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 you 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 frameworks (Spring MVC and Spring WebFlux) while, at the same time, supporting 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 common Spring configuration that is not specific to any one web framework. (This section is equally applicable to Spring’s own web framework variants.)
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) that are 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, and others 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 you can configure a Spring container (a
WebApplicationContext) that contains all of the 'business beans' in your application.
Moving on to specifics, all you need to do is declare a
{api-spring-framework}/web/context/ContextLoaderListener.html[ContextLoaderListener]
in the standard Java EE servlet web.xml file of your web application and add a
contextConfigLocation<context-param/> section (in the same file) that defines which
set of Spring XML configuration files to load.
Consider the following <listener/> configuration:
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>Further consider the following <context-param/> configuration:
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/applicationContext*.xml</param-value>
</context-param>If you do not specify the contextConfigLocation context parameter, the
ContextLoaderListener looks 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, so you can use the
following code snippet to get access to this “business context” ApplicationContext
created by the ContextLoaderListener.
The following example shows how to get the WebApplicationContext:
WebApplicationContext ctx = WebApplicationContextUtils.getWebApplicationContext(servletContext);The
{api-spring-framework}/web/context/support/WebApplicationContextUtils.html[WebApplicationContextUtils]
class is for convenience, so you need not remember the name of the ServletContext
attribute. Its getWebApplicationContext() method returns null if an object
does not exist under the WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
key. Rather than risk getting NullPointerExceptions in your application, it is 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 let you 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. It is an official part of the Java EE umbrella but also individually usable, e.g. through embedding Mojarra or MyFaces within Tomcat.
Please note that recent versions of JSF became closely tied to CDI infrastructure in application servers, with some new JSF functionality only working in such an environment. Spring’s JSF support is not actively evolved anymore and primarily exists for migration purposes when modernizing older JSF-based applications.
The key element in Spring’s JSF integration is the JSF ELResolver mechanism.
SpringBeanFacesELResolver is a JSF compliant ELResolver implementation,
integrating with the standard Unified EL as used by JSF and JSP. It delegates to
Spring’s “business context” WebApplicationContext first and then to the
default resolver of the underlying JSF implementation.
Configuration-wise, you can define SpringBeanFacesELResolver in your JSF
faces-context.xml file, as the following example shows:
<faces-config>
<application>
<el-resolver>org.springframework.web.jsf.el.SpringBeanFacesELResolver</el-resolver>
...
</application>
</faces-config>A custom ELResolver works well when mapping your properties to beans in
faces-config.xml, but, at times, you may need to explicitly grab a bean.
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.
The following example shows how to use FacesContextUtils:
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 let the project grow and become popular among Java web developers.
As a successor to the original Struts 1.x, check out Struts 2.x and the Struts-provided Spring Plugin for the built-in Spring integration.
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 by using a combination of Tapestry for the web user interface and the Spring container for the lower layers.
For more information, see Tapestry’s dedicated integration module for Spring.