Issue: SPR-17086

1. Avoid re-creating the List with delimited byte arrays on every
request if using the default delimiters which don't vary by charset.

2. Replace flatMap with flatMapIterable for splitOnDelimiter.

3. Avoid going through DataBufferUtils#join, and unnecessarily creating
Flux from the List, since the join method needs a list anyway.

Fix ResourceRegionEncoder so that it checks for resource existance
before writing boundaries. Also defer data buffer allocation until
necessary.

Issue: SPR-17419

Issue: SPR-17418

1. Helper method to eliminate duplication in formatting (de-)serialized
   values for logging introduced with prior commit #e62298.

2. Helper method for TRACE vs DEBUG logging with different details.

Issue: SPR-17254

At DEBUG show up to 100 chars, at TRACE show full formatted value.

Note that the formatValue helper method is duplicated a number of times
in this commit. A utility method will likely be added in spring-core
through an extra commit.

Issue: SPR-17254

Issue: SPR-17086

Issue: SPR-17029

Issue: SPR-17012

Issue: SPR-16966

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

Issue: SPR-16968

Update all classes so that inner classes are always last. Also
ensure that utility classes are always final and have a private
constructor and make exceptions final whenever possible.

Issue: SPR-16968

Fix checkstyle violations for javadoc.

Issue: SPR-16968

Issue: SPR-16898

Issue: SPR-16892

Issue: SPR-16606

EncoderHttpMessageWriter checks explicitly for Mono publishers and sets
the content length, if it is known for the given data item.

Issue: SPR-16542

Closes gh-1670

Issue: SPR-16337

Issue: SPR-16365

Use DataBufferUtils.compose instead of writeAggregator to combine
multiple data buffers into one, as the write aggregator would not work
when the initial data buffer did not have enough capacity to contain
all subsequent buffers.

Removed writeAggregator, as it is no longer needed.

Issue: SPR-16365

This commit uses the newly introduced write aggregator from
DataBufferUtils throughout WebFlux.

Issue: SPR-16351

This commit ensures uniform support for decodeToMono across the various
byte and String related decoders.

Issue: SPR-16253

This commit introduces the following changes.

1) It adds a new Spring @NonNull annotation which allows to apply
@NonNullApi semantic on a specific element, like @Nullable does.
Combined with @Nullable, it allows partial null-safety support when
package granularity is too broad.

2) @Nullable and @NonNull can apply to ElementType.TYPE_USE in order
to be used on generic type arguments (SPR-15942).

3) Annotations does not apply to ElementType.TYPE_PARAMETER anymore
since it is not supported yet (applicability for such use case is
controversial and need to be discussed).

4) @NonNullApi does not apply to ElementType.FIELD anymore since in a
lot of use cases (private, protected) it is not part for the public API
+ its usage should remain opt-in. A dedicated @NonNullFields annotation
has been added in order to set fields default to non-nullable.

5) Updated Javadoc and reference documentation.

Issue: SPR-15756

Currently ResourceEncoder and ResourceRegionEncoder use DataBufferUtils
to read resource with an AsynchronousFileChannel if it is a file or
otherwise fallback on getting the channel from the resource.

The same is now required in other places where a Resource needs to be
read and is also generally useful.

Issue: SPR-15773

Issue: SPR-15613

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

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

Starting with removing a package cycle on the use of
ResponseStatusException in the codec package, this commit generally
refines codec exception handling.

The new [Encoding|Decoding]Exception mirror the existing
HttpMessageNot[Readable|Writable]Exception and are used similarly
especially to differentiate betwen 400 and 500 errors when parsing
server request body content.

The commit also aligns some of the exception handling of JSON and XML
on the WebFlux side with that on the Spring MVC side.

Issue: SPR-15516

Issue: SPR-15464

Issue: SPR-15421

Issue: SPR-15318