This commit also removes nullability from two common spots: ResolvableType.getType() and TargetSource.getTarget(), both of which are never effectively null with any regular implementation. For such scenarios, a non-null empty type/target is the cleaner contract.

Issue: SPR-15540

This commit introduces 2 new @Nullable and @NonNullApi
annotations that leverage JSR 305 (dormant but available via
Findbugs jsr305 dependency and already used by libraries
like OkHttp) meta-annotations to specify explicitly
null-safety of Spring Framework parameters and return values.

In order to avoid adding too much annotations, the
default is set at package level with @NonNullApi and
@Nullable annotations are added when needed at parameter or
return value level. These annotations are intended to be used
on Spring Framework itself but also by other Spring projects.

@Nullable annotations have been introduced based on Javadoc
and search of patterns like "return null;". It is expected that
nullability of Spring Framework API will be polished with
complementary commits.

In practice, this will make the whole Spring Framework API
null-safe for Kotlin projects (when KT-10942 will be fixed)
since Kotlin will be able to leverage these annotations to
know if a parameter or a return value is nullable or not. But
this is also useful for Java developers as well since IntelliJ
IDEA, for example, also understands these annotations to
generate warnings when unsafe nullable usages are detected.

Issue: SPR-15540

Prior to this commit, executing tests concurrently in the TestContext
Framework (TCF) was unsupported and typically lead to unpredictable
results.

This commit addresses this core issue by supporting concurrent
execution in the TestContextManager and the DefaultTestContext.

Specifically, the TestContextManager now uses ThreadLocal storage for
the current TestContext, thereby ensuring that any registered
TestExecutionListeners and the TestContextManager itself operate on a
TestContext specific to the current thread.

In order to avoid repeatedly incurring the costs of the overhead of the
TCF bootstrapping process, the original TestContext built by the
TestContextBootstrapper is used as a template which is then passed to
the copy constructor of the concrete implementation of the TestContext
to create the context for the current thread. DefaultTestContext now
implements such a copy constructor, and all concrete implementations of
TestContext are encouraged to do the same.

If the TestContext built by the TestContextBootstrapper does not
provide a copy constructor, thread-safety and support for concurrency
are left completely to the implementation of the concrete TestContext.

Note, however, that this commit does not address any thread-safety or
concurrency issues in the ContextLoader SPI or its implementations.

Issue: SPR-5863

Since the Spring TestContext Framework was introduced in Spring
Framework 2.5, the TestContext class has always been a public class
with package private constructors. The visibility of TestContext's
constructor and methods was intentionally limited in order to hide the
implementation details of the context cache, etc. However, this fact
has made it difficult (if not impossible) to unit test custom
TestExecutionListener implementations.

This commit addresses this issue by converting TestContext into a
public interface with a package private DefaultTestContext
implementation. This enables unit testing of any components that depend
on a TestContext (e.g., TestExecutionListeners) while at the same time
preserving the encapsulation of the inner workings of the TestContext
implementation with regard to context loading and caching.

Issue: SPR-7692

Issue: SPR-8108

Issue: SPR-10373

Prior to this commit the Spring TestContext Framework supported creating
only flat, non-hierarchical contexts. There was no easy way to create
contexts with parent-child relationships.

This commit addresses this issue by introducing a new @ContextHierarchy
annotation that can be used in conjunction with @ContextConfiguration
for declaring hierarchies of application contexts, either within a
single test class or within a test class hierarchy. In addition,
@DirtiesContext now supports a new 'hierarchyMode' attribute for
controlling context cache clearing for context hierarchies.

