Closes gh-23300

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6515 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6625 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

Prefix the opening brace with a single space in some classes.
Replace tab character with space in javadoc of AttributeAccessor.
Also fix some other trivial formatting errors.

Closes gh-1979

Issue: SPR-15918
Issue: SPR-15919
Issue: SPR-15676

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

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

Issue: SPR-15395

This commit also fixes broken javadoc links and code references.

Issue: SPR-13188

Issue: SPR-14349

Prior to this commit, there existed several isEmpty() methods scattered
across various utilities such as ObjectUtils, CollectionUtils, and
StringUtils; however, each of these methods requires a cast to the type
supported for that particular variant.

This commit introduces a general-purpose isEmpty(Object) method in
ObjectUtils that transparently supports multiple object types in a
central location without the need for casts or juggling multiple
utility classes.

Issue: SPR-13119

Fix remaining Java compiler warnings, mainly around missing
generics or deprecated code.

Also add the `-Werror` compiler option to ensure that any future
warnings will fail the build.

Issue: SPR-11064

Issue: SPR-10128

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

Introduce FeatureSpecification interface and implementations

    FeatureSpecification objects decouple the configuration of
    spring container features from the concern of parsing XML
    namespaces, allowing for reuse in code-based configuration
    (see @Feature* annotations below).

    * ComponentScanSpec
    * TxAnnotationDriven
    * MvcAnnotationDriven
    * MvcDefaultServletHandler
    * MvcResources
    * MvcViewControllers

Refactor associated BeanDefinitionParsers to delegate to new impls above

    The following BeanDefinitionParser implementations now deal only
    with the concern of XML parsing.  Validation is handled by their
    corresponding FeatureSpecification object.  Bean definition creation
    and registration is handled by their corresponding
    FeatureSpecificationExecutor type.

    * ComponentScanBeanDefinitionParser
    * AnnotationDrivenBeanDefinitionParser (tx)
    * AnnotationDrivenBeanDefinitionParser (mvc)
    * DefaultServletHandlerBeanDefinitionParser
    * ResourcesBeanDefinitionParser
    * ViewControllerBeanDefinitionParser

Update AopNamespaceUtils to decouple from XML (DOM API)

    Methods necessary for executing TxAnnotationDriven specification
    (and eventually, the AspectJAutoProxy specification) have been
    added that accept boolean arguments for whether to proxy
    target classes and whether to expose the proxy via threadlocal.

    Methods that accepted and introspected DOM Element objects still
    exist but have been deprecated.

Introduce @FeatureConfiguration classes and @Feature methods

    Allow for creation and configuration of FeatureSpecification objects
    at the user level.  A companion for @Configuration classes allowing
    for completely code-driven configuration of the Spring container.

    See changes in ConfigurationClassPostProcessor for implementation
    details.

    See Feature*Tests for usage examples.

    FeatureTestSuite in .integration-tests is a JUnit test suite designed
    to aggregate all BDP and Feature* related tests for a convenient way
    to confirm that Feature-related changes don't break anything.
    Uncomment this test and execute from Eclipse / IDEA. Due to classpath
    issues, this cannot be compiled by Ant/Ivy at the command line.

Introduce @FeatureAnnotation meta-annotation and @ComponentScan impl

    @FeatureAnnotation provides an alternate mechanism for creating
    and executing FeatureSpecification objects.  See @ComponentScan
    and its corresponding ComponentScanAnnotationParser implementation
    for details.  See ComponentScanAnnotationIntegrationTests for usage
    examples

Introduce Default[Formatting]ConversionService implementations

    Allows for convenient instantiation of ConversionService objects
    containing defaults appropriate for most environments.  Replaces
    similar support originally in ConversionServiceFactory (which is now
    deprecated). This change was justified by the need to avoid use
    of FactoryBeans in @Configuration classes (such as
    FormattingConversionServiceFactoryBean). It is strongly preferred
    that users simply instantiate and configure the objects that underlie
    our FactoryBeans. In the case of the ConversionService types, the
    easiest way to do this is to create Default* subtypes. This also
    follows convention with the rest of the framework.

Minor updates to util classes

    All in service of changes above. See diffs for self-explanatory
    details.

    * BeanUtils
    * ObjectUtils
    * ReflectionUtils

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