Expert Spring MVC and Web Flow

■Tip It may be helpful to think of a flow as a method call, just one that can span many requests into the

server. Following this analogy, top-level flows would be public, while inline flows would be private.

Managing FlowExecutions

In this section we will look in detail at the life cycle of a FlowExecutionfrom start to end and

learn how that life cycle is managed. Before discussing how a FlowExecutionis created, it is

worth reviewing how Spring Web Flow is integrated into the various web frameworks.

Integration with Web Frameworks

Chapter 11 introduced the definition of a flow and discussed briefly how FlowExecutions are

managed. In this section we will delve into the FlowExecutionManager, the central façade for

driving the execution of flows,and examine what happens behind the scenes.

Essentially, the FlowExecutionManageris the primary façade through which other systems

like Spring MVC communicate with Spring Web Flow. If Spring Web Flow is a proverbial black

box, then FlowExecutionManageris the lid.

This “handover” to Spring Web Flow is accomplished by calling either the

FlowExecutionManager.launch() orFlowExecutionManager.signalEvent() method, which

instructs Spring Web Flow to initiate processing of an external user event. The launch opera-

tion instructs Web Flow to launch an entirely new flow execution (or user conversation),

and the signalEventoperation instructs Web Flow to signal a user event against the current

state of an existing flow execution. Both methods accept org.springframework.webflow.

ExternalContextand return org.springframework.webflow.ViewSelection. Listing 12-11

shows the ExternalContextinterface façade.

Listing 12-11.Implementation of org.springframework.webflow.ExternalContext

public interface ExternalContext {

Map getRequestParameterMap();

Map getRequestMap();

SharedMap getSessionMap();

SharedMap getApplicationMap();

public interface SharedMap extends Map {

public Object getMutex();

}

}

The ExternalContextinterface enables Spring Web Flow artifacts such as actions to access

information about the calling environment in a standard, normalized fashion, without coupling

those artifacts to a specific deployment environment. It is up to a system calling into Spring

Web Flow to provide an appropriate ExternalContextimplementation. This implementation

may provide access to environment-specific constructs; for example, the servlet-specific

ExternalContext(org.springframework.webflow.context.servlet.ServletExternalContext)

provides access to the native HttpServletRequestand HttpServletResponseServlet artifacts.

Table 12-2 provides a list of ExternalContextadapters provided by Spring Web Flow.

346 CHAPTER 12 ■ADVANCED SPRING WEB FLOW