- Introduced a new @ContextHierarchy annotation.
- Introduced 'name' attribute in @ContextConfiguration.
- Introduced 'name' property in ContextConfigurationAttributes.
- TestContext is now aware of @ContextHierarchy in addition to
  @ContextConfiguration.
- Introduced findAnnotationDeclaringClassForTypes() in AnnotationUtils.
- Introduced resolveContextHierarchyAttributes() in ContextLoaderUtils.
- Introduced buildContextHierarchyMap() in ContextLoaderUtils.
- @ContextConfiguration and @ContextHierarchy may not be used as
  top-level, class-level annotations simultaneously.
- Introduced reference to the parent configuration in
  MergedContextConfiguration and WebMergedContextConfiguration.
- Introduced overloaded buildMergedContextConfiguration() methods in
  ContextLoaderUtils in order to handle context hierarchies separately
  from conventional, non-hierarchical contexts.
- Introduced hashCode() and equals() in ContextConfigurationAttributes.
- ContextLoaderUtils ensures uniqueness of @ContextConfiguration
  elements within a single @ContextHierarchy declaration.
- Introduced CacheAwareContextLoaderDelegate that can be used for
  loading contexts with transparent support for interacting with the
  context cache -- for example, for retrieving the parent application
  context in a context hierarchy.
- TestContext now delegates to CacheAwareContextLoaderDelegate for
  loading contexts.
- Introduced getParentApplicationContext() in MergedContextConfiguration
- The loadContext(MergedContextConfiguration) methods in
  AbstractGenericContextLoader and AbstractGenericWebContextLoader now
  set the parent context as appropriate.
- Introduced 'hierarchyMode' attribute in @DirtiesContext with a
  corresponding HierarchyMode enum that defines EXHAUSTIVE and
  CURRENT_LEVEL cache removal modes.
- ContextCache now internally tracks the relationships between contexts
  that make up a context hierarchy. Furthermore, when a context is
  removed, if it is part of a context hierarchy all corresponding
  contexts will be removed from the cache according to the supplied
  HierarchyMode.
- AbstractGenericWebContextLoader will set a loaded context as the
  ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE in the MockServletContext when
  context hierarchies are used if the context has no parent or if the
  context has a parent that is not a WAC.
- Where appropriate, updated Javadoc to refer to the
  ServletTestExecutionListener, which was introduced in 3.2.0.
- Updated Javadoc to avoid and/or suppress warnings in spring-test.
- Suppressed remaining warnings in code in spring-test.

Issue: SPR-5613, SPR-9863

Issue: SPR-10128

  find . -type f -name "*.java" -or -name "*.aj" | \
    xargs perl -p -i -e "s/[ \t]*$//g" {} \;

Issue: SPR-10127

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

This renaming more intuitively expresses the relationship between
subprojects and the JAR artifacts they produce.

Tracking history across these renames is possible, but it requires
use of the --follow flag to `git log`, for example

    $ git log spring-aop/src/main/java/org/springframework/aop/Advisor.java

will show history up until the renaming event, where

    $ git log --follow spring-aop/src/main/java/org/springframework/aop/Advisor.java

will show history for all changes to the file, before and after the
renaming.

See http://chrisbeams.com/git-diff-across-renamed-directories

- Removed generatesDefaults() and supports() from the SmartContextLoader SPI and modified the DelegatingSmartContextLoader algorithm accordingly.
- DelegatingSmartContextLoader no longer operates on a list of candidate loaders but rather on explicit instances of GenericXmlContextLoader and AnnotationConfigContextLoader.
- Updated Javadoc regarding DelegatingSmartContextLoader as the new default loader.
- Updated and polished Javadoc regarding changes in 3.1.
- Now enforcing @ContextConfiguration's restriction on declaring locations or classes but not both.

[SPR-8549] ContextCache is now keyed by MergedContextConfiguration instead of String; MergedContextConfiguration now implements custom hashCode() and equals() methods and no longer generates a context cache key.

- introduced processContextConfigurationAttributes() method in SmartContextLoader SPI
- refactored AnnotationConfigContextLoader, AbstractContextLoader, AbstractGenericContextLoader, ContextLoaderUtils, and TestContext implementations to take advantage of the SmartContextLoader SPI, MergedContextConfiguration, and ContextConfigurationAttributes
- deleted ResourceTypeAwareContextLoader
- deleted ContextLoaderUtils.LocationsResolver and implementations
- moved context key generation from TestContext to MergedContextConfiguration

[SPR-7960][SPR-8386] Supporting declarative configuration of bean definition profiles in the TestContext framework:
- TextContext now works with MergedContextConfiguration instead of locations and loader
- TextContext now builds context caching key from MergedContextConfiguration
- Test context caching is now based on locations, classes, active profiles, and context loader
- TextContext now delegates to SmartContextLoader or ContextLoader as appropriate
- AbstractContextLoader now implements SmartContextLoader
- AbstractGenericContextLoader now sets active profiles in the GenericApplicationContext 
- Introduced integration tests for profile support in the TCF for both XML and annotation config

[SPR-6184] Extracted ContextLoader resolution functionality from TestContext into a new ContextLoaderUtils utility class.

[SPR-6184] Introduced ResourceTypeAwareContextLoader interface and removed dependency on AnnotationConfigContextLoader in TestContext.

[SPR-6184] AnnotationConfigContextLoader now generates a default configuration class name; fleshed out documentation of AnnotationConfigContextLoader; introduced sub-package and suite for configuration class tests.

[SPR-6104] @TestExecutionListeners now supports a 'listeners' alias for its existing 'value' attribute.

[SPR-5916] ContextLoader class is now inherited from class hierarchy if not specified explicitly via @ContextConfiguration's loader attribute on the current test class.

[SPR-5710] Subclasses of SpringJUnit4ClassRunner can now override the default ContextLoader class via the new getDefaultContextLoaderClassName(Class) method.

EL container integration; support for contextual objects; removal of deprecated Spring 2.0 functionality; Java 5 code style