This chapter details Spring’s integration with third party web frameworks.
One of the core value propositions of the Spring Framework is that of enabling choice. In a general sense, Spring does not force one to use or buy into any particular architecture, technology, or methodology (although it certainly recommends some over others). This freedom to pick and choose the architecture, technology, or methodology that is most relevant to a developer and their development team is arguably most evident in the web area, where Spring provides its own web framework (Spring MVC), while at the same time providing integration with a number of popular third party web frameworks.
Before diving into the integration specifics of each supported web framework, let us first take a look at the Spring configuration that is 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.
On to specifics: all that one need do is to declare a
{api-spring-framework}/web/context/ContextLoaderListener.html[ContextLoaderListener]
in the standard Java EE 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 configuration files to load.
Find below the <listener/> configuration:
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>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
{api-spring-framework}/web/context/WebApplicationContext.html[WebApplicationContext]
object based on the bean definitions and stores it in the ServletContext of the 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
{api-spring-framework}/web/context/support/WebApplicationContextUtils.html[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 and 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.
JavaServer Faces (JSF) is the JCP’s standard component-based, event-driven web user interface framework. As of Java EE 5, it is an official part of the Java EE umbrella.
For a popular JSF runtime as well as for popular JSF component libraries, check out the Apache MyFaces project. The MyFaces project also provides common JSF extensions such as MyFaces Orchestra: a Spring-based JSF extension that provides rich conversation scope support.
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Note
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Spring Web Flow 2.0 provides rich JSF support through its newly established Spring Faces module, both for JSF-centric usage (as described in this section) and for Spring-centric usage (using JSF views within a Spring MVC dispatcher). Check out the Spring Web Flow website for details! |
The key element in Spring’s JSF integration is the JSF ELResolver mechanism.
SpringBeanFacesELResolver is a JSF 1.2+ compliant ELResolver implementation,
integrating with the standard Unified EL as used by JSF 1.2 and JSP 2.1. Like
SpringBeanVariableResolver, it delegates to the Spring’s 'business context'
WebApplicationContext first, then to the default resolver of the underlying JSF
implementation.
Configuration-wise, simply define SpringBeanFacesELResolver in your JSF
faces-context.xml file:
<faces-config>
<application>
<el-resolver>org.springframework.web.jsf.el.SpringBeanFacesELResolver</el-resolver>
...
</application>
</faces-config>A custom VariableResolver works well when mapping one’s properties to beans
in faces-config.xml, but at times one may need to grab a bean explicitly. The
{api-spring-framework}/web/jsf/FacesContextUtils.html[FacesContextUtils]
class makes this easy. It is similar to WebApplicationContextUtils, except that it
takes a FacesContext parameter rather than a ServletContext parameter.
ApplicationContext ctx = FacesContextUtils.getWebApplicationContext(FacesContext.getCurrentInstance());Invented by Craig McClanahan, Struts is an open source project hosted by the Apache Software Foundation. At the time, it greatly simplified the JSP/Servlet programming paradigm and won over many developers who were using proprietary frameworks. It simplified the programming model, it was open source (and thus free as in beer), and it had a large community, which allowed the project to grow and become popular among Java web developers.
Check out the Struts Spring Plugin for the built-in Spring integration shipped with Struts.
From the Tapestry homepage:
Tapestry is a "Component oriented framework for creating dynamic, robust, highly scalable web applications in Java."
While Spring has its own powerful web layer, there are a number of unique advantages to building an enterprise Java application using a combination of Tapestry for the web user interface and the Spring container for the lower layers.
For more information, check out Tapestry’s dedicated integration module for Spring.