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Open Lane: Starting Out With Spring Web Flow

My son’s swim club hosts Minnesota’s largest swim meet. Swim meets such as this provide an opportunity for swimmers already entered into the meet to apply for an exhibition entry into events that do not use all the lanes the pool provides. In Minnesota this is called an open lane swim. The process for managing open lane entries can be quite time consuming. An online system for handling open lanes applications would significantly reduce this.

The initial requirements are:

  • User self service:
    • Create a user.
    • User login and logout.
    • Edit user profile.
    • Search for an event.
    • Manage open lane entry applications (i.e. view, add, delete).
  • Reporting:
    • List all open lane entries by swimmer and event.
  • Data initialization utility – loads meet information into the application’s database.

Since I’ve been using the Spring  framework on and off for some time now I decided to use Spring Web Flow (SWF) to build this application.

Spring Web Flow is a Spring MVC extension that allows implementing the “flows” of a web application. A flow encapsulates a sequence of steps that guide a user through the execution of some business task. It spans multiple HTTP requests, has state, deals with transactional data, is reusable, and may be dynamic and long-running in nature.

The sweet spot for Spring Web Flow are stateful web applications with controlled navigation such as checking in for a flight, applying for a loan, shopping cart checkout, or even adding a confirmation step to a form.

That sounds like just the kind of framework I need. I’ll be using a full open source stack that includes:

  • Apache Tomcat
  • Spring Web Flow
  • JSF using Primefaces
  • Hibernate
  • BIRT
  • An open source RDBMS such as MySQL
Eclipse is my IDE of choice and I should be able to build everything using Maven. While I could use an IDE such as SpringSource Tools Suite (STS) to auto generate and build much of my application I’ve decided to take a ground up approach. I don’t want an auto-magically development environment hiding the minutia from me.  That scenario is great once you’re fluent with the technology. It can be deadly when you first attempt to do something practical with it. When something doesn’t work it’s often frustrating and time consuming to resolve it.
Spring Faces- Hotel Booking Sample Application

Spring Faces- Hotel Booking Sample Application

I start out by downloading the latest version of Spring Web Flow 2.3.1. It contains a sample application called booking-faces. I built the application (to ensure it would in fact build) and then generated an eclipse project using maven:

  • mvn package
  • mvn eclipse:clean
  • mvn eclipse:eclipse

From Eclipse I imported the application into a workspace and using a Tomcat server started up the application. It all worked! I now had a reference point from which I could start coding my application.

Creating Dynamic Web Application

Creating Dynamic Web Application

Using Eclipse Java EE IDE for Web Developers (Indigo SR2) I created a dynamic web project call open-lane.  This gave me an blank web application to start from. I dropped a simple “hello word” static index.html file into webapp, ran it up with Tomcat, and successfully opened the link from from my browser. Now comes the fun part of getting all the necessary SWF bits into the project.

In general, a SWF application contains one or more flows. Flows are comprised of states (steps). SWF is based on a finite-state machine. The idea being that the application can transition (navigate) from one state to another based upon an event. A state typically has a view. I’ll start with the most primitive of applications – an application with one state. My flow definition will look like this.

<?xml version="1.0" encoding="UTF-8"?>

<flow xmlns="http://www.springframework.org/schema/webflow"

	<view-state id="test"></view-state>

To successfully launch an SWF application I need to create a configuration that fulfills the following:

SWF components

Spring Web Flow Components¹

Spring MVC’s DispatcherServlet will be configured to route incoming flow requests to SWF.

<!-- The master configuration file for this Spring web application -->

<!-- The front controller of this Spring Web application, responsible for
	handling all application requests -->
	<servlet-name>Spring MVC Dispatcher Servlet</servlet-name>

The configuration coming from web-application-config.xml maps DispatcherServlet requests to a application resource handler.

<!-- Enables FlowHandler URL mapping -->
<bean class="org.springframework.webflow.mvc.servlet.FlowHandlerAdapter">
    <property name="flowExecutor" ref="flowExecutor" />

Next I define how FlowHandlerAdapter will map a resource path it receives from DispatcherServlet to a flow id. The normal convention has FlowHandlerAdapter searching the flow registry for a matching id. If it finds one then it will either start a new flow or continue an existing one.

<!-- Maps request paths to flows in the flowRegistry;
     e.g. a path of /a/b looks for a flow with id "a/b" -->
<bean class="org.springframework.webflow.mvc.servlet.FlowHandlerMapping">
    <property name="flowRegistry" ref="flowRegistry"/>
    <property name="order" value="0"/>

In order to query a flow from the flow registry one has to exist. This directs the registry to use flows defined in /WEB-INF/flows. For example, WEB-INF/flows/testing/test/test-flow.xml. It also defines the id of the service used to build these flows.

