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This distinction between the root flow and the active flow is modeled by org.springframework. webflow.FlowSession. A stack of FlowSessions are maintained by FlowExecutionImpl. In the simple case of a flow without any subflow states, there will only ever be a single FlowSession. If a flow does contain a subflow state, then each time a subflow is spawned, the top (active) FlowSession is suspended, and a new FlowSession is created and pushed onto the stack (shown in Figure 12-2). As you can see, there is a single FlowExecution representing the entire conversation, but each flow that is spawned (including the parent) receives its own FlowSession. Each instance of FlowSession has its own local data structure that is the basis for flow scope. Figure 12-2 also shows some of the statuses that a FlowSession can have. Table 12-4 lists all of the possible statuses and their relevance or meaning (the statuses are defined in org.springframework. webflow.FlowSessionStatus).
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CHAPTER 12 ADVANCED SPRING WEB FLOW
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FlowExecution <flow start-state= ... > <action-state ....> .... </action-state> <subflow-state ....> <subflow-state ....> SessionA
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SessionB
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SessionC
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(suspended) (suspended)
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</subflow-state ....> </subflow-state ....>
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<view-state> .... </view-state> </flow>
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Figure 12-2. FlowSession life cycle Table 12-4. Status of the FlowSession
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Status
Created Active Paused Suspended Ended
Meaning
The FlowSession has been created, but is not yet active. The FlowSession is currently executing. The FlowSession is awaiting user input.
Event
Creation of a new FlowExecution. When a flow is resumed. When the view is rendered.
The FlowSession is still valid, but not active. When a new subflow is created, the existing FlowSession is suspended. The FlowSession is no longer active. When the end state has been reached.
RequestContext
So far we have looked at FlowExecution which represents the execution of an entire top-level flow including any subflows that are spawned, and FlowSession which represents the execution of a single flow definition. There is one final lifetime that hasn t yet been covered, the lifetime of a request to manipulate a FlowExecution. This is modeled by org.springframework. webflow.RequestContext. You may recognize this as the parameter that is passed into the org.springframework.webflow.Action.execute() method. This object is the one that you will probably be most familiar with if you have implemented a custom Web Flow artifact such as
CHAPTER 12 ADVANCED SPRING WEB FLOW
an Action or a FlowAttributeMapper. It is a convenient fa ade that allows you to access information about the currently executing request and the environment in which it is executing. See Listing 12-5. Listing 12-15. org.springframework.webflow.RequestContext public interface RequestContext { ExternalContext getExternalContext(); FlowExecutionContext getFlowExecutionContext(); Scope getRequestScope(); Scope getFlowScope(); Map getProperties(); Event getLastEvent(); Transition getLastTransition(); void setProperties(Map properties); Map getModel(); } As you can see, you can access the ExternalContext, different levels of data scope, the state of the conversation, and so on. The key point is that the RequestContext represents context about the current request. Table 12-5 summarizes the key artifacts that provide information about the execution of a flow. Table 12-5. Main Artifacts Used for the Execution of a Flow
Artifact
org.springframework.webflow.ExternalContext
Description
Provides access to the external environment in which Spring Web Flow is executing. Defines a logical view selected for rendering. top-level flow, including any spawned subflows.
org.springframework.webflow.ViewSelection
org.springframework.webflow.execution.FlowExecution Represents the entire execution of a
org.springframework.webflow.FlowSession
Represents the execution of a single flow definition (not including subflows). Represents the execution of a single request into Spring Web Flow triggered by the occurrence of an external event.
org.springframework.webflow.RequestContext
FlowExecutionListener
Spring Web Flow defines a rich event model that allows you to observe the life cycle of an executing flow. You can write your own observer by implementing the org.springframework. webflow.execution.FlowExecutionListener interface. See Listing 12-16.
CHAPTER 12 ADVANCED SPRING WEB FLOW
Listing 12-16. org.springframework.webflow.execution.FlowExecutionListener public interface FlowExecutionListener { void requestSubmitted(RequestContext context); void requestProcessed(RequestContext context); void sessionStarting(RequestContext context, State startState, Map input) throws EnterStateVetoException; void sessionStarted(RequestContext context); void eventSignaled(RequestContext context, State state); void stateEntering(RequestContext context, State nextState) throws EnterStateVetoException; void stateEntered(RequestContext context, State previousState, State state); void resumed(RequestContext context); void paused(RequestContext context, ViewSelection selectedView); void sessionEnding(RequestContext context); void sessionEnded(RequestContext context, FlowSession endedSession); } Implementing a FlowExecutionListener allows you to weave in behavior in an interceptor-like way. For example, you could provide a SecurityFlowExecutionListener implementation that throws an EnterStateVetoException preventing a state from entering if the user does not have the necessary permissions. You might develop listeners to apply custom auditing, breadcrumbs (navigation history), or transactional behavior and so on.
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