This chapter shows you how to use the action-state element to control the execution of an action at a point within a flow. It will also show how to use the decision-state element to make a flow routing decision. Finally, several examples of invoking actions from the various points possible within a flow will be discussed. Use the action-state element when you wish to invoke an action, then transition to another state based on the action's outcome: ]]> The full example below illustrates a interview flow that uses the action-state above to determine if more answers are needed to complete the interview: ]]> After the execution of each action, the action-state checks the result to see if matches a declared transition to another state. That means if more than one action is configured they are executed in an ordered chain until one returns a result event that matches a state transition out of the action-state while the rest are ignored. This is a form of the Chain of Responsibility (CoR) pattern. The result of an action's execution is typically the criteria for a transition out of this state. Additional information in the current RequestContext may also be tested as part of custom transitional criteria allowing for sophisticated transition expressions that reason on contextual state. Note also that an action-state just like any other state can have one more on-entry actions that are executed as a list from start to end. Use the decision-state element as an alternative to the action-state to make a routing decision using a convenient if/else syntax. The example below shows the moreAnswersNeeded state above now implemented as a decision state instead of an action-state: ]]> Actions often invoke methods on plain Java objects. When called from action-states and decision-states, these method return values can be used to drive state transitions. Since transitions are triggered by events, a method return value must first be mapped to an Event object. The following table describes how common return value types are mapped to Event objects: Method return type Mapped Event identifier expression java.lang.String the String value java.lang.Boolean yes (for true), no (for false) java.lang.Enum the Enum name any other type success This is illustrated in the example action state below, which invokes a method that returns a boolean value: ]]> While writing action code as POJO logic is the most common, there are several other action implementation options. Sometimes you need to write action code that needs access to the flow context. You can always invoke a POJO and pass it the flowRequestContext as an EL variable. Alternatively, you may implement the Action interface or extend from the MultiAction base class. These options provide stronger type safety when you have a natural coupling between your action code and Spring Web Flow APIs. Examples of each of these approaches are shown below. ]]> ]]> ]]> Actions often invoke services that encapsulate complex business logic. These services may throw business exceptions that the action code should handle. The following example invokes an action that catches a business exception, adds a error message to the context, and returns a result event identifier. The result is treated as a flow event which the calling flow can then respond to. ]]> The following example is functionally equivlant to the last, but implemented as a MultiAction instead of a POJO action. The MultiAction requires its action methods to be of the signature Event ${methodName}(RequestContext), providing stronger type safety, while a POJO action allows for more freedom. ]]> In general it is recommended to catch exceptions in actions and return result events that drive standard transitions, it is also possible to add an exception-handler sub-element to any state type with a bean attribute referencing a bean of type FlowExecutionExceptionHandler. This is an advanced option that if used incorrectly can leave the flow execution in an invalid state. Consider the build-in TransitionExecutingFlowExecutionExceptionHandler as example of a correct implementation. The following example shows an action that creates a new Booking object by invoking a method on a service: ]]> The following example shows a state entry action that sets the special fragments variable that causes the view-state to render a partial fragment of its view: ]]> The following example shows a state exit action that releases a lock on a record being edited: ]]> The following example shows the equivalent object locking behavior using flow start and end actions: ]]> The following example shows a render action that loads a list of hotels to display before the view is rendered: ]]> The following example shows a transition action adds a subflow outcome event attribute to a collection: ]]> The following example shows how to execute a chain of actions in an action-state. The name of each action becomes a qualifier for the action's result event. ]]> In this example, the flow will transition to showResults when thingTwo completes successfully. Sometimes an Action needs to stream a custom response back to the client. An example might be a flow that renders a PDF document when handling a print event. This can be achieved by having the action stream the content then record "Response Complete" status on the ExternalContext. The responseComplete flag tells the pausing view-state not to render the response because another object has taken care of it. ]]> In this example, when the print event is raised the flow will call the printBoardingPassAction. The action will render the PDF then mark the response as complete. Another common task is to use Web Flow to handle multipart file uploads in combination with Spring MVC's MultipartResolver. Once the resolver is set up correctly as described here and the submitting HTML form is configured with enctype="multipart/form-data", you can easily handle the file upload in a transition action. The file upload example below below is not relevant when using Web Flow with JSF. See for details of how to upload files using JSF. Given a form such as: Select file: ]]> and a backing object for handling the upload such as: you can process the upload using a transition action as in the following example: ]]> The MultipartFile will be bound to the FileUploadHandler bean as part of the normal form binding process so that it will be available to process during the execution of the transition action.