the definition of complex flows composed of one or more subflows. Subflows can themselves contain subflows (there is no limit to the depth of subflow nesting). Subflows are best used to model logical sequences of steps when reuse is possible. Consider the Purchase Product use case from 11; it contained a step to capture product shipping information. In a real application this step could be quite complex, consisting of different screens dependent upon the mechanism used to ship the product (air, sea, land, and so on). You can imagine this step being reused elsewhere in the application (wherever shipping information is required), and therefore this self-contained and reusable component is a good candidate for refactoring into its own flow. The steps involved in refactoring logic out of an existing flow and into a subflow are as follows: 1. Isolate the web flow fragment and dependent beans to be modeled as a subflow. 2. Move the fragment and the dependent beans into their own flow definition. 3. Within the new flow definition, define an end state for every logical flow outcome. Each end state should expose any flow attributes that are returned by that outcome (in this case, the shipping information). 4. Replace the factored-out fragment in the original flow with a call to the subflow using the subflow state. Recall the flow definition from 11. The isolated fragment related to entering shipping information is boldface. See Listing 12-5.
CHAPTER 12 ADVANCED SPRING WEB FLOW
Listing 12-3. The Purchase Product Flow Definition <flow start-state="enterPurchaseInformation"> <view-state id="enterPurchaseInformation" view="purchaseForm"> <entry-actions> <action bean="formAction" method="setupForm"/> </entry-actions> <transition on="submit" to="requiresShipping"> <action bean="formAction" method="bindAndValidate"/> </transition> <transition on="cancel" to="cancel"/> </view-state> <decision-state id="requiresShipping"> <if test="${flowScope.purchase.shipping}" then="enterShippingDetails" else="placeOrder"/> </decision-state> <view-state id="enterShippingDetails" view="shippingForm"> <transition on="submit" to="placeOrder"> <action bean="sellItemAction" method="bindAndValidate"/> </transition> </view-state> <action-state id="placeOrder"> <action bean="orderClerk" method="placeOrder(${flowScope.purchase})"/> <transition on="success" to="showCostConfirmation"/> </action-state> <end-state id="showCostConfirmation" view="costConfirmation"/> <end-state id="cancel" view="home"/> <import resource="purchase-flow-context.xml"/> </flow> The preceding fragment uses the purchase bean, which contains all the shipping information as member variables. Since subflows are independent and isolated from their calling flows, the shipping information will be extracted into its own class called Shipping. This new Shipping class will be managed by its own FormAction. Thus, a new flow (as listed in Listing 12-4) is created in /WEB-INF/flows/shipping-flow.xml. Listing 12-4. New Spring Web Flow Subflow <flow start-state="requiresShipping"> <decision-state id="requiresShipping"> <if test="${flowScope.requiresShipping}" then="enterShippingDetails" else="finish"/> </decision-state>
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