This commit introduces support in both servlet and webflux for the
"Accept-Patch" header, which is sent when the client sends unsupported
data in PATCH requests.
See  section 2.2 of RFC 5789.

Closes gh-26759

Closes gh-26470

See gh-23539
Co-authored-by: NSam Brannen <sbrannen@pivotal.io>

See gh-23539

Closes gh-23555

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).

The fix for SPR-17178 switched from debug to warn level warning for
all sub-classes of AbstractHandlerExceptionResolver where the request
concerned the DefaultHandlerExceptionResolver only.

This commit restores the original DEBUG level logging that was in
AbstractHandlerExceptionResolver from before SPR-17178. In addition
DefaultHandlerExceptionResolver registers a warnLogCategory by default
which enables warn logging and hence fulfilling the original goal
for SPR-17178.

Issue: SPR-17383

Issue: SPR-17178

Update code that has accidentally used spaces instead of tabs.
Also remove all trailing whitespace.

Issue: SPR-16968

Issue: SPR-16898

(cherry picked from commit 0795ae5c)

ResponseEntityExceptionHandler rethrows unknown exception (for further processing in DispatcherServlet's HandlerExceptionResolver chain)

Issue: SPR-16743

Issue: SPR-16528

Issue: SPR-16394

Issue: SPR-15869

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

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

HttpMessageConverter's are client and server and arguably shouldn't
contain a server-side concept such a response status.

The status field is recent, it was added to differentiate 400 vs 500
errors with Jackson 2.9+ but there is no need for it since the same
distinction is reflected in raising an HttpMessageNotReadableException
vs a general HttpMessageConversionException.

Issue: SPR-15516

InvalidDefinitionException has been introduced in Jackson 2.9 to be
able to differentiate invalid data sent from the client (should still
generate a 4xx HTTP status code) from server side errors like beans with
no default constructor (should generate a 5xx HTTP status code).

Issue: SPR-14925

Avoid call to sendError when response is committed and log a short
error message instead.

Issue: SPR-14739

Rather than setting the status to 503 directly from the timeout
interceptor which no longer seems to work reliably with Servlet
containers like Jetty even performing an additional ERROR dispatch back
to the original URL, we know rather set the DeferredResult to an
AsyncTimeoutException, which results in a dispatch and standard
handling within Spring MVC. This should be a more reliable way of
dealing with timeouts.

Issue: SPR-14669

Issue: SPR-14430

Issue: SPR-14128

Issue: SPR-13267

Before this change a missing path variable value resulted in a 400
error where in fact the error is due to a mismatch between the
declared @PathVariable and the URI template, i.e. a 500 error.

This change introduced a MissingPathVariableException as a sub-class
of ServletRequestBindingException (the exception previously thrown)
and results in a response status code of 500 by default.

Issue: SPR-13121

Prior to this commit, the DispatcherServlet would send an error
response using the HttpServlet API when no Handler was found to
handle this request.

Now the DispatcherServlet can be configured to throw a
NoHandlerFoundException, when the throwExceptionIfNoHandlerFound
property is set to "true".
Those exceptions can be later on caught by default or custom
HandlerExceptionResolvers/ExceptionHandlers.

Issue: SPR-10481

Issue: SPR-10128

 - Support external Javadoc links using Gradle's javadoc.options.links

 - Fix all other Javadoc warnings, such as typos, references to
   non-existent (or no longer existent) types and members, etc,
   including changes related to the Quartz 2.0 upgrade (SPR-8275) and
   adding the HTTP PATCH method (SPR-7985).

 - Suppress all output for project-level `javadoc` tasks in order to
   hide false-negative warnings about cross-module @see and @link
   references (e.g. spring-core having a @see reference to spring-web).
   Use the `--info` (-i) flag to gradle at any time to see project-level
   javadoc warnings without running the entire `api` task. e.g.
   `gradle :spring-core:javadoc -i`

 - Favor root project level `api` task for detection of legitimate
   Javadoc warnings. There are now zero Javadoc warnings across the
   entirety of spring-framework. Goal: keep it that way.

 - Remove all @link and @see references to types and members that exist
   only in Servlet <= 2.5 and Hibernate <= 4.0, favoring 3.0+ and 4.0+
   respectively. This is necessary because only one version of each of
   these dependencies can be present on the global `api` javadoc task's
   classpath. To that end, the `api` task classpath has now been
   customized to ensure that the Servlet 3 API and Hibernate Core 4 jars
   have precedence.

 - SPR-8896 replaced our dependency on aspectjrt with a dependency on
   aspectjweaver, which is fine from a POM point of view, but causes
   a spurious warning to be emitted from the ant iajc task that it
   "cannot find aspectjrt on the classpath" - even though aspectjweaver
   is perfectly sufficient. In the name of keeping the console quiet, a
   new `rt` configuration has been added, and aspectjrt added as a
   dependency to it. In turn, configurations.rt.asPath is appended to
   the iajc classpath during both compileJava and compileTestJava for
   spring-aspects.

Issue: SPR-10078, SPR-8275, SPR-7985, SPR-8896

DefaultHandlerExceptionResolver and ResponseEntityExceptionHandler now
both set the "javax.servlet.error.exception" request attribute to the
raised exception, allowing custom error pages configured via web.xml.

Issue: SPR-9653

Rename ExceptionHandlerSupport to ResponseEntityExceptionHandler and
emphasize the contrast to DefaultHandlerExceptionResovler -- i.e.
one returns a ResponseEntity and relies on message converters while
the other returns a ModelAndView and relies on view resolution.

Issue: SPR-9290

The DefaultHandlerExceptionResolver now provides an error message in
addition to setting the status of the response.

Issue: SPR-9312

The new class is functionally equivalent to the
DefaultHandlerExceptionResolver (i.e. it translates Spring MVC
exceptions to various status codes) but uses an @ExceptionHandler
returning a ResponseEntity<Object>, which means it can be customized to
write error content to the body of the response.

Issue: SPR-9290

Previously DefaultHandlerExceptionResolver did not handle BindException
but after this change it does. A BindException is raised when an
@ModelAttribute annotated argument is not followed by a BindingResult
argument. Hence this is unlikely to affect browser rendering.
For programmatic clients however this change ensures an unhandled
BindException is at least turned into a 400 error.

Issue: SPR-9310