This first commit for this issue:

- allows JUnit Jupiter to be used for all tests
- adds a dependency on mockito-junit-jupiter
- migrates tests in spring-core to JUnit Jupiter, except parameterized
  tests

The following script was developed in order to semi-automate the
migration process.

https://github.com/sbrannen/junit-converters/blob/master/junit4ToJUnitJupiter.zsh

See gh-23451

See gh-23451

Prior to this commit, StopWatch used System.currentTimeMillis() to
track and report running time in milliseconds.

This commit updates the internals of StopWatch to use System.nanoTime()
instead of System.currentTimeMillis(). Consequently, running time is
now tracked and reported in nanoseconds; however, users still have the
option to retrieve running time in milliseconds or seconds.

Closes gh-23235

Since arbitrary levels of proxies do not occur, this commit replaces
the `while` loop in SerializableTypeWrapper.unwrap() with a simple
`if` statement.

Closes gh-23415

Prior to this commit, AnnotationAttributes#assertNotException checked if
the attribute value was an instance of Exception. Although this was
typically sufficient, the scope was not always broad enough -- for
example, if AnnotationReadingVisitorUtils#convertClassValues stored a
Throwable in the map (such as a LinkageError).

This commit fixes this by checking for an instance of Throwable in
AnnotationAttributes#assertNotException.

Closes gh-23424

Deprecate all mutation methods in `MethodParameter` in favor of factory
methods that return a new instance. Existing code that previously relied
on mutation has been updated to use the replacement methods.

Closes gh-23385

Prior to this commit, the new `TYPE_HIERARCHY_AND_ENCLOSING_CLASSES`
annotation search strategy failed to find annotations on enclosing
classes if the source class was a nested class that itself had no
annotations present.

This commit fixes this by adding special logic to AnnotationsScanner's
isWithoutHierarchy() method to properly support nested classes.

Closes gh-23378

Add a `TYPE_HIERARCHY_AND_ENCLOSING_CLASSES` annotation search strategy
that can be used to search the full type hierarchy as well as any
enclosing classes.

Closes gh-23378

Rename `SearchStrategy.EXHAUSTIVE` from `MergedAnnotations` to
`SearchStrategy.TYPE_HIERARCHY`

See gh-23378

See gh-23352

Closes gh-23352

See gh-23382

See gh-23300

Closes gh-23300

See gh-23321

Fix `isAssignable` for `ResolvableType.forRawClass` so that it can be
used with types backed by a `TypeVarible`. Prior to this commit the
rawClass value was used, which wouldn't always work.

Closes gh-23321

Closes gh-23317

This commit introduces the DataBufferWrapper, a wrapper for DataBuffers,
and uses it in applicable use cases.

See gh-23297

Prior to this commit, a null path supplied to the isPattern(), match(),
and matchStart() methods in AntPathMatcher resulted in a
NullPointerException.

This commit addresses this by treating a `null` path as a non-matching
pattern.

Closes gh-23297

DataBuffers::split (and underlying algorithm) should not be returning
a Flux<DataBuffer>, but rather a Flux<Flux<DataBuffer>>. In other words,
it should not join all data buffers that come before a delimiter.

Providing an implementation of a such a fully reactive split method
proved to be beyond the scope of this release, so this commit removes
the method altogether.

Prior to Spring Framework 5.1.3, MimeTypeUtils.parseMimeTypes() and
MediaType.parseMediaTypes() ignored empty entries, but 5.1.3 introduced
a regression in that an empty entry -- for example, due to a trailing
comma in the list of media types in an HTTP Accept header -- would result
in a "406 Not Acceptable" response status.

This commit fixes this by filtering out empty entries before parsing
them into MimeType and MediaType instances. Empty entries are therefore
effectively ignored.

Fixes gh-23241

Closes gh-23208

Closes gh-23125

Update `MimeTypeUtils` so that the  StringUtils.hasLength check is
performed immediately on the incoming argument, rather than in
`parseMimeTypeInternal`. This restores the `IllegalArgumentException`
rather than the `NullPointerException` which is thrown by the
`ConcurrentHashMap`.

Closes gh-23215
See gh-23211

See gh-23187

Closes gh-23191

See gh-23187

This commit adds two methods to DataBufferUtils:
- one that reads a Flux<DataBuffer> from a Path
- one that writes a Flux<DataBuffer> to a Path

Closes gh-23111

Closes gh-22957

This commit introduces an internal SortedProperties class that is a
specialization of java.util.Properties which sorts properties
alphanumerically based on their keys.

This can be useful when storing a java.util.Properties instance in a
properties file, since it allows such files to be generated in a
repeatable manner with consistent ordering of properties.

Comments in generated properties files can also be optionally omitted.

An instance of SortedProperties can be created via two new
createSortedProperties() factory methods in
org.springframework.core.CollectionFactory.

Closes gh-23018

Rename some `MergedAnnotation` methods to prevent using parent/child
terminology, specifically:

	`getDepth()` has been renamed `getDistance()`
	`getParent()` has been renamed `getMetaSource()`
	`getTypeHierarchy()` has been renamed `getMetaTypes()`

The parent child naming was particularly confusing given that the
parent/child relationships were inverted from the way that a lot of
users think about meta-annotations. For example, a `@RequestMapping`
having a parent of `@GetMapping` feels odd given that `@GetMapping`
is the thing declaring the meta-annotation relationship.

The new method names are designed to align more closely with existing
terms. For example, `getMetaSource` hints at the relationship with
`isMetaAnnotated` and `getSource`.

Closes gh-22946