Support loading WebApplicationContexts in the TCF

[sbrannen]

Prior to this commit, the Spring TestContext Framework only supported
loading an ApplicationContext in integration tests from either XML or
Java Properties files (since Spring 2.5), and Spring 3.1 introduced
support for loading an ApplicationContext in integration tests from
annotated classes (e.g., @configuration classes). All of the
ContextLoader implementations used to provide this support load a
GenericApplicationContext. However, a GenericApplicationContext is not
suitable for testing a web application since a web application relies on
an implementation of WebApplicationContext (WAC).

This commit makes it possible to integration test Spring-powered web
applications by adding the following functionality to the Spring
TestContext Framework.

• Introduced AbstractGenericWebContextLoader and two concrete
  subclasses:
  • XmlWebContextLoader
  • AnnotationConfigWebContextLoader
• Pulled up prepareContext(context, mergedConfig) from
  AbstractGenericContextLoader into AbstractContextLoader to allow it
  to be shared across web and non-web context loaders.
• Introduced AnnotationConfigContextLoaderUtils and refactored
  AnnotationConfigContextLoader accordingly. These utils are also used
  by AnnotationConfigWebContextLoader.
• Introduced a new @WebAppConfiguration annotation to denote that the
  ApplicationContext loaded for a test should be a WAC and to configure
  the base resource path for the root directory of a web application.
• Introduced WebMergedContextConfiguration which extends
  MergedContextConfiguration with support for a baseResourcePath for
  the root directory of a web application.
• ContextLoaderUtils.buildMergedContextConfiguration() now builds a
  WebMergedContextConfiguration instead of a standard
  MergedContextConfiguration if @WebAppConfiguration is present on the
  test class.
• Introduced a configureWebResources() method in
  AbstractGenericWebContextLoader that is responsible for creating a
  MockServletContext with a proper ResourceLoader for the
  resourceBasePath configured in the WebMergedContextConfiguration. The
  resulting mock ServletContext is set in the WAC, and the WAC is
  stored as the Root WAC in the ServletContext.
• Introduced a WebTestExecutionListener that sets up default thread
  local state via RequestContextHolder before each test method by using
  the MockServletContext already present in the WAC and by creating a
  MockHttpServletRequest, MockHttpServletResponse, and
  ServletWebRequest that is set in the RequestContextHolder. WTEL also
  ensures that the MockHttpServletResponse and ServletWebRequest can be
  injected into the test instance (e.g., via @Autowired) and cleans up
  thread locals after each test method.
• WebTestExecutionListener is configured as a default
  TestExecutionListener before DependencyInjectionTestExecutionListener
• Extracted AbstractDelegatingSmartContextLoader from
  DelegatingSmartContextLoader and introduced a new
  WebDelegatingSmartContextLoader.
• ContextLoaderUtils now selects the default delegating ContextLoader
  class name based on the presence of @WebAppConfiguration on the test
  class.
• Tests in the spring-test-mvc module no longer use a custom
  ContextLoader to load a WebApplicationContext. Instead, they now
  rely on new core functionality provided in this commit.

Issue: SPR-5243

[cbeams]

Great to see this, Sam. Thanks!

[rstoyanchev]

Yes, indeed!