<!-- Register all Web Flow definitions under /WEB-INF/flows/**/*-flow.xml -->
<webflow:flow-registry id="flowRegistry"
    <webflow:flow-location-pattern value="/**/*-flow.xml" />

Now things get interesting. SWF provides several flow builder services and you can create your own. The one you choice will bring in the conventions and overhead associated with the service. For example, using a JSF builder service means that:

  • All the necessary JSF libraries and dependencies must be available to the application. This usually means including them in the application packaging.
  • The application is configured for JSF.
  • Views (pages) will be written in JSF.

In other words, welcome to the world of JSF.

To get things started I didn’t want to complicate my first application with JSF or JSP. I’m just trying to ensure that the plumbing is working. While that meant writing a bit of Java code I decided it was the lesser of two evils.

<bean id="viewResolver"
    <property name="viewClass" value="org.bwgz.test.TestUrlBasedView"/>

<!-- Deploy a flow executor -->
<webflow:flow-executor id="flowExecutor" />

<!-- Configure flow builder services -->
<!-- Configure view service -->
<webflow:flow-builder-services id="flowBuilderServices"
    view-factory-creator="mvcViewFactoryCreator" />

<!-- Use the test View Resolver -->
<bean id="mvcViewFactoryCreator"
   <property name="viewResolvers" ref="viewResolver"/>

In this instance the flow builder service will use MvcViewFactoryCreator and it will in turn use UrlBasedViewResolver and my TestUrlBasedView to generate the view. I don’t do much in TestUrlBasedView. Just dump out some variables and get out of Dodge City.

package org.bwgz.test;

import java.util.Iterator;
import java.util.Map;
import java.util.Set;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.view.AbstractUrlBasedView;

public class TestUrlBasedView extends AbstractUrlBasedView {
	protected void renderMergedOutputModel(Map<String, Object> model,
			HttpServletRequest request, HttpServletResponse response) throws Exception {

		Set<?> keys = model.keySet();
		Iterator<?> iterator = keys.iterator();

		System.out.printf("url: %s\n", getUrl());

		while (iterator.hasNext()) {
			Object key = iterator.next();

			System.out.printf("key: %s  value: %s\n", key.toString(), model.get(key));

In my browser I go to http://localhost:8080/open-lane/spring/testing/test and Tomcat dumps the following output to the console.

url: test
key: flashScope  value: map[[empty]]
key: flowRequestContext  value: [RequestControlContextImpl@1455cf4 externalContext = org.springframework.webflow.mvc.servlet.MvcExternalContext@d9b071, currentEvent = [null], requestScope = map[[empty]], attributes = map[[empty]], messageContext = [DefaultMessageContext@1ceebfa sourceMessages = map[[null] -> list[[empty]]]], flowExecution = [FlowExecutionImpl@1e6743e flow = 'testing/test', flowSessions = list[[FlowSessionImpl@d9ceea flow = 'testing/test', state = 'test', scope = map['viewScope' -> map[[empty]]]]]]]
key: flowExecutionKey  value: e1s1
key: flowExecutionUrl  value: /open-lane/spring/testing/test?execution=e1s1
key: currentUser  value: null
key: viewScope  value: map[[empty]]

I could get a little fancier by modifying my flow to:

<view-state id="test" view="/index-static.html">

And adding this line to the end of TestUrlBasedView:

request.getRequestDispatcher(getUrl()).forward(request, response);

That forwards my browser to a static html file in my application.

All the plumbing appears to be in place. Here are all the SWF libraries and dependencies I needed.

  • commons-logging-1.1.1.jar
  • spring-asm-3.1.1.RELEASE.jar
  • spring-beans-3.1.1.RELEASE.jar
  • spring-binding-2.3.1.RELEASE.jar
  • spring-context-3.1.1.RELEASE.jar
  • spring-core-3.1.1.RELEASE.jar
  • spring-expression-3.1.1.RELEASE.jar
  • spring-js-2.3.1.RELEASE.jar
  • spring-web-3.1.1.RELEASE.jar
  • spring-webflow-2.3.1.RELEASE.jar
  • spring-webmvc-3.1.1.RELEASE.jar

Time to get out of the minutia. The next post on Open Lane will introduce JSF into the application.

Source code is available at github.


¹Spring Web Flow Components diagram from Tutorial: Build a Shopping Cart with Spring Web Flow 2.0.

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