19.2 Common configuration

Before diving into the integration specifics of each supported web framework, let us first take a look at the Spring configuration that not specific to any one web framework. (This section is equally applicable to Spring's own web framework, Spring MVC.)

One of the concepts (for want of a better word) espoused by (Spring's) lightweight application model is that of a layered architecture. Remember that in a 'classic' layered architecture, the web layer is but one of many layers... it serves as one of the entry points into a server side application, and it delegates to service objects (facades) defined in a service layer to satisfy business specific (and presentation-technology agnostic) use cases. In Spring, these service objects, any other business-specific objects, data access objects, etc. exist in a distinct 'business context', which contains no web or presentation layer objects (presentation objects such as Spring MVC controllers are typically configured in a distinct 'presentation context'). This section details how one configures a Spring container (a WebApplicationContext) that contains all of the 'business beans' in one's application.

Onto specifics... all that one need do is to declare a ContextLoaderListener in the standard J2EE servlet web.xml file of one's web application, and add a contextConfigLocation <context-param/> section (in the same file) that defines which set of Spring XML cpnfiguration files to load.

Find below the <listener/> configuration:

<listener>
  <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
[Note]Note

Listeners were added to the Servlet API in version 2.3; listener startup order was finally clarified in Servlet 2.4. If you have a Servlet 2.3 container, you can use the ContextLoaderServlet to achieve the same functionality in a 100% portable fashion (with respect to startup order).

Find below the <context-param/> configuration:

<context-param>
  <param-name>contextConfigLocation</param-name>
  <param-value>/WEB-INF/applicationContext*.xml</param-value>
</context-param>

If you don't specify the contextConfigLocation context parameter, the ContextLoaderListener will look for a file called /WEB-INF/applicationContext.xml to load. Once the context files are loaded, Spring creates a WebApplicationContext object based on the bean definitions and stores it in the ServletContext of one's web application.

All Java web frameworks are built on top of the Servlet API, and so one can use the following code snippet to get access to this 'business context' ApplicationContext created by the ContextLoaderListener.

WebApplicationContext ctx = WebApplicationContextUtils.getWebApplicationContext(servletContext);

The WebApplicationContextUtils class is for convenience, so you don't have to remember the name of the ServletContext attribute. Its getWebApplicationContext() method will return null if an object doesn't exist under the WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE key. Rather than risk getting NullPointerExceptions in your application, it's better to use the getRequiredWebApplicationContext() method. This method throws an exception when the ApplicationContext is missing.

Once you have a reference to the WebApplicationContext, you can retrieve beans by their name or type. Most developers retrieve beans by name, then cast them to one of their implemented interfaces.

Fortunately, most of the frameworks in this section have simpler ways of looking up beans. Not only do they make it easy to get beans from a Spring container, but they also allow you to use dependency injection on their controllers. Each web framework section has more detail on its specific integration strategies.